CRWR?Online?Report?10?06?
?
?
?
Hydrologic?Analysis?Before?and?After??
Reservoir?Alteration?at?the?Big?Bend?Reach,?
Rio?Grande/Rio?Bravo?
?
?
?
by?
?
Samuel?Sandoval?Solis,?M.?S.?
?
Benjamin?Reith?
?
and?
Daene?C.?McKinney,?Ph.D.,?PE?
August?2010?
?
?
CENTER?FOR?RESEARCH?IN?WATER?RESOURCES?
?
Bureau?of?Engineering?Research???The?University?of?Texas?at?Austin?
J.J.?Pickle?Research?Campus.???Austin,?TX?78712?4497?
This?document?is?available?online?via?World?Wide?Web?at?
http://www.ce.utexas.edu/centers/crwr/reports/online.html?
Acknowledgments?
The?writers?would?like?to?thank?the?National?Council?of?Science?and?Technology?of?Mexico?
CONACYT?and?the?National?Science?Foundation?for?the?financial?support?provided?to?the?first?and?
second?author,?respectively.?
?
?
?
EXECUTIVE?SUMMARY?
The?Rio?Grande/Bravo?basin?is?located?in?North?America?between?two?riparian?nations,?the?
United?States?(U.S.)?and?Mexico.?The?Big?Bend?reach,?is?located?along?the?Rio?Grande/Bravo,?
between?the?cities?of?Presidio/Ojinaga?to?Amistad?international?Reservoir.?Important?habitats?such?
as?the?Big?Ben?national?and?state?park?in?the?U.S.?and?Maderas?del?Carmen,?Ca?on?de?Santa?Elena?
and?Ocampo?in?Mexico?form?part?of?this?reach,?and?depend?on?the?water?in?the?river?for?their?
environmental?requirements.?The?Treaty?of?1944?establishes?the?water?allocation?in?this?region;?
unfortunately,?this?regulation?does?not?consider?the?environment?as?an?integral?part?of?the?water?
management.?As?a?result,?the?environmental?condition?in?this?reach?has?been?deteriorated.?One?sign?
of?the?deterioration?is?the?continuous?channel?narrowing?in?this?reach.?Prior?1946,?the?Big?Bend?
reach?used?to?maintained?a?wide,?sandy,?multi?thread?channel.?Frequent?floods?moved?sediments?
and? prevented? any? substantial? accumulation.? In? the? 1940?s,? the? construction? of? the? Caballo?
(completed?in?1938)?and?Francisco?I.?Madero?(completed?in?1949)?reservoirs?altered?its?hydraulic?
properties,? becoming? a? narrower? and? single?thread? river.? Channel? narrowing? was? temporally?
interrupted?by?large?floods?that?re?widen?the?channel.?In?addition,?because?of?the?reservoirs,?floods?
were?also?less?frequent?in?time?and?magnitude.??
Two?are?the?main?objectives?of?this?study:?1)?characterize?and?compare?extreme?flood?events,?
normal?and?drought?flows?in?the?Big?Bend?reach?for?two?periods,?prior?the?dams?alteration?(pre?
1946)?and?after?(post?1946);?and?2)?propose?an?annual?hydrograph?that?mimics?the?prior?alteration?
hydrologic? characteristics.? Each?period?has?been?analyzed?to?determine? its? hydrologic?
characteristics.? For? extreme? flood? events? we? are? concerned? with? peak? flows,? frequency,? return?
periods?and?the?volume?of?water?required?to?trigger?river?widening.?For?normal?and?drought?flows,?
three?benchmarks?are?used:?median?monthly?flows,?high?flows?and?small?floods.?The?magnitude,?
timing? and? frequencies? of? these? benchmarks? are? obtained? and? compared? between? periods? of?
analysis.?Based?on?the?analysis?and?comparison?of?both?periods,?an?annual?hydrograph?is?proposed?
in?order?to:?1)?mimic?the?hydrologic?characteristics?of?the?pre?1946?conditions,?and?2)?try?to?restore?
the?environmental?conditions?in?the?Big?Bend?reach.?The?following?sections?are?a?summary?of?the?
results?found.?More?detailed?information?of?the?methods?used?and?results?founded?are?presented?in?
the?main?document.?
Extreme?Flood?Events?
Extreme?flood?events?(EFE)?have?been?analyzed?at?the?Big?Bend?reach?using?Johnson?Ranch?gauge?
station.?These?events?typically?occur?in?late?August,?September?and?early?October.??
Prior? 1946,? four? EFE? occurred? in? late? August? early? September? (1904,? 1932,? 1938,? and? 1942),?
durations?range?from?8?to?30?days?and?peak?daily?flows?from?1,560?to?3,851?m
3
/s;?the?total?flow?in?
any?flooding?event?is?larger?than?1,000X10
6
?m
3
.?These?occurrences,?along?with?frequent?smaller?
floods?moved?sediment?and?prevented?any?substantial?accumulation?in?the?canal,?maintaining?a?
wide,?sandy?multi?threaded?river?with?regular?resetting?events.?
??ii???
After?1946,?four?EFE?occurred?in?September?(1958,?1978,?1990?1991?and?2008),?durations?range?
from?21?to?38?days?and?peak?daily?flows?from?1,410?to?1,850?m
3
/s;?the?total?flow?in?any?flooding?
event?is?larger?than?1,000X10
6
?m
3
.?These?EFE?have?re?widen?the?channel?after?period?of?agradation?
and?channel?narrowing?between?extreme?flood?events.?
After?analyzing?and?comparing?EFE?between?both?periods,?these?are?the?conclusions:?
? EFE?that?re?widen?the?channel?have:?a)?a?peak?flow?larger?than?1,400?m
3
/s?and?b)?a?total?
flow?1,000X10
6
?m
3
?for?the?whole?flooding?event.?
? EFE?post?1946?are?less?frequent?than?pre?1946,?they?happen?in?the?double?of?time?they?used?
to;?once?every?15~16?years?in?the?post?1946?compared?to?once?every?7~8?years?in?the?pre?
1946?period.??
? Peak?flows?post?1946?are?lower?than?post?1946,?about?half?of?the?magnitude?(on?average?
48%)?(Figure?ES?1)?
? EFE?happen?three?weeks?later?in?the?post?1946?period,?from?October?1?7
th
?in?the?pre?1946?
to?October?21?28
th
?in?the?post?1946.?
?
Figure?ES?1.?Log?Pearson?III?distributions,?Pre?1946?and?Post?1946?
Figure?ES?1?show?the?Log?Pearson?type?III?distribution?for?annual?maximum?flows?in?both?periods.?
HEC?SSP,?software?developed?by?the?US?Army?Corps?of?Engineers,?is?used?to?examine?EFE?in?this?
region.?The?Log?Pearson?distributions?show?that?post?1946?maximum?annual?flows?are?about?half?
(48%)?of?the?pre?1946?maximum?annual?flows?for?the?same?return?period.?
Analysis?of?Normal?and?Drought?Conditions?
In?order?to?characterize?normal?and?drought?conditions,?the?mean?daily?flows?at?Johnson?Ranch?
have?been?analyzed.?Flows?have?been?categorized?according?to?the?following?benchmarks.?
10
100
1000
10000
0.0010.010.11
Flow
?
(m
3
/s)
Probability?of?Exceedance
Post?1946
5%?Confidence
95%?Confidence
Pre?1946?
5%?Confidence
95%?Confidence
105 10050
20020
500
Return?Period?(Years)
??iii???
? Median?monthly?flow?is?the?median?value?of?the?mean?daily?flows?for?each?month.?
? High?flows?are?flows?with?a?peak?between?the?75
th
?(56?m
3
/s)?and?95
th
?(224?m
3
/s)?percentile?
of?the?mean?daily?flows?for?the?pre?1946?period.?
? Small?floods?are?flows?with?a?peak?between?the?95
th
?percentile?(224?m
3
/s)?of?the?mean?daily?
flows?for?the?pre?1946?period?and?1,100?m
3
/s,?which?is?a?threshold?to?account?for?the?levees?
upstream? the? Big? Bend? reach? at? Presidio/Ojinaga? cities;? maximum? flow? of? 1,190? m
3
/s?
(42,000?cfs).?
? Large? floods?are?flows?with?a?peak?above?1,100?m
3
/s.?These?large?floods?will?definitely?
threaten?the?safety?of?the?levees?in?the?Ojinaga/Presidio?Valley.?
The?thresholds?for?the?benchmarks?have?been?defined?using?the?pre?1946?data?in?order?to:?1)?
determine? the? characteristics? (frequency,? peak? flows? and? duration)? of? the? benchmarks? that?
occurred?prior?the?alteration?of?the?channel,?and?2)?compare?the?frequency?of?these?benchmarks?
prior?alteration?(pre?1946)?with?the?post?alteration?period?(post?1946).?The?median?values?have?
been?used?to?determine?characteristics?hydrographs?of?high?flows?and?small?floods?for?each?month?
and?period?of?analysis?(pre?1946?and?post?1946).?
Two?hydrologic?periods?have?been?analyzed:?1)?pre?1946,?a?30?year?period?of?analysis?(Jan/1901?to?
Dec/1913?and?Jan/1930?to?Dec/1946);?and?2)?post?1946,?which?data?also?for?30?years?(Jan/1980?to?
Dec/2009).?For?the?post?1946?the?last?thirty?year?period?has?been?chosen?with?the?aim?of?analyze?
the?hydrological?characteristics?of?the?immediate?hydrology.?This?period?contains?a?wide?range?of?
hydrologic?conditions,?from?the?wet?period?of?the?80?s?(1984?1993)?to?the?severe?and?extended?
drought?of?the?90?s?(1994?2007).??
The?Indicators?of?Hydraulic?Alteration?(IHA)?software?has?been?used?to?label?the?different?flows?
along?the?periods?of?analysis.?High?flows?and?small?floods?have?been?characterized?by?determining?
their?hydrograph?for?each?period?of?analysis.?Return?periods?for?high?flows?and?small?flood?have?
been?calculated?using?the?frequency?of?these?benchmarks?in?the?respective?period?of?analysis.?
Normal?Hydrologic?Conditions?
The?daily?mean?flow?values?at?Johnson?Ranch?for?both?periods?has?been?analyzed?in?order?to?
determine?a?characteristic?hydrograph?for?the?following?return?periods?(T):a)?1?year,?b)2?to?3?years,?
c)?5?to10?years,?and?d)?more?than?10?years.?
Prior?1946,?median?monthly?values?varied?from?11?m
3
/s?in?April?to?98?m
3
/s?in?September.?A?median?
of?7?high?flows?occurred?per?year.?High?flows?occurred?every?year?in?July,?August,?October?and?
December?and?once?every?2?3?year?in?the?rest?of?the?months.?A?median?of?2?small?floods?occurred?
every?year,?one?fixed?in?September?(peak?flow=407?m
3
/s)?and?another?small?flood?variable?in?time?
that?usually?happened?in?July?(peak?flow=405?m
3
/s),?August?(peak?flow=396?m
3
/s)?or?October?
(peak?flow=422?m
3
/s).?The?combination?of?two?small?floods?may?have?contributed?to?maintaining?
the?Rio?Grande/Bravo?as?a?wide?sandy?multi?threaded?river.?Figure?ES?2?shows?a?proposed?annual?
hydrograph?for?the?pre?1946?conditions;?it?includes?the?benchmarks?for?1?and?2?3?year?return?
period?(T).?This?hydrograph?have?the?fixed?small?flood?in?September?and?the?moving?small?flood?in?
July.?The?volume?of?the?1?and?2?3?year?return?period?is?1,292X10
6
?m
3
?and?197X10
6
?m
3
?respectively;?
??iv???
thus,?the?annual?hydrograph?shown?in?Figure?ES?2?has?a?volume?of?1,489X10
6
?m
3
.?Values?for?the?
median?monthly?flow?and?the?peak?flows?of?the?benchmarks?are?also?included?in?the?figure.?
?
Figure?ES?2.?Pre?1946?annual?hydrograph?
Post?1946,?median?monthly?values?varied?from?8?m
3
/s?in?April?to?24?m
3
/s?in?September.?A?median?
of?8?high?flows?occurred?per?year.?High?flows?occur?every?year?from?May?to?August;?and?once?every?
2?3?year?in?March,?April,?September?and?October.?A?median?of?0.5?small?floods?occurred?every?year,?
meaning?that?certain?year?one?small?flood?may?occur?September?(peak?flow=341?m
3
/s),?and?the?
following?year?may?not?happen?any?small?flood.?The?reduction?in?the?frequency?and?magnitude?of?
small?floods?contributed?to?the?deterioration?of?the?environmental?conditions?at?the?Big?Bend?reach.?
This?deterioration?has?been?evident?by?the?progressive?channel?narrowing?and?vertical?acreation.?
Figure?ES?3?shows?a?proposed?annual?hydrograph?for?the?post?1946?conditions;?it?includes?the?
benchmarks?for?1?and?2?3?year?return?period?(T).?This?hydrograph?only?have?one?small?flood?in?
September?that?is?smaller?in?magnitude?than?the?pre?1946?value.?The?volume?of?the?1?and?2?3?year?
return?period?is?492X10
6
?m
3
?and?94X10
6
?m
3
?respectively;?thus,?the?annual?hydrograph?shown?in?
Figure?ES?3?has?a?volume?of?586X10
6
?m
3
.?The?median?monthly?values?are?also?smaller,?than?the?pre?
1946?values.?Values?for?the?median?monthly?flow?and?the?peak?flows?of?the?benchmarks?are?also?
included?in?the?figure.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May????????Jun???????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
123
99
405
45
75
102
58
97
407
Small?Flood
2<T<3
Small?Flood
T?1
High?Flows
T?1
27
24 23
11
20
26
44
50
98
51
30
26
56?m
3
/s
224?m
3
/s
High?Flows
2<T<3
??v???
?
Figure?ES?3.?Post?1946?annual?hydrograph?
Comparing?both?hydrographs?it?can?be?concluded?that?part?of?what?happened?every?year?in?the?pre?
1946?period?(annual?hydrograph?with?2?small?floods)?now?it?is?happening?every?2?3?years?in?the?
post?1946?period?(annual?hydrograph?with?only?one?small?flood).?In?fact,?the?post?1946?hydrograph?
represents?the?40%?of?the?pre?1946?volume.?In?the?post?1946?hydrograph?there?is?a?proliferation?of?
high?flows?but?not?of?small?floods.?After?analyzing?different?annual?hydrographs?for?different?return?
periods,?these?are?the?conclusions:?
? The?annual?volume?of?median?monthly?flows?decreased?by?656.8?X10
6?
m
3
,?or?59.0%?of?the?
pre?1946?annual?quantity.?
? Small?floods?in?the?pre?1946?conditions?were?frequent?(peak?flow=400?m
3
/s),?twice?per?
year;?one?fixed?in?September?and?at?least?another?in?July,?August?or?October.?There?is?a?lack?
of?small?floods?in?the?post?1946?conditions,?a?median?of?less?than?one?event?per?year.?
? High?flows?and?small?floods?that?occurred?every?year?(T=1)?in?the?pre?1946?conditions?
happen?every?2?3?years?in?post?1946.?Similarly,?high?flows?and?small?flows?that?occurred?
every?2?3?years?(2<T<3)?in?the?pre?1946?period?happen?every?5?years?in?the?post?1946.?
Drought?Hydrologic?Conditions?
Drought?year?(annual?volume<median)?have?been?identified?for?both?periods;?pre?1946?(1901?
1913?and?1930?1946)?and?post?1946?(1980?2009).?Similarly?to?the?previous?analysis,?daily?mean?
flow?values?at?Johnson?Ranch?for?drought?years?have?been?analyzed?in?order?to?determine?annual?
hydrographs?for?the?following?return?periods?(T):a)?1?year,?b)2?to?3?years,?c)?5?to10?years,?and?d)?
more?than?10?years.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May????????Jun???????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
73
52
342
60
Small?Flood
2<T<3
56?m
3
/s
224?m
3
/s
63
60 55
75
24
2323
1615
11 9
14 1113
11 8
??vi???
During?drought?years?in?the?pre?1946,?median?monthly?values?varied?from?10?m
3
/s?in?April?to?45?
m
3
/s?in?September.?The?annual?volume?of?median?monthly?flows?decreases?during?drought?years?
from?1,130X10
6?
m
3
?to?782X10
6?
m
3
?compared?to?the?pre?1946?normal?hydrologic?conditions.?The?
decrease?in?the?median?monthly?flows?is?the?main?difference?between?the?normal?and?drought?
conditions?of?the?pre?1946.?A?median?of?8?high?flows?still?occurred?per?year?during?drought?years.?
High?flows?occurred?every?year?in?July,?August,?September?and?December?and?once?every?2?3?year?
in?the?rest?of?the?months.?Similarly,?a?median?of?2?small?floods?occurred?every?year,?one?fixed?in?
September?(peak?flow=380?m
3
/s)?and?another?small?flood?variable?in?time?that?usually?happened?in?
August?(peak?flow=270?m
3
/s)?or?October?(peak?flow=422?m
3
/s).?These?two?small?floods?
contributed?to?maintaining?the?Rio?Grande/Bravo?as?a?wide?sandy?multi?threaded?river?in?spite?of?
the? drought? conditions.? Figure? ES?4? shows? a? proposed? annual? hydrograph? for? the? pre?1946?
conditions?during?drought?years;?it?includes?the?benchmarks?for?1?and?2?3?year?return?period?(T).?
This?hydrograph?have?the?fixed?small?flood?in?September?and?the?moving?small?flood?in?October.?
The?volume?of?the?1?and?2?3?year?return?period?is?954X10
6
?m
3
?and?111X10
6
?m
3
?respectively;?thus,?
the?annual?hydrograph?shown?in?Figure?ES?4?has?a?volume?of?1,065X10
6
?m
3
.?Values?for?the?median?
monthly?flow?and?the?peak?flows?of?the?benchmarks?are?also?included?in?the?figure.?
?
Figure?ES?4.?Pre?1946?annual?hydrograph?during?drought?years?
During?drought?years?in?the?post?1946,?median?monthly?values?varied?from?3?m
3
/s?in?April?to?13?
m
3
/s?in?September.?The?annual?volume?of?median?monthly?flows?decreases?during?drought?years?
from?463X10
6?
m
3
?to?250X10
6?
m
3
?compared?to?the?post?1946?normal?hydrologic?conditions.?For?the?
post?1946,?the?decrease?in?the?median?monthly?flows?is?the?most?significant?difference?between?the?
normal?and?drought?conditions.?A?median?of?7?high?flows?occur?per?year?during?drought?years.?High?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May????????Jun???????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
98
380
Small?Flood
2<T<3Small?Flood
T?1
High?Flows
T?1
56?m
3
/s
224?m
3
/s
High?Flows
2<T<3
71
102
91 84
422
42 40
26
25
24 15 20
27 24
45
36
24
21
10
??vii???
flows?occurred?every?year?in?May,?June,?July?and?August?and?once?every?2?3?year?in?September?and?
October.?A?median?of?0?small?floods?occurred?during?drought?years?and?if?they?occurred,?these?event?
happened?mostly?in?June?(peak?flow=310?m
3
/s).?The?characteristics?extended?drought?periods?of?
the?Rio?Grande/Bravo?basin?coupled?with?the?significant?reduction?in?frequency?and?magnitude?of?
small?floods?during?drought?years?definitely?contributed?to?the?deterioration?of?the?environmental?
conditions?at?the?Big?Bend?reach.?During?drought?periods?the?channel? narrowing? and? vertical?
acreation?must?be?exacerbated?due?to?the?scarce?frequency?of?small?floods.?Figure?ES?5?shows?a?
proposed?annual?hydrograph?for?the?post?1946?conditions?during?drought?years;?it?includes?the?
benchmarks?for?1?and?2?3?year?return?period?(T).?This?hydrograph?only?have?one?small?flood?in?June?
that?is?smaller?in?magnitude?than?the?pre?1946?value?and?out?of?timing.?The?volume?of?the?1?and?2?3?
year?return?period?is?282X10
6
?m
3
?and?71X10
6
?m
3
?respectively;?thus,?the?annual?hydrograph?shown?
in?Figure?ES?5?has?a?volume?of?353X10
6
?m
3
.?The?median?monthly?values?are?much?smaller?than?the?
pre?1946?values?during?drought?period.?Values?for?the?median?monthly?flow?and?the?peak?flows?of?
the?benchmarks?are?also?included?in?the?figure.?
?
Figure?ES?5.?Post?1946?annual?hydrograph?during?drought?years?
After?analyzing?the?annual?hydrographs?during?drought?years?for?different?return?periods,?these?are?
the?conclusions:?
? In?pre?1946,?small?floods?occur?during?drought?years,?the?fixed?small?flood?of?September?
(Flow?peak?=?380?m
3
/s)?still?occurs?during?drought?years?preventing?from?agradation?and?
channel?narrowing.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May????????Jun???????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
310
57
Small?Flood
2<T<3
56?m
3
/s
224?m
3
/s
54 60
53
47
63
117
High?Flows
2<T<3
8653 491012813 98
??viii???
? During?drought?years,?small?floods?occurred?more?frequently?in?the?pre?1946?conditions,?a?
median?of?2?events?per?year,?meanwhile?in?the?post?1946?conditions? they? frequency?
dropped?to?0?events?per?year.?
? Median?monthly?flows?decreased?significantly,?by?68%?of?the?pre?1946?annual?quantity.?In?
addition,?the?shape?of?the?median?values?is?altered;?in?the?post?1946?the?highest?median?
value?has?shifted?from?September?to?October.?
Proposed?hydrograph?for?the?Big?Bend?at?Johnson?Ranch?
After?the?extensive?analysis?done?at?Johnson?Ranch,?these?are?the?characteristics?considered?for?
the?proposed?hydrograph:?
? Median?monthly?flows?with?a?distribution?similar?to?the?pre?1946?period.?
? Two? small? floods;? one? small? flood? fixed? in? September? and? another? small? flood? in? July,?
August,?or?October.?The?peak?flow?of?these?floods?must?be?of?400?m
3
/s.?
? Four? high? flow? pulses;? one? in? each? of? the? following? months:? July,?August,?October?and?
December.?
?
Figure?ES?6.?Annual?hydrograph?proposed?for?Johnson?Ranch?
From?1947?to?2009,?the?median?annual?flow?at?Johnson?Ranch?is?744X10
6
?m
3
.?Considering?this,?
an?annual?volume?of?1,000X10
6
?m
3
?is?proposed.?Figure?ES?6?shows?the?annual?hydrograph?proposed?
for?Johnson?Ranch.?Table?ES?1?shows?the?monthly?and?annual?volume?necessary?to?supply?the?
hydrograph?proposed.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
Fixed?Hydrograph
Flexible?Small?Flood
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
One?fixed?Small?Flood
High?Flows
High?Flows
56?m
3
/s
224?m
3
/s
14
13 12
6
10
14
23
26
51
26
16 14
One?Small?Flood
variable?in?time
407
97
405
75
102
45
??ix???
Table?ES?1.?Monthly?volume?of?the?proposed?hydrograph?
?? Volume?(1X106?m3)?
?? Jan? Feb? Mar? Apr May? Jun? Jul? Aug? Sep? Oct Nov? Dec? Sum?
Median? 37? 31? 32? 15? 28? 36? 62? 69? 132 71? 40? 37? 589?
High?Flows? ???? ???? ???? ???? ???? ???? 5? 12? ???? 16? ???? 7? 39?
Small?Floods? ???? ???? ???? ???? ???? ???? 191 ???? 181 ???? ???? ???? 372?
Total? 37? 31? 32? 15? 28? 36? 257 81? 313 86? 40? 43? 1000?
?
Compared? to? the? post?1946? conditions?(Figure?ES?3),?main?differences? are? in? the? median?
monthly? flows,? mostly? from? July? to? October,? and? in? considering? 2? small? floods? of? 400? m
3
/s?
approximately.?
Conclusions?
The?characterization?of?extreme?flood?events?(EFE),?normal?and?drought?conditions?and?the?
comparison?of?their?results?has?show?how?significantly?different?is?the?hydrology?prior?and?after?
alteration.? The? change? in? the? hydrology? is? the? result? of? the? infrastructure?and?human?water?
management?that?has?completely?modified?the?hydrologic?characteristics?of?the?river.?For?EFE,?the?
quantity,?frequency,?and?timing?have?been?severely?altered,?magnitudes? of? peak? flows? have?
decreased?by?50%?and?return?periods?have?doubled.?During?normal?and?drought?conditions,?the?
annual?flow?volumes?have?decreased?by?alarming?59%?and?68%,?respectively.?A?lack?of?small?floods?
is?a?constant?in?the?post?1946?period.?As?a?result,?the?environment?has?been?deteriorated;?one?sign?
of?this?is?the?continuous?narrowing?of?the?channel.?In?order?to?revert?this?trend,?an?annual?
hydrograph?has?been?proposed?for?environmental?restoration.?
The?hydrograph?proposed?tries?to?match?as?much?as?possible?the?pre?1946?characteristics:?i)?
two?small?floods,?ii)?several?high?flows?and?iii)?median?monthly?flow?similarly?distributed?as?the?
pre?1946?normal?conditions.?In?fact,?the?proposed?hydrograph?(Figure?ES?6)?is?about?the?same?
volume?as?the?drought?years??hydrograph?(Figure?ES?4)?for?the?pre?1946.?Is?it?fair?(or?correct)?to?
propose?a?hydrograph?that?used?to?be?for?drought?conditions??Should?we?ask?for?the?pre?1946?
normal?condition?(Figure?ES?2)?hydrograph??Is?it?enough?water?left?in?the?current?system?to?supply?
any?of?the?two?hydrographs??How?much?water?is?still?available?in?the?system??How?much?of?the?
available?water?can?be?managed?by?the?infrastructure?to?meet?this?environmental?flow?restoration??
Unfortunately,?there?is?no?answer?for?any?of?the?previous?questions?yet.?However,?research?has?
been?done?to?determine?environmental?restoration?flows?and?how?to?operate?the?system?in?order?to?
meet?these?flows.?This?report?forms?part?of?this?effort?to?determine?the?environmental?restoration?
flows.? Results? presented? here? are? not? definitive;? on? the? contrary,?they?are?intended?to?be?the?
foundation? of? discussions? and? constructive? opinions? from? academic,? scientists? and? the?
environmental?community.?This?is?just?one?step?of?several?that?will?be?needed?to?determine?the?
required?flows?for?environmental?restoration?at?the?Big?Bend?reach.?
?
?
??x???
?
?
?
??i???
TABLE?OF?CONTENTS?
Section? page 
Executive Summary.................................................................................................................... i?
Table of Contents ........................................................................................................................ i?
1.? Background .........................................................................................................................5?
1.1? Introduction?............................................................................................................................................................?5?
1.2? Hydrologic Period of Analysis?......................................................................................................................?7?
1.3? Data Sources?........................................................................................................................................................?8?
2.? Extreme Flood events .......................................................................................................9?
2.1.? Before 1946?...........................................................................................................................................................?9?
2.2.? After 1946?.............................................................................................................................................................?10?
2.3.? Comparison?.........................................................................................................................................................?12?
2.4.? Conclusions?.........................................................................................................................................................?13?
3.? Normal Hydrologic Conditions .....................................................................................14?
3.1.? Monthly Medians?...............................................................................................................................................?14?
3.1.1.? Before 1946?.............................................................................................................................................?14?
3.1.2.? After 1946?.................................................................................................................................................?15?
3.1.3.? Comparison?.............................................................................................................................................?16?
3.2.? High Flows?...........................................................................................................................................................?17?
3.2.1.? Before 1946?.............................................................................................................................................?18?
3.2.2.? After 1946?.................................................................................................................................................?19?
3.2.3.? Comparison?.............................................................................................................................................?20?
3.3.? Small Floods?.......................................................................................................................................................?23?
3.3.1.? Before 1946?.............................................................................................................................................?23?
3.3.2.? After 1946?.................................................................................................................................................?23?
??ii???
3.3.3.? Comparison?.............................................................................................................................................?25?
3.4.? Integration of Results?......................................................................................................................................?26?
3.4.1.? Before 1946?.............................................................................................................................................?26?
3.4.2.? After 1946?.................................................................................................................................................?29?
3.4.3.? Comparison?.............................................................................................................................................?33?
3.5.? Conclusions?.........................................................................................................................................................?36?
4.? Drought Periods ...............................................................................................................38?
4.1.? Monthly Medians?...............................................................................................................................................?40?
4.1.1.? Before 1946?.............................................................................................................................................?40?
4.1.2.? After 1946?.................................................................................................................................................?41?
4.1.3.? Comparison?.............................................................................................................................................?42?
4.2.? High Flows?...........................................................................................................................................................?44?
4.2.1.? Before 1946?.............................................................................................................................................?44?
4.2.2.? After 1946?.................................................................................................................................................?45?
4.2.3.? Comparison?.............................................................................................................................................?47?
4.3.? Small Floods?.......................................................................................................................................................?49?
4.3.1.? Before 1946?.............................................................................................................................................?49?
4.3.2.? After 1946?.................................................................................................................................................?49?
4.3.3.? Comparison?.............................................................................................................................................?51?
4.4.? Integration of Results?......................................................................................................................................?52?
4.4.1.? Before 1946?.............................................................................................................................................?52?
4.4.2.? After 1946?.................................................................................................................................................?56?
4.4.3.? Comparison?.............................................................................................................................................?60?
4.5.? Conclusions?.........................................................................................................................................................?62?
5.? Conclusions.......................................................................................................................64?
5.1.? Extreme Flood Events?....................................................................................................................................?64?
??iii???
5.2.? Normal Hydrologic Conditions?...................................................................................................................?64?
5.3.? Drought Periods?................................................................................................................................................?65?
6.? Prescribed Annual Hydrograph ...................................................................................67?
References  ...........................................................................................................................71?
Appendix A.? Rio Grande/Bravo at Johnson Ranch Streamflow ...............................72?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
??iv???
 
 
?
??5???
1. BACKGROUND?
?
1.1 INTRODUCTION?
?
The?Rio?Grande/Bravo?basin?is?a?transboundary?basin?between?Mexico?and?the?United?States?
(U.S.)?(Figure?1?1).?The?Rio?Grande/Bravo?is?the?principal?source?of?water?for?over?ten?million?
people?in?the?Rio?Grande/Bravo?basin.?Withdrawals?and?diversions?have?been?created?to?use?water?
for?drinking,?irrigation,?sanitation,?manufacturing,?and?power?generation.?In?its?northern?stretches,?
above? Ojinaga/Presidio,? the? Rio? Grande/Bravo? is? primarily? fed? by? snowmelt? from? the? Rocky?
Mountains.?To?the?south,?below?Ojinaga/Presidio,?the?river?is?fed? by? tributaries? in? the? basin,?
primarily?the?Rio?Conchos.?Between?these?sources?lies?the?Big?Bend,?a?490?km?stretch?between?the?
cities?of?Ojinaga/Presidio?to?Amistad?reservoir.?Environmental?flows?are?of?particular?concern?in?
the?Big?Bend?region?due?to?the?wildlife?reserves?in?the?area.?Important?environmental?habitats?such?
as?the?Big?Bend?national?and?state?park?in?the?US,?the?Northern?Chihuahuan?desert,?the?Maderas?del?
Carmen,? the? Ca?on? de? Santa? Elena? and? Ocampo? natural? reserves? in? Mexico? are? ecologically?
threatened?because?of?the?lack?of?environmental?water?management?policies?in?the?basin?
Historically,?this?basin?has?been?manipulated?in?an?exclusive?human? water? resource?
management,?not?considering?the?environmental?needs?for?the?native?ecosystems.?The?Convention?
of?1906?signed?between?Mexico?and?the?U.S.?(IBWC?1906)?and?the?Rio?Grande?Compact?(TCEQ?
1939)?ratified?in?1939?between?the?States?of?Colorado,?New?Mexico?and?Texas?prove?the?fact?that?
the?water?in?this?basin?was?thought?to?be?used?exclusively?for?human?benefit.?The?water?allocation?
in?these?agreements?obeys?exclusively?the?human?concerns,?leaving? out? the? natural? water?
requirements?of?the?basin.?As?a?result?of?the?previous?agreements,?the?quantity?of?water?in?the?Rio?
Grande/Bravo?reach?between?El?Paso/Cd.?Juarez?to?Presidio/Ojinaga?were?reduced?so?dramatically,?
that?this?reach?is?now?referred?as?the??forgotten?river??where?almost?no?water?flows.??
Another?example?is?the?signature?of?the?treaty?of?1944?between?Mexico?and?the?U.S.?(IBWC?
1944).?In?this?treaty,?the?environmental?issues?are?not?considered?and?the?priority?assigned?to?the?
environment?is?not?even?specified,?leaving?it?in?disadvantage?with?respect?to?the?other?water?uses.?
In?addition,?because?of?the?water?scarcity?in?the?basin?and?the?high?competition?among?water?users?
for?the?resource,?the?environmental?requirements?has?been?denied?or?not?even?considered?for?a?
long?time.?
?
?
?
??6???
?
Figure?1?1:?Rio?Grande/Bravo?Basin?
?
Flooding?is?an?integral?part?of?the?environmental?cycle.?Many?ecological? functions? are?
performed?when?floodplains?become?inundated?with?water.?High?flows?provide?a?spawning?cue?for?
fish.?The?shallow,?nutrient?rich,?floodplains?also?provide?an?ideal?habitat?for?feeding?and?spawning.?
Additionally,?flooding?helps?maintain?plant?diversity.?Periods?of?inundation?allow?for?water?tolerant?
plants?to?thrive?while?killing?those?who?are?less?suited.?In?the?absence?of?regular?flooding,?drought?
resistant?plants?dominate?the?riparian?vegetation.?Inundation?also?improves?soil?and?water?quality.?
Floods?deliver?nutrients?to?floodplains?while?washing?away?old?topsoil.?Inundation?increases?water?
purification?by?providing?greater?dispersion?of?pollutants?and?more?contact?with?vegetation?(Postel?
and?Ritcher?2003).?
Dean?and?Schmidt?(2010)?extensively?examined?the?relationship?between?channel?widening?and?
extreme?flood?events?by?using?aerial?photographs?of?the?Big?Bend?reach?to?establish?a?relationship?
between?floods?events?and?channel?width.?Figure?1?2?shows?the?channel?width?as?function?of?time,?
as?well?as?some?of?the?feedback?mechanisms?that?influence?the?channel?width.?Dean?and?Schmidt?
(2010)?conclude?that?due?to?reductions?in?mean?and?peak?stream?flows,?the?channel?of?the?lower?Rio?
Grande/Bravo?has?narrowed?by?more?than?50%?since?40?s??
??7???
Reductions?in?mean?flow?and?extreme?flood?events?are?product?of?dam?construction.?Prior?1946,?
the?Big?Bend?reach?used?to?maintained?a?wide,?sandy,?multi?thread?channel?(Dean?and?Schmidt?
2010).? Frequent? floods? moved? sediments? and? prevented? any? substantial? accumulation.? In? the?
1940?s,?the?construction?of?the?Caballo?(completed?in?1938)?and?Francisco?I.?Madero?(completed?in?
1949)? reservoirs? altered? its? hydraulic? properties,? becoming? narrower? and? single?thread? river.?
Channel?narrowing?has?been?temporally?interrupted?by?large?floods?that?re?widen?the?channel.?Not?
only?mean?flows?but?also?extreme?flood?events?have?been?reduced?in?frequency?and?magnitude.?
?
?
Figure?1?2:?Reconstructed?channel?widths?(Dean?and?Schmidt,?2010)?
?
This?document?describes?the?hydrologic?characteristics?of?the?Rio?Grande/Bravo?in?the?Big?
Bend?reach?for?two?periods:?1)?before?1946,?when?the?river?used?to?be?wide,?sandy,?and?multi?
threaded?river;?and?2)?after?1946,?when?the?river?become?narrower?and?single?thread?channel.?The?
hydrologic?characteristics?analyzed?in?this?document?are:?a)?extreme?flood?events;?b)?normal?flows?
and?c)?drought?flows.?
?
1.2 HYDROLOGIC?PERIOD?OF?ANALYSIS?
?
The? length? of? the? hydrologic? period? of? analysis? considered? in? this? study? is? 93? years,? from?
January? 1901? to? December? 1913? and? from? January? 1930? to? December? 2009.Prior? to? 1936,? no?
stream?flow?data?exists?at?Johnson?Ranch;?thus?the?data?has?been?calculated?using?a?regression?
based?on?flows?at?the?gage?station?Rio?Grande?below?Ojinaga?which?exist?for?this?time?period.??A?
more?detailed?explanation?about?the?calculated?data?is?provided?in?Appendix?A.?Also,?between?1914?
and?1929?no?data?was?recorded?at?either?station,?so?no?data?is?available?for?this?time?period.?
??8???
The?primary?year?of?concern?is?1946.?In?the?1940?s?the?construction?of?the?Caballo?(completed?
in?1938)?and?Francisco?I.?Madero?(completed?in?1949)?reservoirs?altered?the?hydraulic?properties?
of?the?channel?at?the?Big?Bend.?These?alterations?are?reflected?in?both?the?recorded?flows?and?
channel?geomorphology?as?presented?in?Dean?and?Schmidt?(2010).?This?study?compares?the?periods?
of?pre?alteration?(1901?1946)?and?post?alteration?(1947?2009)?flow?conditions.?
For?normal?and?drought?conditions,?the?period?of?1980?2009?is?analyzed.?The?main?objective?is?
to?compare?the?pre?alteration?hydrology?with?the?last?30?year?hydrology?at?the?Big?Bend?reach.?Both?
periods?of?analysis?were?selected?to?match?the?period?length?prior?to?1946?(1901?1913,?1930?
1946),?which?is?also?30?years.?All?water?years?begin?in?January.?
?
1.3 DATA?SOURCES?
?
Johnson?Ranch?gage?station,?located?in?the?Big?Bend,?is?used?as?the?point?of?analysis?for?the?
project.? Mean? daily? discharge? data? for? this? and? other? control? points? can? be? gathered? from? the?
International?Water?Boundary?Commission?(IWBC)?database?(IBWC?2009).?
?
??
??9???
2. EXTREME?FLOOD?EVENTS?
?
In?this?section,?extreme?flood?events?are?analyzed?at?the?Big?Bend?reach?using?Johnson?Ranch?
gauge?station.?Extreme?flow?events?at?Johnson?Ranch?come?from?the?Rio?Conchos,?which?is?a?major?
tributary?of?the?Rio?Grande/Bravo.?This?tributary?receives?the?majority?of?the?runoff?in?the?Mexican?
portion?of?the?basin.?During?the?monsoon?season,?runoff?causes?high?flows?in?the?Rio?Conchos?and?
soon?after?in?the?Rio?Grande/Bravo.?These?extreme?floods?typically?occur?in?late?September?to?early?
October.?HEC?SSP,?software?developed?by?the?US?Army?Corps?of?Engineers,?is?used?to?examine?
extreme?flood?events?in?this?region.?
?
2.1. BEFORE?1946?
?
Prior?to?1946,?there?are?four?extreme?flooding?events,?occurring?in?1904,?1932,?1938,?and?1942?
as?shown?in?Figure?2?1.?Table?2?1?presents?a?summary?of?the?main?characteristic?of?the?flooding?
events.?There?is?little?uniformity?between?the?incidents?as?durations?range?from?8?to?30?days?and?
peak?daily?flows?from?1,560?m
3
/s?(55,091?cfs)?to?3,851?m
3
/s?(135,997?cfs).?However,?the?total?flow?
in?any?flooding?event?is?larger?than?approximately?1,000X10
6
?m
3
?(810,713?acre?feet).?Flood?events?
begin?in?late?August?to?early?September.?These?occurrences,?along?with?frequent?smaller?floods?
moved?sediment?and?prevented?any?substantial?accumulation?in?the?canal,?maintaining?a?wide,?
sandy?multi?threaded?river?with?regular?resetting?events?(Schmidt?2010).?
?
?
Table?2?1:?Summary?of?extreme?events?pre?1946?
?? Extreme?Flooding?Event?
1904? 1932? 1938? 1942?
Start?Date? 9/5? 9/10? 9/3? 8/22?
End?Date? 9/22? 10/11? 10/4? 10/4?
Duration?(days)? 8? 30? 14? 19?
Total?Flow?(1X10
6
m
3
)? 1249? 1349? 986? 1368?
Peak?Flow?(m
3
/s)? 3851? 2282? 1560? 1610?
Return?Period?(years)? 126? 23? 9? 10?
?
?
?
?
??10???
?
Figure?2?1:?Reconstructed?discharge?data?for?Johnson?Ranch?1900?1946?
?
2.2. AFTER?1946?
?
After?1946,?there?are?four?extreme?flooding?events,?occurring?in?1958,?1978,?1990?1991?and?
2008? (as? shown? in? Figure? 2?2).? Table? 2?2? shows? a? summary? of? these? events.? The? peak? flow?
threshold?for?widening?for?a?one?year?event?is?1,400?m
3
/s,?with?a?typical?duration?of?approximately?
three?weeks.?Three?of?the?four?flood?events?begin?in?late?September?with?the?other?occurring?earlier?
in?the?month.?
Following?the?construction?of?dams?in?the?basin,?peak?flows?decreased?compared?to?the?pre?
1946?state.?The?absence?of?regular?flooding,?led?to?accumulation?of?sediment?and?rapid?channel?
agradation.?After?1946,?progressive?narrowing?of?the?channel?cross?section?continued,?interrupted?
intermittently? by? three? extreme? flooding? events? causing? widening.? Similar? to? the? pre? 1946?
conditions,?the?total?flow?in?any?flooding?event?is?larger?than?1,000X?10
6
?m
3
/s?(810,713?acre?feet),?
approximately.?The?largest?widening?event?in?1990?1991?corresponds?with?high?flow?events?in?
consecutive?years.?
0
500
1000
1500
2000
2500
3000
3500
4000
1900 1903 1907 1911 1915 1919 1923 1927 1931 1935 1939 1943 1947
Flow
?
(m
3
/s)
No??Data
1904
1932
1938
1942
Estimated Existing
??11???
?
?
Figure?2?2:?Discharge?data?for?Johnson?Ranch?plotted?against?channel?width?1946?2009?(Dean?and?
Schmidt?2010)?
?
Table?2?2:?Summary?of?extreme?events?post?1946?
?? Extreme?Flooding?Event?
1958? 1978? 1990? 1991? 2008?
Start?Date? 9/24? 9/25? 9/20? 9/4? 9/3?
End?Date? 10/14? 10/14? 10/11? 10/11? 10/3?
Duration?(days)? 21? 20? 22? 38? 31?
Total?Flow?(1X10
6
m
3
)? 1580? 1159? 968? 1573? 2144?
Peak?Flow?(m
3
/s)? 1520? 1850? 1410? 1030? 1490?
Widening?(m)? 9? 15? 29? N/A?
Return?Period?(years)? 48? 82? 40? 19? 46?
?
?
?
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0
10
20
30
40
50
60
70
80
90
100
1946 1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002 2006
Flow
?
(m
3
/s)
Active
?
Channel
?
Width
?
(m)
Channel?Width
Flow
1958
1978
1990?1991
2008
??12???
2.3. COMPARISON?
?
In? order? to? compare? the? extreme? flood? events,? a? Log?Pearson? Type? III? analysis? has? been?
performed?to?both?periods:?pre?1946?and?post?1946.?This?analysis?compares?the?maximum?annual?
flows?corresponding?to?the?expected?return?period.?This?analysis?used?a?regional?skew?of??0.2?and?a?
mean?standard?error?of?0.35?(USGS?1996).?Figure?2?3?show?the?flow?probability?of?exceedance?
curves?using?the?Log?Pearson?type?III?analysis?for?both?periods.?
The?Log?Pearson?Type?III?analysis?shows?an?alteration?in?the?Rio?Grande/Bravo?basin.?Table?2?3?
compares?the?expected?annual?maximum?flow?of?both?periods?of?analysis.?Peak?flows?post?1946?are?
lower?than?post?1946,?about?half?of?the?magnitude?(on?average?48%).?For?instance,?the?20?year?
return?period?flow?for?pre?1946?is?2,347?m
3
/s?while?for?the?post?1946?is?only?1,056?m
3
/s?which?
represents?45%?of?the?pre?1946?conditions.?In?addition,?the?duration?of?the?floods?is?more?uniform?
in?the?post?1946?period,?with?all?roughly?3?weeks?in?length.?For?the?pre?1946?period?this?can?vary?
from?8?days?to?a?month.?Moreover,?flooding?events?occur?on?average?two?to?three?weeks?later?in?the?
post?1946?conditions,?from?September?1?7
th
?in?the?pre?1946?to?September?21?28
th
?in?the?post?1946.?
These?discrepancies?suggest?that?flood?waters?were?stored?in?reservoirs?and?released?at?a?later?time?
and?in?more?uniform?quantities.?
?
?
Figure?2?3:?Summary?of?Log?Pearson?III?analysis?
10
100
1000
10000
0.0010.010.11
Flow
?
(m
3
/s)
Probability?of?Exceedance
Post?1946
5%?Confidence
95%?Confidence
Pre?1946?
5%?Confidence
95%?Confidence
105
100
50 20020 500
Return?Period?(Years)
??13???
Table?2?3:?Summary?of?Log?Pearson?III?analysis?
Return?Period?
(years)?
Expected?Annual?Max?Flow?(m
3
/s)?
1900?1946? 1947?2009? Reduction?
1?
38? 46? ????
2?
531? 264? 50%?
5?
1121? 523? 53%?
10?
1617? 764? 53%?
20?
2347? 1056? 55%?
50?
2957? 1539? 48%?
100?
3617? 1993? 45%?
200?
4326? 2538? 41%?
500?
5339? 3424? 36%?
48%?
?
?
2.4. CONCLUSIONS?
?
From?the?previous?analysis?of?the?flood?events?and?the?maximum?annual?flows,?it?can?be?concluded?
that:?
? Pre?1946?extreme?events?(flow?>?1,400?m
3
/s)?occurred?more?frequently,?at?least?4?events?in?
30? years? approximately? one? event? every? 7~8? years;? meanwhile? for?the?post?1946?
conditions,?extreme?events?occurred?less?frequently,?4?events?in?63?years,?approximately?
one?event?every?15~16?years.?
? Expected?flows?for?a?given?return?period?decreased?by?an?average?of?48%?after?1946.?
? Total?flow?for?the?floods?the?periods?are?approximately?the?same?around?1,000X?10
6
?m
3
?
(810,713?acre?feet).?
? Post?1946?durations?are?typically?3?weeks;?while?pre?1946?are?more?variable?ranging?from?
8?to?30?days.?
? Events?happen?two?to?three?weeks?later?after?1946,?from?September?1?7
th
?in?the?pre?1946?to?
September? 21?28
th
?in?the?post?1946.?This?is?likely?explained?by?the?timing?of?reservoir?
releases.?
? Peak?flows?decreased?by?an?average?of?37.5%?post?1946,?for?the?eight? extreme? events?
studied.?
?
??
??14???
3. NORMAL?HYDROLOGIC?CONDITIONS?
?
In?this?section?are?analyzed?the?daily?mean?flow?values?at?Johnson?Ranch.?Flows?have?been?
categorized?in:?median?flows,?high?flows,?small?floods?and?large?floods.??
? A?median?monthly?flow?is?the?median?value?of?all?the?daily?mean?flows?for?the?month?and?
the?hydrologic?period?analyzed.?
? A?high?flow?is?a?flow?with?a?peak?between?the?75
th
?and?95
th
?percentile?of?daily?flows?for?
the?pre?1946?hydrologic?period?of?analysis.?
? A?small?flood?is?a?flow?with?a?peak?between?the?95th?percentile?of?daily?flows?for?the?
pre?1946?period?and?1,110?m
3
/s,?which?is?a?threshold?to?account?for?the?levees?flow?
capacity? upstream? the? Big? Bend? reach? at? Presidio/Ojinaga? cities;? maximum? flow? of?
1,190?m3/s?(42,000?cfs).?
? A?large?flood?is?a?flow?with?a?peak?flow?greater?than?1,100?m
3
/s?(38,846?cfs).?These?large?
floods?will?definitely?threaten?the?safety?of?the?levees?in?the?Ojinaga/Presidio?Valley.?
Flows?bigger?than?this?threshold?have?been?analyzed?in?the?previous?section.?
Two?hydrologic?periods?are?analyzed?in?this?section:?1)?before?1946,?which?has?data?for?30?
years,?from?January?1901?to?December?1913?and?from?January?1930?to?December?1946;?and?2)?
after?1946,?which?data?also?for?30?years,?from?January?1980?to?December?2009.?For?the?after?1946?
period? we? chose? the? last? thirty? years? because? we? would? like? to? analyze? the? hydrological?
characteristics?of?the?immediate?hydrology.?By?doing?this?is?possible?to?compare?the?hydrologic?
characteristics?when?the?Rio?Grande/Bravo?was?a?wide?sandy?multi?threaded?river,?with?the?actual?
conditions,? which? are? a? narrower? single?thread? river? with? cohesive?vertical?banks?(Dean?and?
Schmidt?2010).?In?addition,?this?period?contains?a?wide?range?of?hydrologic?conditions,?from?the?wet?
period?of?the?80?s?(1984?1993)?to?the?severe?and?extended?drought?of?the?90?s?(1994?2007).?The?
Indicators?of?Hydraulic?Alteration?(IHA)?software?is?used?to?label?the?different?flows?along?the?
periods?of?analysis.?The?frequency?of?each?event?(i.e.?high?flow,?small?and?large?floods)?has?been?
determined?for?each?period?of?analysis.?Return?periods?for?each?event?have?been?calculated?using?
the?frequency?of?each?event.?
?
3.1. MONTHLY?MEDIANS?
?
The?median?monthly?flow?is?the?median?value?of?the?daily?mean?flows?for?each?month.?One?value?
for?each?month?has?been?calculated?for?pre?1946?and?post?1946?periods.?
3.1.1. BEFORE?1946?
?
Table?3?1?provides?a?summary?of?median?monthly?flows?prior?to?1946.??The?peak?monthly?
median?flow?occurred?in?September?at?98?m3/s?(3,443?cfs)?and?the?minimum?in?April?at?11?m3/s?
??15???
(403?cfs).?In?September,?it?was?expected?that?flows?exceeded?226?m3/s?(7,971?cfs)?once?every?four?
years.?Similarly?in?April,?low?flows?below?5?m3/s?(162?cfs)?were?likely?to?occur?once?every?four?
years.?
Table?3?1:?Median?monthly?flows,?pre?1946?period?
?? Pre?1946?Monthly?Flows?(m
3
/s)?
?? 75th Median 25th
Jan? 34 27 14
Feb? 32 24 17
Mar? 31 23 14
Apr? 23 11 5
May? 54 20 7
Jun? 75 26 9
Jul? 108 44 21
Aug? 125 50 21
Sep? 226 98 38
Oct? 102 51 27
Nov?43 30 16
Dec? 39 26 14
?
3.1.2. AFTER?1946?
?
Table?3?2?provides?summary?of?median?monthly?flows?of?the?post?1940?period?(last?30?years).?
The?peak?monthly?median?flow?also?occurs?in?September?at?24?m
3
/s?(851?cfs)?and?the?minimum?in?
April?at?8?m
3
/s?(279?cfs).??In?September?it?is?expected?that?flows?will?exceed?67?m
3
/s?(2,355?cfs)?
once?every?four?years.?Similarly?in?April,?low?flows?below?3?m
3
/s?(95?cfs)?are?likely?to?occur?once?
every?four?years.?
Table?3?2:?Median?monthly?flows,?1980?2009?period?
?? 1980?2009??Monthly?Flows?(m3/s)
?? 75th Median 25th
Jan? 19 11 7
Feb? 19 9 6
Mar? 18 10 4
Apr? 18 8 3
May? 30 13 3
Jun? 38 15 4
Jul? 42 16 7
Aug? 65 23 11
Sep? 67 24 8
Oct? 48 23 11
Nov?23 14 8
Dec? 18 11 7
?
??16???
?
3.1.3. COMPARISON?
?
Median?monthly?flows?at?Johnson?Ranch?have?declined?since?1946.?Figure?3?1?shows?monthly?
medians,?25
th
?and?75
th
?percentile?flows?for?pre?1946?and?post?1946?(1980?2009)?periods.?Median?
values?are?compared?in?Table?3?3.?The?annual?median?flow?volume?for?the?pre?1946?and?post?1946?
are?1130?X10
6?
m
3
?and?464?X10
6?
m
3
,?respectively.?Comparing?these?annual?volumes?of?the?periods,?
the?resulting?deficit?is?666?X10
6?
m
3
?of?water,?or?59.0%?of?the?pre?1946?quantity.?Much?of?this?deficit?
(62%)?comes?from?high?flow?months?July?through?October.?The?absence?of?high?flows?post?1980?
suggest?that?these?high?flow?events?are?being?stored?in?basin?reservoirs.?
?
??
Figure?3?1:?Comparison?of?median?monthly?flows,?Normal?Conditions?
?
?
?
0
50
100
150
200
250
JFMAMJJASOND
Discharge
?
(m
3
/s)
Pre?1946
1980?2009
25th?percentile
25th?percentile
75th?percentile
75th?percentile
??17???
?
Table?3?3:?Monthly?median?flows?and?deficits?
Monthly?Median?Flows?(m
3
/s)
Pre?1946 Post?1980 Deficit %?Deficit
Jan? 27 11 16 61
Feb? 24 9 15 62
Mar? 23 10 13 58
Apr? 11 8 3 31
May? 20 13 7 33
Jun? 26 15 11 43
Jul? 44 16 28 64
Aug? 50 23 27 55
Sep? 98 24 74 75
Oct? 51 23 28 55
Nov?30 14 16 53
Dec? 26 11 15 58
?
3.2. HIGH?FLOWS?
?
High?flows?are?defined?as?those?that?exceed?the?75?percentile?but?are?lower?than?the?95
th
?
percentile?of?daily?flows?for?the?pre?1946?period.?The?75
th
?and?95
th
?percentile?flows?in?the?pre?1946?
period?are?56?m
3
/s?(1,978?cfs)?and?224?m
3
/s?(7,910?cfs),?respectively.?These?thresholds?are?also?
used?in?the?post?1946?period?in?order?to?evaluate?how?frequently?pre?1946?high?flows?occurred?in?
the?post?1946?period.?
Flows? that? satisfy? the? previous? thresholds? have? been? identified,? selected? and? analyzed?
individually?by?month?and?period?of?analysis?(pre?1946?or?post?1946).?When?analyzing?high?flows?
for?every?month,?it?was?evident?that?two?kinds?of?high?flows?occurred?in?this?region:?1)?small?high?
flows?pulses?that?occurred?all?year?long?as?a?result?of?small?precipitation?in?the?region?or?in?the?
upper?basin,?and?2)?large?high?flows?that?occurred?in?the?monsoon?season?(Jun?Oct)?as?a?result?of?
moderate?rains?in?the?upper?basin.?For?each?kind?of?high?flow?(small?or?large),?the?median?of?the?
flows?was?estimated?in?order?to?obtain?a?characteristic?hydrograph.?This?section?presents?the?high?
flow?hydrographs?for?each?month,?period?(pre?1946?or?post?1946)?and?type?of?high?flow?(small?or?
large).?For?the?pre?1946?the?small?high?flows?have?a?peak?lower?than?125?m
3
/s;?meanwhile?for?the?
post?1946,?small?high?flows?have?a?peak?lower?than?100?m
3
/s.?
?
?
?
?
??18???
?
3.2.1. BEFORE?1946?
?
Table?3?4?shows?the?high?flow?pulses?with?peaks?lower?than?125?m
3
/s?(4,414?cfs)?for?the?pre?
1946?period?for?different?return?periods?(T).?Small?high?flow?pulses?that?happen:?
? Every?year?(T<1.5?years)?occur?in?July,?August,?October?and?December;?with?a?duration?
of?1,?3,?4?and?3?days?and?peak?flows?of?75,?102,?97?and?45?m
3
/s,?respectively.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?the?rest?of?the?months?(January,?
February,?March,?April,?May,?June?and?November),?with?a?duration?of?3?days?and?peak?
flows?of?37,?40,?41,?52,?123,?99,?and?58?m
3
/s,?respectively.?
?
Table?3?4:?High?flow?pulses?lower?than?125?m
3
/s,?Pre?1946?period?
?? High?Flow?Pulses?(m
3
/s)
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep? Oct?Nov Dec
1? 33? 34? 33? 47 48 52 75 57 ???? 58? 44 36
2? 37? 40? 41? 52 123 99 ??? 102 ???? 97? 58 45
3? 32? 34? 33? 47 48 52 ??? 57 ???? 73? 44 36
4? ???? ???? ???? ??? ??? ??? ??? ??? ???? 58? ??? ???
5? ???? ???? ???? ??? ??? ??? ??? ??? ???? ???? ??? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ???? ???? ??? ???
Duration?(days)? 3? 3? 3? 3 3 3 1 3 ???? 4? 3 3
Peak?(m
3
/s)? 37? 40? 41? 52? 123? 99? 75? 102? ???? 97? 58? 45?
Frequency? 0.31 0.59? 0.48 0.59 0.45 0.52 0.76 1.00 ???? 0.72? 0.48 0.76
T?(years)? 3.2? 1.7? 2.1? 1.7? 2.2? 1.9? 1.3? 1.0? ???? 1.4? 2.1? 1.3?
?
Table?3?5?shows?the?high?flow?pulses?larger?than?125?m
3
/s?(4,414?cfs)?for?the?prior?1946?period?
for?different?return?periods?(T).?Large?high?flow?pulses?that?happen:?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June?and?July?with?a?duration?of?5?and?
8?days?and?peak?flows?of?152?and?195?m
3
/s,?respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?August?and?September?with?duration?of?
7?and?5?days?and?peak?flows?of?169?and?202?m
3
/s.?
?
?
?
?
?
??19???
Table?3?5:?High?flow?pulses?larger?than?125?m
3
/s,?Pre?1946?period?
?? High?Flow?Pulses?(m
3
/s)
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? ??? 118 139 123 138 ???? ???? ???
2? ???? ???? ???? ??? ??? 127 169 135 168 ???? ???? ???
3? ???? ???? ???? ??? ??? 152 186 143 202 ???? ???? ???
4? ???? ???? ???? ??? ??? 127 195 169 168 ???? ???? ???
5? ???? ???? ???? ??? ??? 118 169 143 138 ???? ???? ???
6? ???? ???? ???? ??? ??? ??? 139 135 ??? ???? ???? ???
7? ???? ???? ???? ??? ??? ??? 128 123 ??? ???? ???? ???
8? ???? ???? ???? ??? ??? ??? 118 ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? ??? 5 8 7 5 ???? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ???? ???? 152? 195? 169? 202? ???? ???? ????
Frequency? ???? ???? ???? ??? ??? 0.10 0.10 0.07 0.07 ???? ???? ???
T?(years)? ???? ???? ???? ???? ???? 9.7? 9.7? 14.5? 14.5? ???? ???? ????
?
3.2.2. AFTER?1946?
?
Table?3?6?shows?the?high?flow?pulses?lower?than?100?m
3
/s?(3,531?cfs)?for?the?after?1946?period?
for?different?return?periods?(T).?Small?high?flow?pulses?that?happen:?
? Every?year?(T<1.5?years)?occur?in?May,?June,?July?and?August,?with?a?duration?of?5,?3,?3,?
and?3?days?and?peak?flows?of?60,?63,?60?and?55?m
3
/s,?respectively.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?April,?September?and?October,?
with?a?duration?of?3?days?and?peak?flows?of?52,?50?to?61?m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?February,?March,?November?and?
December?with?a?duration?of?3?days?and?peak?flows?of?45,?73,?57? to? 56? m
3
/s,?
respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?January?with?duration?of?4?days?and?a?
peak?flow?of?84?m
3
/s.?
Table?3?6:?High?flow?pulses?lower?than?100?m
3
/s,?Post?1946?period?
?? High?Flow?Pulses?(m
3
/s)
Day? Jan? Feb? Mar Apr May Jun Jul Aug Sep? Oct?Nov Dec
1? 50?37?43?353034293131?3?3448
2? 84? 45? 73? 52 38 63 60 55 50? 61? 57 56
3? 73?37?43?356034293131?3?3448
4? 50? ???? ???? ??? 38 ??? ??? ??? ???? ???? ??? ???
5? ???? ???? ???? ??? 30 ??? ??? ??? ???? ???? ??? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ???? ???? ??? ???
Duration?(days)? 4? 3? 3? 3 5 3 3 3 3 3? 3? 3
Peak?(m
3
/s)? 84? 45? 73? 52? 60? 63? 60? 55? 50? 61? 57? 56?
Frequency? 0.07 0.13? 0.27 0.43 1.03 1.20 0.93 0.97 0.53? 0.53? 0.20 0.13
T?(years)? 15.0? 7.5? 3.8? 2.3? 1.0? 0.8? 1.1? 1.0? 1.9? 1.9? 5.0? 7.5?
??20???
?
Table?3?7?shows?the?high?flow?pulses?larger?than?100?m
3
/s?(3,531?cfs)?for?the?after?1946?period?
for?different?return?periods?(T).?High?flow?pulses?that?happen:?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?July?and?August,?with?a?duration?
of?5?days?and?peak?flows?of?119?and?142??m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June,?September?and?October?with?a?
duration?of?3,?5?and?3?days?and?peak?flows?of?191,?127?and?112?m
3
/s,?respectively.?
?
Table?3?7:?High?flow?pulses?larger?than?100?m
3
/s,?Post?1946?period?
?? High?Flow?Pulses?(m
3
/s)
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? ??? 116 33 73 66 49? ???? ???
2? ???? ???? ???? ??? ??? 191 79 107 82 112? ???? ???
3? ???? ???? ???? ??? ??? 116 119 142 127 49? ???? ???
4? ???? ???? ???? ??? ??? ??? 79 107 82 ???? ???? ???
5? ???? ???? ???? ??? ??? ??? 33 73 66 ???? ???? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? ??? 3 5 5 5 3? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ???? ???? 191? 119? 142? 127? 112? ???? ????
Frequency? ???? ???? ???? ??? ??? 0.13 0.40 0.43 0.20 0.23? ???? ???
T?(years)? ???? ???? ???? ???? ???? 7.5? 2.5? 2.3? 5.0? 4.3? ???? ????
?
?
3.2.3. COMPARISON?
?
Examining? the? magnitude? and? frequency? of? high? flow? events? provides? further? evidence? of?
alteration.?Table?3?8?and?Figure?3?2?show?a?comparison?of?small?high?flows?(peak?flow?<?125?m3/s).?
The?majority?of?months?see?reductions?up?to?51%?with?a?few?notable?exceptions.?For?the?period?of?
Dec?Mar,?the?intensity?of?flow?pulses?actually?increased,?up?to?127%?in?January,?probably?caused?by?
dam?releases.?Prior?to?1946,?there?was?a?median?of?7?small?high?flow?pulse?events?per?year?(average?
of?7.4?events?per?year).?Post?1946,?the?frequency?dropped?to?6.5?events?per?year?(average?of?6.5?
events?per?year).?For?both?time?frames,?August?had?the?most?pulses?and?January?the?least.?
?
?
?
?
??21???
Table?3?8:?Magnitude?and?frequency?of?high?flow?pulses?smaller?than?125?m3/s?
Small?High?Flow?Pulses?
Peak?Flow?(m
3
/s) Return?Period?(years)?
Pre?1946? Post?1980 %?Reduction Pre?1946 Post?1980?
Jan? 37? 84 ?127 3.2 15?
Feb? 40? 45 ?12 1.7 7.5?
Mar? 41? 73 ?77 2.1 3.8?
Apr? 52? 52 0 1.7 2.3?
May? 123? 60 51 2.2 1?
Jun? 99? 63 36 1.9 0.8?
Jul? 75? 60 20 1.3 1.1?
Aug? 102? 55 46 1.0 1?
Sep? 91? 50 45 1.3 1.9?
Oct? 97? 61 37 1.4 1.9?
Nov? 58? 57 2 2.1 5?
Dec? 45? 56 ?26 1.3 7.5?
?
?
Figure?3?2:?Duration,?magnitude?and?frequency?of?high?flow?pulses?smaller?than?125?m
3
/s?
?
A?summary?of?large?high?flow?pulses?(peak?flow>?than?100?m3/s)?is?shown?below?in?Table?3?9?
and?Figure?3?3.?The?maximum?annual?high?pulse?has?decreased?from?202?m3/s?pre?1946?to?127?
m3/s?post?1980.?The?majority?of?months?see?reductions?up?to?39%?with?the?exception?of?June?
which?increases?26%.?Prior?to?1946,?there?was?a?median?of?0?small?high?flow?pulse?events?per?year?
(average?of?0.3?events?per?year).?Post?1980,?the?frequency?increased?to?1?event?per?year?(average?of?
1.4?events?per?year).?Large?high?flow?pulses?(peak?flow?>100?m3/s)?in?the?pre?1946?period?are?less?
frequent?than?in?the?post?1980?period.?For?instance,?in?July?and?August?large?high?flow?pulses?
occurred?once?every?2?or?3?years?in?the?post?1946?period,?meanwhile?they?occurred?once?every?5?to?
10?years?in?the?pre?1946?periods.?This?means?that?large?high?flow?pulses?(peak?flow?>?100?m3/s)?
20
40
60
80
100
120
140
123451234123412341234561234123412341234123451234123
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Da
5<T<10
2<T<3
T?1
T?1
T?1
T?1
2<T<3
2<T<3
2<T<3
2<T<3
2<T<3
5<T<10
5<T<10
5<T<10
T?1 T?1
T?1
T?1
2<T<3
2<T<3
2<T<3
T>10
2<T<3
??22???
were?not?very?common?in?the?pre?1946?conditions?as?they?are?now?in?the?post?1946?conditions;?this?
may?be?as?a?consequence?of?reservoir?releases?upstream.?
?
Table?3?9:?Peak?flows?and?frequency?of?high?flow?pulses?larger?than?125?m
3
/s?
Large?High?Flow?Pulses?
Peak?Flow?(m
3
/s) Return?Period?(years)?
Pre?1946? Post?1980 %?Reduction Pre?1946 Post?1980?
Jan?
Feb?
Mar?
Apr?
May?
Jun? 152? 191 ?26% 9.7 7.5?
Jul? 195? 119 39% 9.7 2.5?
Aug? 169? 142 16% 14.5 2.3?
Sep? 202? 127 37% 14.5 5?
Oct? 112 4.3?
Nov?
Dec?
?
?
?
Figure?3?3:?Duration,?magnitude?and?frequency?of?high?flow?pulses?larger?than?100?m
3
/s?
?
20
40
60
80
100
120
140
160
180
200
220
1234561234567891234567812345678123457
Jun Jul Aug Sep Oct
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Days
2<T<3
2<T<3
5<T<10
5<T<10
5<T<10
5<T<10
5<T<10
T>10
T>10
??23???
3.3. SMALL?FLOODS?
?
Small?Floods?are?those?that?have?a?peak?flow?greater?than?the?95?percentile?of?daily?flows?for?the?
whole? period,? but? smaller? than? 1,100? m
3
/s? (38,846? cfs),? which? is? a? flow? smaller? than? the?
Presidio/Ojinaga?levees?flow?design,?1,190?m
3
/s?(42,000?cfs).?The?95th?percentile?flow,?224?m
3
/s?
(7,910?cfs)?has?been?determined?using?the?date?of?the?pre?1946?period.?This?thresholds?is?also?used?
in?the?post?1946?period?in?order?to?evaluate?the?how?frequently?high?flows?occurred?in?this?period.?
Flows?that?satisfy?the?previous?thresholds?have?been?identified,?selected?and?analyzed?individually?
by?month?and?period?of?analysis?(pre1946?or?post?1946).?For?each?month,?the?median?of?the?flows?
was?estimated?in?order?to?obtain?a?characteristic?small?flood.?This? section? presents? the?
characteristics?small?floods?for?each?month?and?period?of?analysis?(pre?1946?or?post?1946).?
?
3.3.1. BEFORE?1946?
?
Table?3?10?shows?the?small?floods?for?the?pre?1946?period?and?for?different?return?periods?(T).?
Small?floods?that?happen:?
? Every?year?(T<1.5?years)?occur?in?September?with?a?duration?of?11?days?and?a?median?
peak?flow?of?407?m
3
/s.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?July,?August,?and?October?with?
durations?of?9,?13,?and?7days?and?peak?flows?of?405,?396?and?422?m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?May,?June,?and?November?with?a?
durations?3,?8,?and?6?days?and?peak?flows?of?252,?433,?and?284?m
3
/s,?respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?April?and?December?with?durations?of?3?
and?6?days?and?peak?flows?of?402?and?353?m
3
/s,?respectively.???
?
3.3.2. AFTER?1946?
?
Table?3?11?shows?the?small?floods?for?the?post?1946?period?and?for?different?return?periods?(T).?
Small?floods?that?happen:?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?September?with?a?duration?of?5?
days?and?a?peak?flow?of?342?m
3
/s.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June,?July,?August,?and?October?with??
durations? of? 6,? 7,? 9,? and? 9? days? and? peak? flows? of? 265,? 293,? 326? and? 438? m
3
/s,?
respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?February,?May,?and?November?with?
durations?of?5?days?and?peak?flows?of?289,?203,?and?239?m
3
/s?respectively.?
??24???
Table?3?10:?Small?floods?(224?m
3
/s?<flow<1,100?m
3
/s),?Pre?1946?period?
?? Small?Floods?(m
3
/s)?
Day? Jan? Feb? Mar? Apr? May Jun? Jul? Aug? Sep? Oct? Nov? Dec?
1? ???? ???? ???? 278? 59? 52? 190? 133? 154? 159? 167? 157?
2? ???? ???? ???? 402? 252? 330? 223? 166? 189? 193? 188? 261?
3? ???? ???? ???? 278? 59? 433? 273? 201? 203? 301? 284? 353?
4? ???? ???? ???? ???? ???? 330? 320? 215? 255? 422? 259? 289?
5? ???? ???? ???? ???? ???? 267? 405? 247? 322? 301? 188? 261?
6? ???? ???? ???? ???? ???? 177? 320? 316? 407? 193? 167? 157?
7? ???? ???? ???? ???? ???? 159? 273? 396? 322? 159? ???? ????
8? ???? ???? ???? ???? ???? 141? 223? 316? 255? ???? ???? ????
9? ???? ???? ???? ???? ???? ???? 190? 247? 203? ???? ???? ????
10? ???? ???? ???? ???? ???? ???? ???? 215? 189? ???? ???? ????
11? ???? ???? ???? ???? ???? ???? ???? 201? 154? ???? ???? ????
12? ???? ???? ???? ???? ???? ???? ???? 166? ???? ???? ???? ????
13? ???? ???? ???? ???? ???? ???? ???? 133? ???? ???? ???? ????
Duration?(days)? ???? ???? ???? 3? 3? 8? 9? 13? 11? 7? 6? 6?
Peak?(m
3
/s)? ???? ???? ???? 402? 252? 433? 405? 396? 407? 422? 284? 353?
Frequency? ???? ???? ???? 0.03 0.10 0.21 0.48 0.41 1.00 0.31? 0.10? 0.07?
T?(years)? ???? ???? ???? 29? 9.7? 4.8? 2.1? 2.4? 1? 3.2? 9.7? 14.5?
?
Table?3?11:?Small?floods?(224?m
3
/s?<flow<1,100?m
3
/s),?Post?1946?period?
?? Small?Floods?(m
3
/s)?
Day? Jan? Feb? Mar? Apr May Jun? Jul? Aug? Sep? Oct? Nov? Dec
1? ???? 123? ???? ???? 112? 63? 110? 91? 94? 70? 95? ????
2? ???? 176? ???? ???? 137? 101? 153? 114? 198? 90? 207? ????
3? ???? 289? ???? ???? 203? 265? 211? 179? 342? 111? 239? ????
4? ???? 176? ???? ???? 137? 162? 293? 210? 198? 193? 207? ????
5? ???? 123? ???? ???? 112? 101? 211? 326? 94? 438? 95? ????
6? ???? ???? ???? ???? ???? 63? 153? 210? ???? 193? ???? ????
7? ???? ???? ???? ???? ???? ???? 110? 179? ???? 111? ???? ????
8? ???? ???? ???? ???? ???? ???? ???? 114? ???? 90? ???? ????
9? ???? ???? ???? ???? ???? ???? ???? 91? ???? 70? ???? ????
10? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ???? ????
Duration?
(days)? ???? 5? ???? ???? 5? 6? 7? 9? 5? 9? 5? ????
Peak?(m
3
/s)? ???? 289? ???? ???? 203? 265? 293? 326? 342? 438? 239? ????
Frequency? ???? 0.03? ???? ???? 0.03 0.27 0.2? 0.23 0.33 0.2? 0.03? ????
T?(years)? ???? 30? ???? ???? 30? 3.5? 5? 4.3? 3? 5? 30? ????
?
??25???
3.3.3. COMPARISON?
A?summary?of?small?flooding?events?is?show?below?in?Table?3?12?and?Figure?3?4.?Note?that?one?
small?flood?in?the?pre?1946?occurred?every?September;?meanwhile?in?the?post?1946?conditions?it?
occurs?once?every?3?years.?Prior?to?1946,?there?was?a?median?of?2?small?flood?events?per?year?
(average?of?2.7?events?per?year).?Post?1980,?the?frequency?drops?by?more?than?half?to?0.5?events?per?
year?(average?of?1.4?events?per?year).?The?maximum?flows?for?each?period?are?similar?with?a?pre?
1946?of?433?m
3
/s?and?a?post?1980?of?438?m
3
/s.?For?the?pre?1946?maximum?value?occurs?in?June,?
while?the?post?1980?occurs?in?October.?There?is?a?decrease?in?6?of?the?7?months,?up?to?39%,?where?
small?floods?are?present?in?both.?For?both?periods,?small?floods?are?most?common?in?September.??
Table?3?12:?Peak?flows?and?frequency?of?small?floods?
?? Small?Floods
Peak?Flow?(m
3
/s) Return?Period?(years)?
?? Pre?1946? Post?1980 %?Reduction Pre?1946? Post?1980?
Jan?
Feb? ?? 289 30?
Mar?
Apr? 402? 29.0 ??
May? 252? 203 20% 9.7 30?
Jun? 433? 265 39% 4.8 3.8?
Jul? 405? 293 28% 2.1 5?
Aug? 396? 326 18% 2.4 4.3?
Sep? 407? 342? 16%? 1.0? 3?
Oct? 422? 438 ?4% 3.2 5?
Nov? 284? 247 13% 9.7 30?
Dec? 353? 14.5 ??
?
?
Figure?3?4:?Duration,?magnitude?and?frequency?of?small?floods?
?
?
20
70
120
170
220
270
320
370
420
470
1351313513579246810246810121424681012246810246135
Feb Apr May Jun Jul Aug Sep Oct Nov Dec
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Days
T?1
2<T<3
2<T<3
2<T<3
5<T<10
5<T<10
5<T<10
T>10
T>10
2<T<3
5<T<10
5<T<10
5<T<10
5<T<10
T>10
T>10
T>10
??26???
?
3.4. INTEGRATION?OF?RESULTS?
?
This?section?integrates?the?results?of?median?monthly?flows,?high?flows?and?small?flood?for?the?
pre?1946?and?post?1946?conditions.??
?
3.4.1. BEFORE?1946?
?
Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?year?return?period?in?the?pre?1946?
conditions.?Table?3?13?shows?the?volume?necessary?to?satisfy?this?annual?hydrograph,?which?is?
1,292X10
6
?m
3
?per?year.?Notice?that?one?small?flood?in?September?always?happened?in?the?pre?1946?
conditions.?
?
?
Figure?3?5:?Annual?Hydrograph,?Return?Period?=?1?year,?pre?1946?conditions?
?
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T?=?1?year
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
??27???
?
Table?3?13:?Annual?hydrograph?volume?for?1?year?return?period?flows,?pre?1946?conditions?
?? Volume?(1X10
6
m
3
)
?? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct? Nov?Dec? Sum
Median? 72? 59? 61? 30 53 68 118 133 253 136? 77? 71? 1130
High?Flows? ???? ???? ???? ??? ??? ??? 3 6 ??? 7 ???? 10? 26
Small?Floods? ???? ???? ???? ??? ??? ??? ??? ??? 136 ???? ???? ???? 136
Total? 72? 59? 61? 30 53 68 121 139 389 143? 77? 81? 1292
?
Figure?3?6?shows?the?annual?hydrograph?for?flows?with?one?and?two?to?three?year?return?period?
in?the?pre?1946?conditions.?Results?show?that?small?floods?happened?in?July,?August?and?November,?
once?every?2?3?years.?Considering?that?one?small?flood?always?occurred?in?September,?the?previous?
results?suggest?that?two?small?floods?happened?every?year,?one?in?September?and?another?in?July,?
August? or? October.? The? combination? of? two? small? floods,? one? fixed? in? September? and? another?
moving?in?July,?August?or?October?may?have?contributed?to?maintaining?the?Rio?Grande/Bravo?as?a?
wide?sandy?multi?threaded?river.?
?
?
Figure?3?6:?Annual?Hydrograph,?Return?Period?=?2?3?years,?pre?1946?conditions?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T?=?1?year
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
Small?Floods
2<T<3
High?Flows
2<T<3
High?Flows
2<T<3
??28???
?
Figure?3?7?shows?the?annual?hydrograph?for?flows?with?one?and?five?to?ten?year?return?period?in?
the?pre?1946?conditions.?Results?show?that?small?floods?did?not?occur?so?frequently?outside?the?
monsoon?season?(June?to?October)?nor?large?high?flows?(Peak?flow?>?125?m
3
/s).?Small?floods?and?
high?flows?showed?in?Figure?3?7?used?to?occur?once?every?five?to?ten?years.?
?
?
Figure?3?7:?Annual?Hydrograph,?Return?Period?=?5?10?years,?pre?1946?conditions?
?
Figure?3?8?shows?the?annual?hydrograph?for?flows?with?one?and?ten?or?more?year?return?period?
in?the?pre?1946?conditions.?Similarly?that?in?the?previous?analysis,?results?show?that?small?floods?
did?not?occur?so?frequently?outside?the?monsoon?season?(June?to?October)?nor?large?high?flows?
(Peak?flow?>?125?m
3
/s).?Small?floods?and?high?flows?showed?in?Figure?3?8?used?to?occur?once?every?
ten?years?or?more.?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T?=?1?year
5<T<10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Small?Floods
5<T<10
High?Flows
5<T<10
Small?Floods
5<T<10
??29???
?
?
Figure?3?8:?Annual?Hydrograph,?Return?Period?>10?years,?pre?1946?conditions?
?
3.4.2. AFTER?1946?
?
Figure?3?9?shows?the?annual?hydrograph?for?flows?with?one?year?return?period?in?the?post?1946?
conditions.?Table?3?14?shows?the?volume?for?each?month?according?to?the?1?year?return?period?
hydrograph;?the?total?volume?in?the?hydrograph?is?small?(492X10
6
?m
3
/year)?compared?to?the?pre?
1946?conditions?(1,471X10
6
?m
3
/year).?Every?year?only?4?high?flows?occur;?these?high?flows?can?be?
the?result?of?storage?release?from?Luis?L.?Leon?reservoir?to?be?prepared?for?the?monsoon?season?(in?
May?and?June)?or?to?spill?the?water?stored?in?the?flooding?storage?capacity?(in?July?and?August).?
Notice?that?no?small?flood?occurs?every?year?in?the?post?1946?conditions.?This?might?be?the?cause?
for?the?reduction?in?channel?width?in?the?post?1946?period.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T?=?1?year
T>10
Small?Floods
High?Flows
Jan????????Feb Mar?????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov?????????Dec
Small?Floods
T>10
High?Flows
T>10
Small?Floods
T>10
??30???
?
Figure?3?9:?Annual?Hydrograph,?Return?Period?=?1?year,?post?1946?conditions?
?
Table?3?14:?Annual?hydrograph?volume?for?1?year?return?period?flows,?post?1946?conditions?
?? Volume?(1X10
6
m
3
)
?? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct Nov?Dec?Sum
Median? 28? 23? 25? 21 35 39 43 60 62 61 36? 30? 463
High?Flows? ???? ???? ???? ??? 11 7 6 4 ??? ??? ???? ???? 29
Small?Floods? ???? ???? ???? ??? ??? ??? ??? ??? ??? ??? ???? ???? 0
Total? 28? 23? 25? 21 46 46 49 64 62 61 36? 30? 492?
?
Figure?3?10Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?two?to?three?year?
return?period?in?the?post?1946?conditions.?Results?show?that?small?floods?occur?once?every?2?or?3?
years?in?June?and?September.?This?result?combined?with?the?1?year?return?hydrograph?mean?that?
one?year?there?may?be?no?small?flood?such?as?in?the?1?year?return?period?hydrograph?(Figure?3?9),?
the?following?year?one?small?flood?may?occur?in?June?with?a?peak?of?265?m3/s?and?the?next?year?one?
small?flood?may?occur?in?September?with?a?peak?of?341?m3/s.?Notice?that?the?small?flood?that?used?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
T?1
??31???
to?happened?every?year?in?September?(407?m
3
/s)?for?the?pre?1946?conditions,?now?is?happening?
once?every?3?years?with?a?smaller?peak?magnitude?(342?m
3
/s).?
?
?
Figure?3?10:?Annual?Hydrograph,?Return?Period?=?2?3?years,?pre?1946?conditions?
?
Figure?3?11Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?five?to?ten?year?
return?period?in?the?post?1946?conditions.?Small?floods?occur?once?every?5?years?in?July,?August?and?
October.?Notice?that?small?floods?that?used?to?happen?every?2?3?years?in?July?August?and?October?
(405,?396?and?422?m
3
/s)?for?the?pre?1946?conditions,?they?are?occurring?in?the?double?of?time?in?the?
post?1946?period,?every?5?years,?with?smaller?peak?magnitudes?(293,?326?and?438?m
3
/s)?for?July?
and?August.?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
2<T<3
Small?Floods
2<T<3
High?Flows
2<T<3
??32???
?
Figure?3?11:?Annual?Hydrograph,?Return?Period?=?5?10?years,?pre?1946?conditions?
?
Figure?3?12Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?ten?year?return?
period?in?the?post?1946?conditions.?Similarly?to?the?pre?1946?period,?small?floods?do?not?occur?so?
frequently?outside?the?monsoon?season?(June?to?October)?nor?high?flows.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
5<T<10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
5<T<10
Small?Floods
5<T<10
High?Flows
5<T<10
??33???
?
Figure?3?12:?Annual?Hydrograph,?Return?Period?>10?years,?pre?1946?conditions?
?
3.4.3. COMPARISON?
?
Figure?3?13?shows?the?annual?hydrograph?for?flows?with?one?year?return?period?for?the?pre?
1946?and?post?1946?conditions.?Even?though?there?is?the?same?number?of?high?flows,?there?is?
different?timing?and?magnitude?of?these.?Most?importantly,?one?small?flood?with?a?median?peak?of?
407?m
3
/s?occurred?every?year?in?the?pre?1946?conditions?while?in?the?post?1946?condition?it?does?
not?occur.?Also?notice?the?big?variability?of?median?monthly?flows?in?the?pre?1946?conditions,?the?
ratio?of?the?maximum?value?(September=97?m
3
/s)?and?the?minimum?value?(April=11?m
3
/s)?8.8?is?
times;?meanwhile?in?the?post?1946?conditions,?the?same?ratio?of?maximum?value?(September=24?
m
3
/s)?and?the?minimum?value?(April=88?m
3
/s)?is?3?times?only.??
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
T>10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
T>10
Small?Floods
T>10
??34???
?
Figure?3?13:?Annual?Hydrograph,?Return?Period?=?1?year,?post?and?pre?1946?conditions?
?
Figure?3?14?shows?annual?hydrograph?for?flows?with?one?year?return?period?for?the?pre?1946?
and? 2?3? year? return? period? for? the? post?1946? conditions.? The? hydrograph? for? the? pre?1946?
condition?considers?that?two?small?floods?happened?every?year,?the?one?fixed?in?September,?and?the?
one? variable? in? time? that? this? occasion? occurred? in? July.? For? the? post?1946? Hydrograph,? it? is?
considered?that?both?small?floods?occurred?that?year,?in?June?and?September.?High?flows?in?May,?
June,?July,?August?and?October?occur?in?both?hydrographs.?Figure?3?14?demonstrate?how?high?flows?
and?small?floods?that?occurred?every?year?in?the?pre?1946?conditions,?they?are?occurring?every?2?3?
in? the? post?1946? conditions.? Not? only? small? floods? do? not? occur? so? often? in? the? post?1946?
conditions,?but?also?their?magnitude?(median?peak)?is?smaller?than?in?the?pre?1946?conditions.?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Same?number?of?high?flows,
different?timing
One?Small?Flood
every?year
??35???
?
Figure?3?14:?Comparison?of?annual?hydrograph,?Pre?1946?T=1?year?Vs.?Post??1946?2<T<3?
?
Figure?3?15?shows?annual?hydrograph?for?flows?with?2?3?year?return?period?for?the?pre?1946?
and? 5?10? year? return? period? for? the? post?1946? conditions.? The? hydrograph? for? the? pre?1946?
condition?considers?that?three?small?floods?happened,?the?one?fixed?in?September,?and?two?variables?
in?time?that?this?occasion?occurred?in?August?and?October.?For?the?post?1946?Hydrograph,?it?is?
considered?that?two?small?floods?occurred?that?year,?in?August?and?October.?High?flows?in?February,?
May,?June,?July,?August,?October,?November?and?December?occur?in?both?hydrographs.?Figure?3?15?
demonstrate? how? high? flows? and? small? floods? that? occurred? every? 2?3? years? in? the? pre?1946?
conditions,?they?are?occurring?every?5?in?the?post?1946?conditions.?Same?conclusion?can?be?derived?
from?these?results,?not?only?small?floods?do?not?occur?so?often?in?the?post?1946?conditions,?but?also?
their?magnitude?(median?peak)?is?smaller?than?in?the?pre?1946?conditions.?Notice?that?the?fixed?
small?flood?of?September?may?not?occur?every?year?in?the?post?1946?conditions.?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
pre?1946?T=1
post?1946?2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Same?number?of?high?flows,
different?magnitude
Small?Floods:
? Post?1946?every?3?years
? Pre?1946?every?year
Same?number?of?small?
floods,?different return?
period?and?magnitude
??36???
?
Figure?3?15:?Comparison?of?annual?hydrograph,?Pre?1946?2<T<3?year?Vs.?Post??1946?5<T<10?
?
3.5. CONCLUSIONS?
?
From?the?previous?analysis?of?the?normal?flows,?it?can?be?concluded?that?when?comparing?pre?
1946?to?post?1980:?
? Pre?1946?conditions?have?more?frequent?small?floods?(median?peak?flow?=?400?m
3
/s),?one?
fixed?every?year?in?September,?and?at?least?one?small?flood?in?one?of?the?following?months:?
July,?August?and?October.?
? High? flows? and? small? floods? that? occurred? every? year? in? the? pre?1946? condition,? they?
happened?every?2?3?years?in?the?post?1946?conditions.??
? Similarly,? high? flows? and? small? flows? that? occurred? every? 2?3? years? in? the? pre?1946?
condition,?they?happened?every?5?years?in?the?post?1946?conditions.?
? Median? annual? flows? decreased? by? 656.8? X10
6?
m
3
,? or? 59.0%? of? the? pre?1946? annual?
quantity.?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
Pre?1946?2<T<3
Post?1946?5<T<10
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Small?Floods
High?Flows
Same?number?of?high?flows,
different?magnitude
Post?1946?conditions:
2?Small?Floods?every?5?years
September?small?flood?
not?a?constant?in?the?post?
1946 conditions
Pre?1946?conditions:
3?Small?Floods?every?2?3?years
??37???
? September? has? the? largest? median? monthly? flows,? while? April? has?the?lowest?for?both?
conditions?
? Small?flow?pulse?event?decrease?in?magnitude?for?a?majority?of?the?year?by?an?average?of?
34%.?However,?in?the?post?1980?winter?months,?there?is?an?average?increase?of?61%.?Events?
occur?with?roughly?the?same?frequency?for?both?periods?(7.4?against?6.4?per?year).?
? Large?high?flow?pulse?events?are?four?times?more?likely?to?occur?in?the?post?1946?conditions?
but?have?an?average?17%?less?flow.?
? Small? floods? are? two? times? less? likely? to? occur? in? the? post?1946? conditions? and? have? a?
smaller?median?peak?flow?magnitude.?
?
??
??38???
4. DROUGHT?PERIODS?
?
In?this?section?are?analyzed?the?daily?mean?flow?values?of?drought?years?for?both?periods.?Pre?
1946?period?include?mean?daily?flow?data?for?29?years,?from?1901?to?1913?and?from?1931?to?1946.?
Post?1946?period?include?data?for?30?years,?from?1980?to?2009.?Drought?years?are?defined?as?those?
when?the?annual?flow?volume?is?smaller?than?the?median?annual?volume?for?the?period?of?analysis.?
Table?4?1?shows?the?median,?mean?and?standard?deviation?for?both?periods.?The?median?annual?
streamflow?has?been?reduced?by?half,?from?1,318X10
6
?m
3
?on?the?pre?1946?to?676X10
6
?m
3
?on?the?
post?1946.?
Table?4?1:?Annual?statistics,?Pre?1946?and?Post?1946?
?? Percentile?(1X10
6
?m
3
/s)? Average? Std.?Dev.?
?? 0.75? 0.5?(Median)? 0.25? (1X10
6
?m
3
/s)? (1X10
6
?m
3
/s)?
Pre?1946? 2530? 1318? 1053 1892 1104?
Post?1946? 1157? 676? 404 1000 837?
Deficit?
1373? 642? 649 892 267?
(54%)? (49%)? (62%) (47%) (24%)?
?
Figure?4?1?and?Figure?4?2?show?the?annual?streamflow?volume?at?Johnson? Ranch? for? both?
periods.?For?the?pre?1946?period,?the?daily?mean?flow?of?the?following?years?is?analyzed:?1901,?
1903,?1910,?1913,?1931,?1934?1937,?1939?1940?and?1943?1946.?For?the?post?1946?period,?the?
daily?mean?flow?of?the?following?years?is?analyzed:?1983,?1994?2003,?2005?2007?and?2009.?
A?similar?analysis?has?been?done?for?mean?daily?flows?during?drought?years.?Flows?have?been?
categorized?in:?median?flows,?high?flows,?small?floods?and?large?floods?using?the?same?thresholds?
calculated?in?the?previous?section.?
? A?median?monthly?flow?is?the?median?value?of?all?the?daily?mean?flows?for?the?month?and?
the?drought?years?of?the?hydrologic?period?analyzed.?
? A?high?flow?is?a?flow?with?a?peak?between?the?75
th
?(56?m
3
/s)?and?the?95
th
?(224?m
3
/s)?
percentile?of?daily?flows.?
? A?small?flood?is?a?flow?with?a?peak?between?the?95
th
?(224?m
3
/s)?and?1,100?m
3
/s.?
? A?large?flood?is?a?flow?with?a?peak?larger?than?1,100?m
3
/s.?
In?this?section?the?objective?is?to?characterize?these?flows?during?drought?years.?The?Indicators?
of?Hydraulic?Alteration?(IHA)?software?is?used?to?label?the?different?flows?along?the?periods?of?
analysis.?The?frequency?of?each?event?(i.e.?high?flow,?small?and?large?floods)?has?been?determined?
for?each?period?of?analysis.?Return?periods?for?each?event?have?been?calculated?using?the?frequency?
of?each?event.?
??39???
?
Figure?4?1:?Johnson?Ranch?annual?flow?volume,?pre?1946?
?
?
Figure?4?2:?Johnson?Ranch?annual?flow?volume,?post?1946?
?
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946
Annual
?
Flow
?
Volume
?
(1X10
6
m
3
)
Percentile?0.75
Annual?median
Percentile?0.25
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
Annual
?
Flow
?
Volume
?
(1X10
6
m
3
)
Percentile?0.75
Annual?median
Percentile?0.25
??40???
4.1. MONTHLY?MEDIANS?
?
The?median?monthly?flow?is?the?median?value?of?the?daily?mean?flows?for?each?month?of?the?
selected?drought?years.?One?value?for?each?month?has?been?calculated?for?pre?1946?and?post?1946?
periods?under?drought?conditions.?
?
4.1.1. BEFORE?1946?
?
Table?4?2?provides?a?summary?of?median?monthly?flows?during?drought?years?prior?to?1946.?
The?peak?monthly?median?flow?occurred?in?September?at?45?m
3
/s?(1,590?cfs)?and?the?minimum?in?
April?at?10?m
3
/s?(353?cfs).?Figure?4?3?shows?the?median?monthly?flow?under?drought?and?normal?
conditions? for? the? pre?1946? period.? Main? differences? in? the? median? flows? are? from? June? to?
November.??
Table?4?2:?Median?monthly?flows,?pre?1946?period?
?? Pre?1946?Monthly?Flows?(m
3
/s) Median Deficit?
?? 75th? Median 25th Norm.?Cond. (m
3
/s)? (%)?
Jan? 33? 26 14 27 0 1?
Feb? 33? 25 18 24 ?1? ????
Mar? 33? 24 16 23 ?2? ????
Apr? 17? 10 4 11 2 14?
May? 32? 15 6 20 5 24?
Jun? 34? 20 9 26 6 24?
Jul? 52? 27 15 44 17? 38?
Aug? 46? 24 13 50 26? 52?
Sep? 96? 45 20 98 52? 54?
Oct? 63? 36 19 51 15? 30?
Nov? 32? 24 15 30 6 21?
Dec? 28? 21 9 26 5 20?
?
??41???
?
Figure?4?3:?Median?monthly?flows?under?drought?conditions,?pre?1946?
4.1.2. AFTER?1946?
?
Table?4?3?provides?a?summary?of?median?monthly?flows?during?drought?years?after?1946?
(1980?2009).?The?peak?monthly?median?flow?occurred?in?October?at?13?m
3
/s?(460?cfs)?and?the?
minimum?in?April?at?3?m
3
/s?(105?cfs).?Figure?4?4?shows?the?median?monthly?flow?under?drought?
and?normal?conditions?for?the?post?1946?period.?
Table?4?3:?Median?monthly?flows,?post?1946?period?
??
Post?1946?Monthly?Flows?
(m
3
/s)? Median?
? 75th? Median 25th Norm.?Cond.?
Jan? 11? 8 7 11
Feb? 8? 6 5 9
Mar? 7? 5 3 10
Apr? 6? 3 2 8
May? 16? 4 2 13
Jun? 17? 9 2 15
Jul? 17? 10 4 16
Aug? 23? 12 7 23
Sep? 16? 8 3 24
Oct? 24? 13 6 23
Nov?12? 9 6 14
Dec? 9? 8 6 11
0
20
40
60
80
100
120
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Median
?
Monthly
?
streamflow
?
(m
3
/s)
75th
Median
25th
Median?Norm.?Cond.
??42???
?
Figure?4?4:?Median?monthly?flows?under?drought?conditions,?post?1946?
?
4.1.3. COMPARISON?
?
Figure?4?5?shows?monthly?medians,?25
th
?and?75
th
?percentile?flows?for?pre?1946?and?the?post?
1946?(1980?2009).?Median?values?are?compared?in?Table?4?4.?During?droughts,?there?is?a?significant?
difference?between?pre?1946?and?post?1946?conditions.?The?annual?median?flow?volume?for?the?
pre?1946?and?post?1946?are?782X10
6?
m
3
?and?251X10
6?
m
3
,?respectively.?Comparing?these?annual?
volumes? of? the? periods,? the? resulting? deficit? is? 531X10
6?
m
3
?of?water,?or?68%?of?the?pre?1946?
quantity.?Much?of?this?deficit?(65%)?comes?from?high?flow?months?June?through?November.?Notice?
that?in?the?pre?1946?conditions,?the?median?monthly?values?preserve?the?seasonal?shape,?with?high?
values?during?monsoon?season?(June?to?October)?and?low?values?during?winter?and?spring.?On?the?
contrary,?in?the?post?1946?conditions,?the?annual?hydrograph?is?completely?distorted.?The?peak?
monthly?flow?has?shifted?to?October,?and?there?is?a?decrease?in? the? median? monthly? flow? in?
September,?that?used?to?be?the?month?with?the?highest?median?monthly?value.??
?
?
?
?
0
5
10
15
20
25
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Median
?
Monthly
?
streamflow
?
(m
3
/s)
75th
Median
25th
Median?Norm.?Cond.
??43???
?
Table?4?4:?Comparison?of?monthly?median?flows?
?? Monthly?Median?Flows?during?droughts?(m
3
/s)?
?? Pre?1946? Post?1980? Deficit? %?Deficit?
Jan? 26 8 18 70
Feb? 25 6 19 75
Mar? 24 5 20 81
Apr? 10 3 7 67
May? 15 4 11 75
Jun? 20 9 11 54
Jul? 27 10 17 63
Aug? 24 12 12 51
Sep? 45 8 37 82
Oct? 36 13 23 64
Nov? 24 9 14 60
Dec? 21 8 13 63
?
?
Figure?4?5:?Comparison?of?median?monthly?flows?under?drought?conditions?
?
?
?
0
20
40
60
80
100
120
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Median
?
Monthly
?
streamflow
?
(m
3
/s)
Pre?1946
Post?1946
75th?percentile
75th?percentile
25th?Percentile
25th?percentile
??44???
4.2. HIGH?FLOWS?
?
High?flows?are?defined?as?those?whose?peak?flow?exceed?the?75?percentile?(56?m
3
/s)?but?are?
lower?than?the?95
th
?percentile?(224?m
3
/s)?of?daily?flows?for?the?whole?period.?These?thresholds?
already?established?in?the?normal?conditions?section?are?used?in?order?to?evaluate?how?frequently?
high?flows?occurred?during?drought?in?the?pre?1946?and?post?1946?periods.?
Similar?to?the?previous?section,?each?high?flow? have? been?identified,?selected?and?analyzed?
individually?by?month?and?period?of?analysis?(pre1946?or?post?1946).??
In?this?section?is?presented?the?characteristics?high?flows?during?drought?years?for?each?month,?
period?(pre?1946?or?post?1946)?and?type?of?high?flow?(small?or?large).?For?the?pre?1946?the?small?
high?flows?have?a?peak?lower?than?125?m
3
/s;?meanwhile?for?the?post?1946,?small?flows?have?a?peak?
lower?than?100?m
3
/s.?
?
4.2.1. BEFORE?1946?
?
Table?4?5?shows?the?high?flow?pulses?with?peaks?lower?than?125?m
3
/s?(4,414?cfs)?for?drought?
years?prior?1946?for?different?return?periods?(T).?Small?high?flow?pulses?that?happen:?
? Every?year?(T<1.5?years)?occur?July,?August,?September?and?December?with?a?duration?
of?3?days?and?peak?flows?of?71,?102,?91?and?40?m
3
/s,?respectively.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?the?rest?of?the?months?(January,?
February,?March,?April,?May,?June,?October?and?November),?with?a?duration?of?3,?3,?3,?3,?
3,?3,?4,?and?3,?days?and?peak?flows? of? 36,? 40,? 38,? 61,? 116,? 98,? 84? and? 42? m
3
/s,?
respectively.?
?
Table?4?5:?High?flow?pulses?during?drought?years?lower?than?125?m
3
/s,?Pre?1946?period?
?? Small?high?flow?pulses during?drought?years?(m
3
/s)?
Day? Jan Feb? Mar Apr May Jun Jul Aug Sep? Oct?Nov Dec
1? 31?28?31?533853415550?58?3430
2? 36? 40? 38? 61 116 98 70.5 102 91? 84? 42 40
3? 31?28?31?533853415550?64?3430
4? ???? ???? ???? ??? ??? ??? ??? ??? ???? 58? ??? ???
5? ???? ???? ???? ??? ??? ??? ??? ??? ???? ???? ??? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ???? ???? ??? ???
Duration?(days)? 3? 3? 3? 3 3 3 3 3 3? 4? 3 3
Peak?(m
3
/s)? 36? 40? 38? 61 116 98 71 102 91? 84? 42 40
Frequency? 0.40? 0.47? 0.47 0.33 0.53 0.60 0.76 1.00 1.13? 0.60? 0.47 0.73
T?(years)? 2.5 2.1? 2.1 3.0 1.9 1.7 1.3 1.0 0.9? 1.7? 2.1 1.4
?
??45???
Table?4?6?shows?the?high?flow?pulses?with?peaks?larger?than?125?m
3
/s?(4,414?cfs)?for?the?prior?
1946?period?for?different?return?periods?(T).?Large?high?flow?pulses?that?happen:?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?July?with?a?duration?of?8?days?and?a?
peak?flow?of?188?m
3
/s.?
?
Table?4?6:?High?flow?pulses?during?drought?year?larger?than?125?m
3
/s,?Pre?1946?period?
?? Large?high flow?pulses during?drought?years?(m
3
/s)?
Day? Jan? Feb? Mar Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? ??? ??? 139 ??? ??? ???? ???? ???
2? ???? ???? ???? ??? ??? ??? 166 ??? ??? ???? ???? ???
3? ???? ???? ???? ??? ??? ??? 180 ??? ??? ???? ???? ???
4? ???? ???? ???? ??? ??? ??? 188 ??? ??? ???? ???? ???
5? ???? ???? ???? ??? ??? ??? 166 ??? ??? ???? ???? ???
6? ???? ???? ???? ??? ??? ??? 139 ??? ??? ???? ???? ???
7? ???? ???? ???? ??? ??? ??? 124 ??? ??? ???? ???? ???
8? ???? ???? ???? ??? ??? ??? 109 ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? ??? ??? 8 ??? ??? ???? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ??? ??? ??? 188 ??? ??? ???? ???? ???
Frequency? ???? ???? ???? ??? ??? ??? 0.10 ??? ??? ???? ???? ???
T?(years)? ???? ???? ???? ??? ??? ??? 9.7 ??? ??? ???? ???? ???
?
4.2.2. AFTER?1946?
?
Table?4?7?shows?the?high?flow?pulses?lower?than?100?m
3
/s?(3,531?cfs)?for?the?after?1946?period?
for?different?return?periods?(T).?High?flow?pulses?that?happen:?
? Every?year?(T<1.5?years)?occur?May,?June,?July?and?August,?with?a?duration?of?5,?3,?3,?
and?3?days?and?peak?flows?of?57,?54,?60?and?53?m
3
/s,?respectively.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?September?and?October?with?a?
duration?of?3?days?and?peak?flows?of?47?and?63?m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?March,?April?and?November?with?a?
duration?of?3?days?and?peak?flows?of?69,?32?and?62?m
3
/s,?respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?February?with?duration?of?3?days?and?a?
peak?flow?of?46?m
3
/s.?
?
?
?
?
??46???
Table?4?7:?High?flow?pulses?during?drought?years?lower?than?100?m
3
/s,?Post?1946?period?
?? Small?high?flow?pulses during?drought?years (m
3
/s)?
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? 10? 33? 13 14 26 25 29 24 25? 20? ???
2? ???? 46? 69? 32 32 54 60 53 47 63? 62? ???
3? ???? 10? 33? 13 57 26 25 29 24 25? 20? ???
4? ???? ???? ???? ??? 32 ??? ??? ??? ??? ???? ???? ???
5? ???? ???? ???? ??? 14 ??? ??? ??? ??? ???? ???? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
Duration?(days)? ???? 3? 3? 3 5 3 3 3 3 3? 3? ???
Peak?(m
3
/s)? ???? 46? 69? 32 57 54 60 53 47 63? 62? ???
Frequency? ???? 0.07? 0.20? 0.20 1.07 1.13 1.20 1.13 0.47 0.53? 0.20? ???
T?(years)? ???? 15.0? 5.0? 5.0 0.9 0.9 0.8 0.9 2.1 1.9? 5.0? ???
?
Table?4?8?shows?the?high?flow?pulses?larger?than?100?m
3
/s?(3,531?cfs)?for?the?after?1946?period?
for?different?return?periods?(T).?High?flow?pulses?that?happen:?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?July?and?October?with?a?duration?
of?3?days?and?peak?flows?of?111?and?117?m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June,?August?and?September?with?a?
duration?of?3,?5?and?5?days?and?peak?flows?of?174,?173?and?127?m
3
/s,?respectively.?
?
Table?4?8:?High?flow?pulses?during?drought?years?larger?than?100?m
3
/s,?Post?1946?period?
?? Large?high?flow?pulses during?drought?years (m
3
/s)?
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? ??? 77 60 57 102 46? ???? ???
2? ???? ???? ???? ??? ??? 174 111 104 114 117? ???? ???
3? ???? ???? ???? ??? ??? 77 60 173 127 46? ???? ???
4? ???? ???? ???? ??? ??? ??? ??? 104 114 ???? ???? ???
5? ???? ???? ???? ??? ??? ??? ??? 57 102 ???? ???? ???
6? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? ??? 3 3 5 5 3? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ??? ??? 174 111 173 127 117? ???? ???
Frequency? ???? ???? ???? ??? ??? 0.27 0.33 0.20 0.20 0.40? ???? ???
T?(years)? ???? ???? ???? ??? ??? 3.8 3.0 5.0 5.0 2.5? ???? ???
?
?
?
?
?
??47???
4.2.3. COMPARISON?
?
Table?4?9?and?Figure?4?6?show?a?comparison?of?small?high?flows?(peak?flow?<?125?m3/s).?The?
majority?of?months?see?reductions?up?to?51%?(in?May)?with?a?few?notable?exceptions.?For?the?period?
of?Nov?Mar,?the?intensity?of?flow?pulses?actually?increased,?up?to?81%?in?March,?probably?caused?by?
dam?releases.?Prior?to?1946,?there?was?an?average?of?8.3?small?high?flow?pulse?events?per?year.?Post?
1980,?the?frequency?dropped?to?6.1?per?year.??
Table?4?9:?Magnitude?and?frequency?of?high?flow?pulses?during?drought?years?smaller?than?125?m3/s?
?? Small?high?flow?pulses?during?drought?years?
Peak?Flow?(m
3
/s) Return?Period?(years)?
?? Pre?1946? Post?1946? %?Reduction? ? Pre?1946? Post?1980?
Jan? 36? ??? ??? 2.5 ????
Feb? 40? 46 ?15% 2.1 15.0?
Mar? 38? 69 ?81% 2.1 5.0?
Apr? 61? 32 48% 3.0 5.0?
May? 116? 57 51% 1.9 0.9?
Jun? 98? 54 45% 1.7 0.9?
Jul? 71? 60 15% 1.3 0.8?
Aug? 102? 53 49% 1.0 0.9?
Sep? 91? 47 48% 0.9 2.1?
Oct? 84? 63 25% 1.7 1.9?
Nov? 42? 62 ?46% 2.1 5.0?
Dec? 40? ??? ??? 1.4 ????
?
?
Figure?4?6:?Duration,?magnitude?and?frequency?of?high?flow?pulses?during?drought?years?smaller?than?
125?m
3
/s?
0
20
40
60
80
100
120
140
12341234123412341234561234123412341234123451234123
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Days
5<T<10
T?1
2<T<3
T?1
2<T<3
T>10
T?1
T?1
T?1
2<T<3
2<T<3
2<T<3
2<T<3
2<T<3
2<T<3
2<T<3
T?1
T?1
T?1
2<T<3
5<T<10
5<T<10
??48???
A?summary?of?large?high?flow?pulses?(peak?flow>?than?100?m3/s)?is?shown?below?in?Table?4?10?
and?Figure?4?7.?In?the?pre?1946?period,?only?large?high?flows?occurred?in?July.?On?the?contrary,?in?
the?post?1946?period?large?high?flows?occurred?from?June?to?October.?This?means?that?large?high?
flow?pulses?(peak?flow?>?100?m3/s)?were?not?very?common?in?the?pre?1946?conditions?as?they?are?
now?in?the?post?1946?conditions;?this?may?be?as?a?consequence?of?reservoir?releases?upstream.For?
instance,? in? July? large? high? flow? pulses? occurred? once? every? 3? years? in? the? post?1946? period,?
meanwhile?they?occurred?once?every?10?years?in?the?pre?1946?periods.??
?
Table?4?10:?Magnitude?and?frequency?of?high?flow?pulses?during?drought?years?larger?than?100?m
3
/s?
?? Large?high?flow?pulses?during?drought?years?
Peak?Flow?(m
3
/s) Return?Period?(years)?
?? Pre?1946? Post?1980? %?Reduction ? Pre?1946? Post?1980?
Jan? ???? ??? ??? ??? ????
Feb? ???? ??? ??? ??? ????
Mar? ???? ??? ??? ??? ????
Apr? ???? ??? ??? ??? ????
May? ???? ??? ??? ??? ????
Jun? ???? 174 ??? ??? 3.8?
Jul? 188? 111 41% 9.7 3.0?
Aug? ???? 173 ??? ??? 5.0?
Sep? ???? 127 ??? ??? 5.0?
Oct? ???? 117 ??? ??? 2.5?
Nov? ???? ??? ??? ??? ????
Dec? ???? ??? ??? ??? ????
?
?
Figure?4?7:?Duration,?magnitude?and?frequency?of?high?flow?pulses?during?drought?years?larger?than?
125?m
3
/s?
0
20
40
60
80
100
120
140
160
180
200
1234123456789123456123456123
Jun Jul Aug Sep Oct
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Days
5<T<10
T>10
5<T<10
5<T<10
2<T<3
2<T<3
??49???
4.3. SMALL?FLOODS?
?
Small?Floods?are?those?that?have?a?peak?flow?greater?than?the?95?percentile?(224?m
3
/s)?but?
smaller?than?1,100?m
3
/s?(38,846?cfs),?which?is?a?below?the?Presidio/Ojinaga?levees?flow?design,?
1,190?m
3
/s?(42,000?cfs).?The?95th?percentile?flow,?224?m
3
/s?(7,910?cfs),?has?been?determined?using?
the?date?of?the?pre?1946?period.?This?thresholds?are?used?in?both?periods?to?evaluate?the?how?
frequently?high?flows?occurred?in?drought?years.?Flows?that?satisfy?the?previous?thresholds?have?
been?identified,?selected?and?analyzed?individually?by?month?and?period?of?analysis?(pre1946?or?
post?1946)?during?drought?years.?For?each?month,?the?median?of?the?flows?was?estimated?in?order?
to?obtain?a?characteristic?small?flood.?This?section?presents?the?characteristics?small?floods?for?each?
month?and?period?of?analysis?(pre?1946?or?post?1946)?during?drought?years.?
?
4.3.1. BEFORE?1946?
?
Table?4?11?shows?the?small?floods?for?drought?years?of?pre?1946?period?and?for?different?return?
periods?(T).?Small?floods?that?happen:?
? Every?year?(T<1.5?years)?occur?in?September?with?a?duration?of?11?days?and?a?median?
peak?flow?of?380?m
3
/s.?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?occur?in?October?with?durations?of?7days?
and?a?median?peak?flow?of?422?m
3
/s,?respectively.?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June,?July?and?August?with?a?duration?
of?8,?7?and?11?days?and?median?peak?flows?of?632,?323,?and?270?m
3
/s,?respectively.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?May?with?duration?of?3?and?a?median?
peak?flow?of?247?m
3
/s.?
?
4.3.2. AFTER?1946?
?
Table?4?12?shows?the?small?floods?for?drought?years?of?post?1946?period?and?for?different?
return?periods?(T).?Small?floods?that?happen:?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?occur?in?June?with?a?duration?of?6?days?and?a?
median?peak?flow?of?310?m
3
/s.?
? Every?10?years?or?more?(T?>?10?years)?occur?in?August,?September?and?October?with?
durations? of? 9,? 5? and? 7? days? and? median? peak? flows? of? 402,? 352,? and? 599? m
3
/s?
respectively.?
?
??50???
?
?
Table?4?11:?Small?floods?(224?m
3
/s?<flow<1,100?m
3
/s)?during?drought?years,?Pre?1946?period?
?? Small?floods during?drought?years(m
3
/s)?
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? 49 49 78 90 113 66.3? ???? ???
2? ???? ???? ???? ??? 247 339 175 129 139 140? ???? ???
3? ???? ???? ???? ??? 49 632 258 172 150 229? ???? ???
4? ???? ???? ???? ??? ??? 339 323 193 204 422? ???? ???
5? ???? ???? ???? ??? ??? 265 258 215 262 229? ???? ???
6? ???? ???? ???? ??? ??? 136 175 270 380 140? ???? ???
7? ???? ???? ???? ??? ??? 121 78 215 262 66.3? ???? ???
8? ???? ???? ???? ??? ??? 105 ??? 193 204 ???? ???? ???
9? ???? ???? ???? ??? ??? ??? ??? 172 150 ???? ???? ???
10? ???? ???? ???? ??? ??? ??? ??? 129 139 ???? ???? ???
11? ???? ???? ???? ??? ??? ??? ??? 90 113 ???? ???? ???
12? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
13? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? 3 8 7 11 11 7? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ???? 247? 632? 323? 270? 380? 422? ???? ????
Frequency? ???? ???? ???? ??? 0.07 0.13 0.13 0.20 0.87 0.33? ???? ???
T?(years)? ???? ???? ???? ???? 15.0? 7.5? 7.5? 5.0? 1.2? 3.0? ???? ????
?
Table?4?12:?Small?floods?(224?m
3
/s?<flow<1,100?m
3
/s)?during?drought?years,?Post?1946?period?
?? Small?floods?during?drought?years (m
3
/s)?
Day? Jan? Feb? Mar? Apr May Jun Jul Aug Sep Oct?Nov?Dec
1? ???? ???? ???? ??? ??? 25 ??? 62 197 38? ???? ???
2? ???? ???? ???? ??? ??? 37 ??? 164 274 168? ???? ???
3? ???? ???? ???? ??? ??? 310 ??? 232 352 388? ???? ???
4? ???? ???? ???? ??? ??? 130 ??? 404 274 599? ???? ???
5? ???? ???? ???? ??? ??? 37 ??? 232 197 388? ???? ???
6? ???? ???? ???? ??? ??? 25 ??? 164 ??? 168? ???? ???
7? ???? ???? ???? ??? ??? ??? ??? 62 ??? 38? ???? ???
8? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
9? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
10? ???? ???? ???? ??? ??? ??? ??? ??? ??? ???? ???? ???
Duration?(days)? ???? ???? ???? ??? ??? 6 ??? 9 5 7? ???? ???
Peak?(m
3
/s)? ???? ???? ???? ???? ???? 310? ???? 404? 352? 599? ???? ????
Frequency? ???? ???? ???? ??? ??? 0.27 ??? 0.07 0.07 0.07? ???? ???
T?(years)? ???? ???? ???? ???? ???? 3.8? ???? 15.0? 15.0? 15.0? ???? ????
?
?
?
??51???
?
4.3.3. COMPARISON?
?
A?summary?of?small?flooding?events?is?show?below?in?Table?4?13?and?Figure?4?8.?The?maximum?
flows?for?each?period?are?similar?with?a?pre?1946?of?632?m3/s?and?a?post?1980?of?599?m3/s;?
however,?the?pre?1946?maximum?value?occurs?in?June,?while?the?post?1980?occurs?in?October.?Also?
notice?that?during?drought?conditions?one?small?flood?in?the?pre?1946?occurred?every?September?
(380? m
3
/s),? meanwhile? in? the? post?1946? conditions? it? occurs? once? every? 15? years.?There?is?a?
decrease?in?2?months,?up?to?50%,?where?small?floods?are?present?in?both.?During?drought?periods,?
prior?to?1946?there?was?an?average?of?1.7?small?flood?events?per?year?meanwhile?in?the?post?1980,?
the?frequency?drops?by?more?than?half?to?0.5?per?year.?
?
Table?4?13:?Peak?flows?and?frequency?of?small?floods?during?drought?years?
?? Small?High?Flow?Pulses?
Peak?Flow?(m
3
/s) Return?Period?(years)?
?? Pre?1946? Post?1980? %?Reduction ? Pre?1946? Post?1980?
Jan? ???? ???? ??? ??? ????
Feb? ???? ???? ??? ??? ????
Mar? ???? ???? ??? ??? ????
Apr? ???? ???? ??? ??? ????
May? 247? ???? ??? 15.0 ????
Jun? 632? 310? 51% 7.5 3.8?
Jul? 323? ???? ??? 7.5 ????
Aug? 270? 404? ?50% 5.0 15.0?
Sep? 380? 352? 7% 1.2 15.0?
Oct? 422? 599? ?42% 3.0 15.0?
Nov? ???? ???? ??? ??? ????
Dec? ???? ???? ??? ??? ????
?
??52???
?
Figure?4?8:?Duration,?magnitude?and?frequency?of?small?floods?during?drought?years?
?
4.4. INTEGRATION?OF?RESULTS?
?
This?section?integrates?the?results?of?median?monthly?flows,?high?flows?and?small?flood?for?the?
pre?1946?and?post?1946?conditions?during?drought?conditions.??
?
4.4.1. BEFORE?1946?
?
Figure?4?9?shows?the?annual?hydrograph?for?flows?with?one?year?return?period?during?drought?
years?in?the?pre?1946?conditions.?Table?4?14?shows?the?volume?necessary?to?satisfy?the?annual?
hydrograph.? These? results? show? that? in? order? to? satisfy? the? pre?1946? annual? conditions,? it? is?
necessary?a?volume?of?954X10
6
?m
3
?per?year.??
?
Table?4?14:?Annual?hydrograph?volume?for?T?=?1?year?during?drought?conditions,?pre?1946?
?? Volume?(1X10
6
?m
3
)?
?? Jan? Feb? Mar? Apr May Jun Jul Aug Sep? Oct Nov? Dec? Sum
Median? 71? 61? 66? 25? 40? 52? 73 63? 117 96? 61? 57? 782?
High?Flows? ???? ???? ???? ???? ???? ???? 6? 12? 5? ???? ???? 9? 32?
Small?Floods? ???? ???? ???? ???? ???? ???? ???? ???? 140 ???? ???? ???? 140?
Total? 71? 61? 66? 25? 40? 52? 79 76? 262 96? 61? 65? 954?
?
0
100
200
300
400
500
600
700
131357924682468101224681012246810
May Jun Jul Aug Sep Oct
Flow
?
(m
3
/s)
Pre?1946
Post?1946
Days
T>10
5<T<10
T?1
2<T<3
5<T<10
5<T<10
5<T<10
T>10
T>10
T>10
??53???
?
Figure?4?9:?Annual?Hydrograph?for?T?=?1?year?in?drought?years,?pre?1946?conditions?
?
There?are?several?similarities?with?the?hydrograph?for?normal?conditions?(Figure?3?5):?1)?one?
small?flood?occurs?in?September?of?about?the?same?size,?the?peaks?flows?are?380?m
3
/s?during?
drought?years?and?407m
3
/s?during?normal?conditions;?2)?four?high?flow?pulses?practically?in?the?
same? months,? July,? August? and? December? for? both? conditions? (Normal? and? droughts),? October?
during?normal?conditions?and?September?during?drought?conditions.?The?main?difference?between?
both?hydrographs?is?in?the?median?flows,?mostly?in?four?months,?from?July?to?October.?The?annual?
median? flow? volume? for? normal? and? drought? conditions? are? 1,130X10
6?
m
3
?and?782X10
6?
m
3
,?
respectively.?Comparing?these?annual?volumes,?the?resulting?deficit?is?348X10
6?
m
3
?of?water,?or?31%?
less?than?volume?during?normal?conditions.?
Figure?4?10Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?two?to?three?years?
return?period?in?the?pre?1946?conditions?during?drought?years.?Results?are?consistent?with?the?
hydrograph?of?normal?conditions?(Figure?3?6),?high?flows?start?happening?in?all?months.?In?addition,?
during?drought?years?a?small?flood?may?occur?in?October?with?a?bigger?peak?that?the?fixed?small?
flood?of?September.?Even?though?the?overall?year?might?be?catalogued?as?a?drought?year,?it?was?
possible?to?have?two?small?floods?during?drought?years.?The?occurrence?of?small?flood?even?in?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
??54???
drought?years?may?have?contributed?to?maintaining?the?Rio?Grande/Bravo?as?a?wide?sandy?multi?
threaded?river.?
?
?
Figure?4?10:?Annual?Hydrograph?during?drought?years,?Return?Period?=?2?3?years,?pre?1946?
conditions?
?
Figure?4?11Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?five?to?ten?year?
return? period? in? the? pre?1946?conditions?during?drought?years.?Similarly?to?normal?conditions?
(Figure?3?7),?small?floods?start?appearing?outside?the?monsoon?season,?in?this?case?in?May.?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
Small?Floods
2<T<3
High?Flows
2<T<3
High?Flows
2<T<3
??55???
?
Figure?4?11:?Annual?Hydrograph,?Return?Period?=?5?10?years,?pre?1946?conditions?
?
Figure?4?12Figure?3?5?shows?the?annual?hydrograph?for?flows?with?one?and?five?to?ten?year?
return? period? in? the? pre?1946?conditions?during?drought?years.?Similarly?to?normal?conditions?
(Figure?3?8),?small?floods?and?high?flows?start?occurring?outside?the?monsoon?season.?Notice?that?
small?floods?used?to?even?during?drought?years.?
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
T>?10?years
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Small?Floods
5<T<10
??56???
?
Figure?4?12:?Annual?Hydrograph,?Return?Period?>10?years,?pre?1946?conditions?
?
4.4.2. AFTER?1946?
?
Figure?4?13?shows?the?annual?hydrograph?during?drought?years?for?flows?with?one?year?return?
period?in?the?post?1946?conditions.?Error!?Reference?source?not?found.?shows?the?volume?for?
each?month?according?to?the?1?year?return?period?hydrograph;?the?total?volume?in?the?hydrograph?
is?very?small?(390X10
6
?m
3
/year)?compared?to?the?pre?1946?conditions?(954X10
6
?m
3
/year).?Every?
year?only?4?high?flows?occur,?similarly?as?what?happens?during?normal?conditions.?Notice?that?no?
small?flood?occurs?during?drought?years?in?the?post?1946?conditions.?The?absence?of?this?small?flood?
might?be?one?of?the?causes?for?the?reduction?in?channel?width?in?the?post?1946?period.?
?
0
100
200
300
400
500
600
700
Flow
?
(m
3
/s)
T=1?year
5<T<10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Small?Floods
T>10
High?Flows
T>10
??57???
?
Figure?4?13:?Annual?Hydrograph?during?drought?years,?Return?Period?=?1?year,?post?1946?conditions?
?
Table?4?15:?Annual?hydrograph?volume?for?T?=?1?year?during?drought?years,?post?1946?
?? Volume?(1X10
6
?m
3
)?
?? Jan? Feb? Mar? Apr May Jun Jul Aug Sep? Oct? Nov? Dec? Sum
Median? 21? 15? 12? 8? 10? 24? 27 31? 22? 34? 24? 21? 251?
High?Flows? ???? ???? ???? ???? 11? 7? 7? 7? ???? ???? ???? ???? 31?
Small?Floods? ???? ???? ???? ???? ???? ???? ???? ???? 108 ???? ???? ???? 108?
Total? 21? 15? 12? 8? 21? 31? 34 38? 130 34? 24? 21? 390?
?
Figure?4?14Figure?3?5?shows?the?annual?hydrograph?during?drought?years?for?flows?with?one?
and?two?to?three?year?return?period?in?the?post?1946?conditions.?Results?show?that?more?high?flows?
occur?during?drought?years?but?no?small?flood?is?happening?in?this?return?period.??
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
T?1
??58???
?
?
Figure?4?14:?Annual?hydrograph?during?drought?years,?Return?Period?=?2?3?years,?pre?1946?
?
Figure?4?15Figure?3?5?shows?the?annual?hydrograph?during?drought?years?for?flows?with?one?
and?five?to?ten?year?return?period?in?the?post?1946?conditions.?Up?to?this?point,?one?small?flood?
occurs?in?June?with?a?peak?flow?of?310?m
3
/s?and?a?return?period?of?4?years.?This?result?is?very?
alarming;?basically?it?is?necessary?to?wait?3?year?for?a?small?flood?under?normal?conditions?and?up?to?
4?years?under?drought?conditions?in?the?post?1946?period.??
?
?
?
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
2<T<3
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
2<T<3
??59???
?
?
Figure?4?15:?Annual?hydrograph?during?drought?years,?Return?Period?=?5?10?years,?pre?1946?
?
Figure?4?16Figure?3?5?shows?the?annual?hydrograph?during?drought?years?for?flows?with?one?
and?ten?year?return?period?in?the?post?1946?conditions.?This?Figure?show?that?eventually,?small?
floods?will?occur?in?their?natural?pattern,?in?August,?September?and?October.?Floods?that?used?to?
happen? every? year? (T=1? year)? or? once? every? 2?3? years? in? the? pre?1946? conditions,? now? are?
happening?every?ten?years?or?more,?during?drought?years.?
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
T=1?year
5<T<10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
5<T<10
Small?Floods
5<T<10
High?Flows
5<T<10
??60???
?
Figure?4?16:?Annual?hydrograph?during?drought?years,?Return?Period?>10?years,?pre?1946?
?
4.4.3. COMPARISON?
?
Figure?4?17?shows?the?annual?hydrograph?during?drought?years?for?flows?with?one?year?return?
period?for?the?pre?1946?and?post?1946?conditions.??
Even?though?the?analysis?is?done?during?drought?years,?conclusions?are?the?same?as?during?
normal?conditions,?there?is?the?same?number?of?high?flows?but?smaller?magnitude?and?different?
timing.?Most?importantly,?one?small?flood?with?a?median?peak?of?380?m
3
/s?occurred?every?year?in?
the?pre?1946?conditions?while?in?the?post?1946?condition?it?does?not?occur.?Also,?the?distribution?of?
the?medians?flows?is?altered;?meanwhile?in?the?pre?1946?September?has?the?largest?median?flow?
(45?m
3
/s),?in?the?post?1946?October?has?the?largest?median?flow?(13?m
3
/s).?
?
?
0
100
200
300
400
500
600
700
Flow
?
(m
3
/s)
T=1?year
T>10
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
High?Flows
T>10
Small?Floods
T>10
??61???
?
?
Figure?4?17:?Annual?hydrograph?during?drought?years,?T?=?1?year,?post?and?pre?1946?
?
Figure?4?18?shows?annual?hydrograph?during?drought?years?for?flows?with?one?year?return?
period?for?the?pre?1946?and?ten?years?or?more?return?period?for?the?post?1946?conditions.?The?
hydrograph?for?the?pre?1946?condition?considers?that?two?small?floods?happened?every?two?three?
years,?the?one?fixed?in?September,?and?another?occurred?in?October.?For?the?post?1946?hydrograph,?
it? is? considered? two? small? floods? occurred? that? year,? in? September? and? October.?High?flows?in?
February,?May,?June,?July?and?August?occur?in?both?hydrographs.?Figure?4?18?demonstrate?how?high?
flows?and?small?floods?that?occurred?during?drought?years?every?two?three?years?in?the?pre?1946?
conditions,?they?are?occurring?every?ten?years?or?more?in?the?post?1946?conditions;?small?floods?
barely?occur?in?the?post?1946?conditions?during?drought?years.?
?
?
?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
Pre?1946?(T=1)
Post?1946?(T=1)
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Same?number?of?high?flows,
different?timing
One?Small?Flood
every?year
??62???
?
Figure?4?18:?Comparison?of?annual?hydrograph?during?drought?years,?Pre?1946?T=1?year?Vs.?Post??
1946?T>10?years?
?
4.5. CONCLUSIONS?
?
From?the?previous?analysis?of?the?drought?years,?it?can?be?concluded?that?when?comparing?pre?
1946?to?post?1980:?
? Small? floods? do? occur? during? drought? years? in? the? pre?1946? conditions,? they? were? a?
characteristic?flow?event?during?drought?years.?In?the?pre?1946?conditions,?the?fixed?small?
flood?of?September?(Flow?peak?=?380?m
3
/s)?still?occurs?during?drought?years?preventing?
from?agradation?and?channel?reduction.?
? Median? monthly? flows? decreased? significantly,? by? 531X10
6?
m
3
,? or? 68%? of? the? pre?1946?
annual?volume.?In?addition,?the?shape?of?the?median?values?is?altered.?During?drought?years?
the?pre?1946?median?values?follow?the?normal?condition?pattern;?in?contrast.?the?post?1946?
the?highest?median?values?has?shifted?from?September?to?October.?
0
100
200
300
400
500
600
700
Flow
?
(m
3
/s)
Pre?1946?(2<T<3)
Post?1946?(T>10)
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul??????????Aug?????????Sep????????Oct??????????Nov???????Dec
Small?Floods
High?Flows
Same?number?of?high?flows,
different?magnitude
Small?flood?frequency:
Pre?1946?once?every?year
Post?1946?once?every?10?years
??63???
? During? drought? years,? small? high? flow? pulse? occurred? more? frequently?in?the?pre?1946?
conditions,?a?median?of?8?small?high?flow?pulse?per?year,?compare?to?6?events?per?year?in?the?
post?1946?conditions.??
? During?drought?years,?large?high?flow?pulse?were?not?common?in?the?pre?1946?conditions,?it?
only?happened?once?in?the?pre?1946?period,?meanwhile?large?flow?pulses?happened?once?
every?two?years?in?the?post?1946?conditions.?
? During?drought?years,?small?floods?occurred?more?frequently?in?the?pre?1946?conditions,?a?
median?of?2?events?per?year,?meanwhile?in?the?post?1946?conditions?the?median?dropped?to?
0?events?per?year.?
?
??
??64???
5. CONCLUSIONS?
?
5.1. EXTREME?FLOOD?EVENTS?
?
From?analysis?of?extreme?flood?events,?it?can?be?concluded?that:?
? The?characteristics?of?extreme?flood?events?that?re?widen?the?channel?are:?a)?a?peak?flow?
larger?than?1,400?m
3
/s?and?b)?a?total?flow?of?1,000X10
6?
m
3
?for?the?whole?flooding?event.?
? For?the?same?storm?frequency?(return?period),?the?magnitude?of?the?flow?has?decreased?by?
50%?
? Return?periods?for?extreme?events?have?doubled?
? Pre?1946?:?one?event?every?7~8?years?
? Post?1946:?one?event?every?15~16?years?
? Flooding?events?occur?on?average?two?to?three?weeks?later?in?the?post?1946?conditions,?
from?October?1?7
th
?in?the?pre?1946?to?October?21?28
th
?in?the?post?1946.?
? Pre?1946?events?have?greater?variation;?durations?range?from?8?to?30?days?and?peak?flows?
from?1560?to?3851?m
3
/s?
? Post?1946?events?are?more?uniform;?durations?are?typically?3?weeks?and?peak?flows?range?
from?1030?to?1850?m
3
/s?
?
5.2. NORMAL?HYDROLOGIC?CONDITIONS?
?
From?analysis?of?mean?daily?flow?at?Johnson?Ranch?gage?station?in?both?periods?of?analysis,?it?
can?be?concluded?that:??
? The?volume?of?the?median?monthly?flow?for?the?pre?1946?and?post?1946?are?1130X10
6?
m
3
/year?and?464X10
6?
m
3
/year,?respectively?
? The?annual?deficit?is?666X10
6?
m
3
?of?water,?or?59%?of?the?pre?1946?quantity.?The?majority?of?
the?deficit?comes?from?the?high?flow?months?of?August,?September,?and?October?
? Pre?1946?conditions?have?more?frequent?small?floods?(median?peak?flow?=?400?m3/s),?one?
fixed?every?year?in?September,?and?at?least?one?small?flood?in?one?of?the?following?months:?
July,?August?and?October.?
??65???
? High?flows?and?small?floods?that?occurred?every?year?in?the?pre?1946?condition,?they?
happened?every?2?3?years?in?the?post?1946?conditions.??
? Similarly,?high?flows?and?small?flows?that?occurred?every?2?3?years?in?the?pre?1946?
condition,?they?happened?every?5?years?in?the?post?1946?conditions.?
?Pre?1946?
? A?small?flood?occurs?every?year?(T<1.5)?in?September?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?small?floods?occur?in?July,?August,?and?October?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?small?floods?occur?in?May,?June,?and?November??
?Post?1980?
? There?are?no?expected?annual?(T<1.5?years)?small?floods???
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?a?small?flood?occurs?in?September?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?small?floods?occur?in?June,?July,?August,?and?October?
?
5.3. DROUGHT?PERIODS?
?
From?analysis?of?mean?daily?flow?during?drought?years?at?Johnson?Ranch?gage?station?in?both?
periods?of?analysis,?it?can?be?concluded?that:??
? The?volume?of?the?median?monthly?flow?during?drought?years?for?the?pre?1946?and?post?
1946?are?782X10
6?
m
3
/year?and?251X10
6?
m
3
/year,?respectively.?The?annual?deficit?is?
531X10
6?
m
3
?of?water,?or?68%?of?the?pre?1946?quantity.?In?addition,?the?shape?of?the?median?
values?is?altered;?in?the?post?1946?the?highest?median?value?has?shifted?from?September?to?
October.?
? Small?floods?do?occur?during?drought?years?in?the?pre?1946?conditions,?the?fixed?small?flood?
of?September?(Flow?peak?=?380?m
3
/s)?still?occurs?during?drought?years.?Small?floods?
occurred?more?frequently?in?the?pre?1946?conditions,?a?median?of?2?events?per?year,?
meanwhile?in?the?post?1946?conditions?the?median?dropped?to?0?events?per?year.?
Pre?1946?
? A?small?flood?occurs?every?year?(T<1.5)?in?September?
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?small?floods?occur?in?October?
? Every?5?to?10?years?(3.5?years?<?T?<?10)?small?floods?occur?in?June,?July?and?August?
??66???
Post?1980?
? There?are?no?expected?annual?(T<1.5?years)?small?floods???
? Every?2?to?3?years?(1.5?years?<?T?<?3.5?years)?a?small?flood?occurs?in?June?
? Every?10?years?or?more?(T?>?10?years)?small?floods?occur?in?August,?September?and?October?
?
?
?
??
??67???
6. PRESCRIBED?ANNUAL?HYDROGRAPH?
?
After? the? extensive? analysis? done? at? Johnson? Ranch? gage? station,? these? are? the? essential?
characteristics?for?a?prescribed?hydrograph:?
? Median?monthly?flows? with?a?distribution?similar?to?the?pre?1946?period.?These? values?
might?be?scaled,?as?it?happens?during?drought?years;?however,?the?distribution?must?be?
similar?to?the?one?found?in?the?pre?1946.?
? Two? small? floods.? One? small? flood? fixed? in? September? and? another? small? flood? in? July,?
August,?or?October.?The?peak?flow?of?these?floods?must?be?of?400?m
3
/s.?
? Four? high? flow? pulses;? one? in? each? of? the? following? months:? July,?August,?October?and?
December.?
In?order?to?propose?an?annual?volume?for?the?proposed?hydrograph,?first,?it?is?necessary?to?
analyze?the?annual?volumes?in?the?Rio?Grande/Bravo?at?Johnson?Ranch?and?in?the?Rio?Conchos?at?
Ojinaga.??
Figure?6?1?shows?the?annual?flow?volume?at?Johnson?Ranch?and?Rio?Conchos?at?Ojinaga.?Values?
for?the?Rio?Conchos?start?in?1955?because?data?is?available?since?that?year.?Notice?that?the?Rio?
Conchos?contributes?with?a?significant?amount?of?water?at?Johnson?Ranch.?A?median?value?78%?of?
the?flow?at?Johnson?Ranch?comes?from?the?Rio?Conchos.?Table?6?1?shows?the?annual?statistics?for?
both?sites.?
?
Figure?6?1:?Annual?flow?of?Rio?Grande/Bravo?at?Johnson?Ranch?and?Rio?Conchos?at?Ojinaga?
?
1946.5 1956.5 1966.5 1976.5 1986.5 1996.5 2006.5
0
500
1000
1500
2000
2500
3000
3500
1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
Annual
?
Flow
?
Volume
?
(1X10
6
m
3
) Johnson?Ranch
Rio?Conchos
Median
75
th
Percentile
25
th
Percentile
??68???
Table?6?1:?Annual?hydrograph?volume?during?drought?years?for?a?1?year?return?period,?post?1946?
?? ?? Percentile??(1X10
6
?m
3
/s)? ?? Average? Std.?Dev.
?? Period? 0.75? 0.5?(Median)? 0.25? ?? (1X10
6
?m
3
/s)? (1X10
6
?m
3
/s)?
Johnson?Ranch? 1947?2009? 1121? 744? 501? ?? 954? 682
Rio?Conchos? 1955?2009? 962 650? 396 749 574
?
Figure?6?1?shows?several?hydrologic?periods,?the?extended?drought?of?the?50?s?(1947?to?1957),?
the?short?drought?of?the?60?s?(1961?to?1965),?the?wet?period?of?the?70?s?(1968?to?1981),?80?s?and?
early?90?s?(1984?to?1993),?and?the?severe?and?extended?drought?of?the?09?s?(1994?to?2007).?The?
median?annual?flow?at?Johnson?Ranch?is?744X10
6
?m
3
.?Given?that?half?of?the?years?the?annual?volume?
is?larger?than?the?median,?an?annual?volume?of?1,000X10
6
?m
3
?is?proposed?as?an?initial?value.?
For? the? pre?1946? hydrographs? shown?in?the?Executive?Summary?section? (Figure? ES?2? and?
Figure?ES?4),?approximately?75%?of?the?annual?volume?is?used?to?supply?the?median?monthly?flows,?
and?25%?to?supply?high?flows?and?small?floods.?Table?6?2?show?the?distribution?median?monthly?
flows?of?the?pre?1946?period??
Table?6?2:?Median?monthly?flow?distribution,?prescribed?hydrograph?
?? Median?monthly?flow?distribution??(%)?
?? Jan? Feb? Mar? Apr? May? Jun? Jul? Aug? Sep? Oct? Nov? Dec? Sum?
Median? 6? 5? 5? 3? 5? 6? 10? 12? 22? 12? 7? 6? 100?
?
Table?6?3?show?the?daily?flows?for?the?small?floods?considered:?a)?one?in?September?with?
duration?of?11?days?and?a?peak?of?407?m
3
/s;?and?b)?another?flood?that?varies?in?time,?with?duration?
of?9?days?and?a?peak?flow?of?405?m
3
/s.?These?small?floods?have?been?extracted?from?the?pre?1946?
period.?
Table?6?3:?Small?floods,?prescribed?hydrograph?
Day?
September Variable
(m
3
/s) (m
3
/s)
1 154 190
2 189 223
3 203 273
4 255 320
5 322 405
6 407 320
7 322 273
8 255 223
9 203 190
10 189 ???
11 154 ???
?
??69???
Table?6?4?show?the?daily?flows?for?the?high?flows?considered?in? July,? August,? October? and?
December.?Only?this?4?high?flows?will?be?considered?initially,?the?rest?of?the?high?floods?are?expected?
to?be?meet?by?the?system.?
Table?6?4:?Small?floods,?prescribed?hydrograph?
Day?
July August October December
(m
3
/s) (m
3
/s) (m
3
/s) (m
3
/s)
1? 75 57 58 36
2? ??? 102 97 45
3? ??? 57 73 36
4? ??? ??? 58 ???
5? ??? ??? ??? ???
6? ??? ??? ??? ???
?
Figure?6?2?show?the?proposed?hydrograph?for?the?Rio?Grande/Bravo?at?Johnson?Ranch.?Table?
6?5?shows?the?volume?of?water?to?supply?the?median?monthly?flow,?high?flows?and?small?floods.?
Table?6?6?shows?the?median?monthly?flows?of?the?hydrograph?proposed.?
?
Figure?6?2:?Annual?hydrograph?proposed?in?the?Rio?Grande/Bravo?at?Johnson?Ranch?
0
50
100
150
200
250
300
350
400
450
500
Flow
?
(m
3
/s)
Fixed?Hydrograph
Flexible?Small?Flood
Small?Floods
High?Flows
Jan?????????Feb Mar????????Apr?????????May???????Jun??????????Jul?????????Aug????????Sep??????Oct???????????Nov????????Dec
Small?Floods:
September
Flexible (Jul,?Sug?or?Oct)
High?Flows
High?Flows
56?m
3
/s
224?m
3
/s
14
13 12
6
10 14
23
26
51
26
16
14
??70???
Table?6?5:?Monthly?volume,?prescribed?hydrograph?
?? Volume?(1X106?m3)?
?? Jan? Feb? Mar? Apr May? Jun? Jul? Aug? Sep? Oct Nov? Dec? Sum?
Median? 37? 31? 32? 15? 28? 36? 62? 69? 132 71? 40? 37? 589?
High?Flows? ???? ???? ???? ???? ???? ???? 5? 12? ???? 16? ???? 7? 39?
Small?Floods? ???? ???? ???? ???? ???? ???? 191 ???? 181 ???? ???? ???? 372?
Total? 37? 31? 32? 15? 28? 36? 257 81? 313 86? 40? 43? 1000?
?
Table?6?6:?Median?monthly?flows,?prescribed?hydrograph?
?? Median?Monthly?
?? Flows?(m
3
/s)?
Jan 14
Feb 13
Mar 12
Apr 6
May 10
Jun 14
Jul 23
Aug 26
Sep 51
Oct 26
Nov 15
Dec 14
?
?
??
??71???
REFERENCES?
?
Dean,?D.J.,?Schmidt,?J.C.,?(2010),??The?role?of?feedback?mechanisms?in?historic?channel?changes?of?the?????????
lower?Rio?Grande?in?the?Big?Bend?region??Journal?of??Geomorphology??
IBWC???International?Boundary?and?Water?Commission?(1906).??Convention?between?the?United?
States?and?Mexico.?Equitable?distribution?of?the?Waters?of?the?Rio?Grande??
http://www.ibwc.state.gov/Files/1906Conv.pdf??
IBWC??International?Boundary?and?Water?Commission?(1944).??Treaty?between?the?Unites?States?of?
America?and?Mexico.??www.ibwc.state.gov/Files/1944Treaty.pdf?
IBWC???International?Boundary?and?Water?Commission?(2009).??Rio?Grande?Historical?Mean?Daily?
Discharge?Data.??<http://www.ibwc.state.gov/Water_Data/histflo1.htm>?(Dec.?31,?2009).?
Postel,?A.?and?Richter,?B.?(2003).???Rivers?for?Life.?Managing?water?for?people?and?nature.??Island?
Press?
Sandoval?Solis,?S.?(2009)??Water?Planning?and?Management?for?Large?Scale?River?Basin.?Case?of?
Study:?the?Rio?Grande/Bravo?Transboundary?Basin?.?Dissertation?Proposal.?The?University?
of?Texas?at?Austin.?
Texas?Commission?on?Environmental?Quality?(TCEQ)?(1939).?Rio?Grande?Compact.?Water?Code,?
Title?3,?Chapter?41.?http://tlo2.tlc.state.tx.us/statutes/docs/WA/content/pdf/wa.003.00.?
000041.00.pdf??
Teasley,?R.?L.?and?D.?C.?McKinney.??(2005).??Modeling?the?Forgotten?River?Segment?of?the?Rio?
Grande/Bravo?Basin.???CRWR?Online?Report?05?12,?Center?for?Research?in?Water?Resources,?
University?of?Texas?at?Austin?<http://www.crwr.utexas.edu/reports/pdf/2005/rtp05-12.pdf>?(Jun?26,?
2006).?
USGS???United?States?Geological?Survey?(1996).?Techniques?to?estimate?generalized?skew?
coefficients?of?annual?peak?streamflow?for?natural?basins?in?Texas.?Water?Resources?
Investigations?Report?96?4117.??
?
?
?
??72???
Appendix?A. RIO?GRANDE/BRAVO?AT?JOHNSON?RANCH?STREAMFLOW?
?
A?simple?linear?regression?has?been?used?to?estimate?the?mean?daily?flow?at?Johnson?Ranch?from?
May?1
st
?1900?to?March?31
st
?1913?and?from?January?1
st
?1931?to?December?31
st
?1935.?The?software?
HEC?DSSVue?was?used?to?calculate?the?intercept?B
0
?and?the?slope?B
1
?used?in?the?linear?regression?
equation.?Records?from?the?gage?station?Rio?Grande?Below?Ojinaga?are?used?to?estimate?the?flow?at?
Johnson?Ranch?for?every?day?t.??
nullnullnullnullnullnullnull
null
nullnull
null
nullnull
null
null nullnullnullnullnullnull?nullnullnullnullnull?nullnullnullnullnullnullnull
null
?
Where:?
B
0?
=?0.57597?
B
1?
=?0.91238?
?
Due?to?space?limitations,?raw?data?is?excluded?from?the?report.??Data?for?Rio?Grande/Bravo?at?
Johnson?Ranch?can?be?found?on?the?IBWC?website?at?http://www.ibwc.gov/wad/DDQJOHNS.htm.????
?
?