Identification of Additional Recharge During High Flow Events

Edwards Aquifer, Uvalde County, Texas

(by Allan Standen, Daniel B. Stephens and Associates)

The Edwards Aquifer is the major water supply source for the City of San Antonio and many counties in Central Texas.  The Aquifer is locally strongly karstified and a major fault system (Balcones Fault) has impacted the karstification of the Cretaceous age limestones.  This system is referred to as the Edwards Balcones Fault Zone (BFZ) Aquifer.  Quantification of the volume of recharge to this system is critical to management of the water resource.

The Frio River system consists of the Frio and Dry Frio Rivers.  The USGS has conducted gain-loss studies during low to moderate flow conditions along these two rivers and has demonstrated that these stream reaches are losing across the Edwards BFZ because of the abundant karst features within the drainages.  These losses are considered recharge (short distance, minimal ET, minimal bank storage) in groundwater models. 

Each river has a stream gage above the Edwards BFZ Aquifer and they share a stream gage below the confluence of the Frio and Dry Frio rivers in Uvalde.  Approximately 88% of the stream flow measurements in Uvalde are zero, suggesting that most flows through the upper two stream gages are lost to recharge.

This study reviews 50 years of daily stream flows and precipitation station data (3 sites) and approximately 25 years worth of daily groundwater levels from wells adjacent to the rivers.  The goal was to identify additional recharge by determining maximum flow through the upper gages resulting in zero flow in the gage below the recharge zone.  This potentially represents the maximum recharge rate along that stream reach.  Previous precipitation frequency three to six months prior to the runoff events was also considered.