Ground Recharge in the Central High Plains of Texas: Roberts and Hemphill Counties

Abstract

Reliable estimates of recharge are important for assessing and managing groundwater resources, especially in regions like the High Plains aquifer of Texas where declining groundwater levels due to large-scale pumping make recharge estimation even more critical.

The purpose of this study was to estimate groundwater recharge in the vicinity of Roberts County. Three basic approaches were used to estimate recharge:

  1. Chloride mass balance in groundwater,
  2. Chloride mass balance in the unsaturated zone, and
  3. Numerical modeling of recharge in the unsaturated zone.

Groundwater chloride concentrations were used to evaluate regional recharge rates based on the chloride mass balance approach in Roberts County. Additionally, a limited number of groundwater well samples were analyzed for tritium-helium ages to supplement the regional groundwater chloride mass balance analysis.

The chloride mass balance approach was also applied to the unsaturated zone to provide point recharge estimates in different land use settings. A total of 19 boreholes were drilled from 2006 through 2008 in different locations (14 in Roberts and 5 in Hemphill counties) to depths ranging from 18.5 to 88 ft (5.6 to 26.8 m). Natural rangeland represents the dominant land use in these counties, and nine boreholes were located in this setting. Two boreholes were situated beneath dryland agriculture, and three boreholes beneath irrigated agriculture. One borehole was drilled in a dry drainage channel, and four boreholes were drilled adjacent to stock impoundments that pond water in Roberts County.

Soil samples were collected in the field for laboratory measurement of soil physics (water content and matric potential head) and environmental tracers (chloride, fluoride, nitrate, and sulfate). Groundwater recharge was estimated using the chloride mass balance or chloride front displacement approach. Groundwater recharge in Roberts County was also estimated using unsaturated zone modeling based on meteorological data from 1961 through 1990, representative online soils data from SSURGO, and representative vegetation types. Sensitivity analyses were conducted to estimate maximum recharge based on bare sand and to evaluate soil texture and vegetation controls on recharge.

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