Relationship of Flood Peaks to 24-Hour Rainfall in the eastern Texas Hill Country, South-Central Texas

The eastern Texas Hill Country is noted for its extreme storm rainfall amounts and the resulting flood discharges. The resulting flash f loods contribute to Texas leading the U.S. in automobile-related, flood fatalities. These f loods produce considerable groundwater recharge to the Edwards Aquifer. A better knowledge of relations between precipitation and peak flood discharges can improve f lood hazard warning and provide estimates of groundwater recharge from ungauged ephemeral streams. The purpose of this study was to analyze the relationship between 24-hour precipitation amounts and peak discharge for four ephemeral streams, with drainage areas between 0.3 to 18.8 mi.2 on the Texas State University Freeman Ranch located approximately 6 miles west of downtown San Marcos, TX. We used the USGS Slope-area method (Dalrymple and Benson 1967) to calculate peak discharge and on-site as well as nearby and NWS, CoCoRaHS precipitation data. Reference unit runoff and flood volume runoff were determined from nearby USGS gauges for similar sized drainages. A precipitation minimum of 4 in in 24 hrs. was required to produce quickflow, a value that is affected by antecedent precipitation in the previous 1 to 3 days. Approximately 370 ac-ft of runoff would be produced by each 1,000 ft3 /sec peak stream flow with approximately fifteen percent becoming groundwater recharge in basins without dams, and a majority of which becomes recharge on streams with flood detention dams.