Linking Biologic Metrics to Hydrologic Characteristics in Austin, Texas Streams

Urbanization can alter the hydrology of creeks as land use changes as represented by increases in impervious cover. Flow regime change is modeled directly on the basis of impervious cover in the City of Austin; however, it would be helpful for planning purposes if other aspects of the environmental integrity of urban creeks could be modeled on the same basis. Benthic macroinvertebrates and diatoms are used as biological indicators of creek health, but an adequate direct relationship between the impervious cover and these biological metrics has not been found. Relationships between flow and biological health were investigated in order to form an indirect link so that changes in biological health could be predicted by changes to impervious cover. Pearson correlations and step-wise regression was performed on 35 hydrologic metrics and 25 biological metrics using sub-daily and daily flow data from the US Geologic Survey in 14 creeks in Austin, Texas. Data were grouped by impervious cover period or water year. The sub-daily flow data grouped by impervious cover period created the best model using mean--area (avg neg flowrate changes/watershed area),FHn (avg times mean flow > 75%-tile), and TQmean (fraction of time daily mean flow > mean for period) as the predictive hydrologic metrics to describe biological health of the creeks. Correlations were strongest between hydrology and the sensitive taxa biologic metrics. As the hydrology becomes flashy, one result of an increase in impervious cover, the sensitive taxa decrease at a site thereby decreasing the overall biological health of a site. It is recommended that this indirect link be used in water quality improvement projects to predict how the biological health of a creek in Austin will be affected by changes in land use.