Estimating Potential Effects on Streamflow from Nearby Wells in the Lower San Antonio River Basin

Science Center Objects

The USGS Texas Water Science Center (TXWSC) developed a groundwater-flow model to estimate the potential for streamflow depletion in the Lower San Antonio River based on changes in groundwater pumping in the watershed. Results from the project can be used by the San Antonio River Authority (SARA) to better understand aquifer interactions with the river and potential groundwater pumping effects on future streamflows.

TXWSC is investigating streamflow depletion potential for the lower San Antonio River basin due to low rainfall and increased pumping conditions within subsurface depths of municipal-well withdrawals. A transient groundwater model is being developed and calibrated using MODFLOW NWT and the Streamflow-Routing (SFR) Package. Two scenarios with lower quartile precipitation (during the calibration period of 2006-2013) and a basin-wide increase of 1% pumping and 25% pumping are being investigated.

The two specific predictive model simulations are being designed and run to investigate streamflow depletion over time under different pumping conditions in order to identify river reaches that are particularly vulnerable to depletion from groundwater pumping. The model includes reaches within the Lower San Antonio River basin which traverse the Edwards-Trinity, Texas Coastal Uplands, and Texas Coastal Lowlands aquifer systems from the basin headwaters until the eventual convergence of the lower San Antonio River with the Guadalupe River. This model simulates groundwater flow within the Texas Coastal Uplands and Texas Coastal Lowlands aquifer systems in Wilson, Karnes and Goliad Counties for the time period of 2006-2013.

The model is focused on groundwater-flow within the domestic well and municipal-well supply depths within the subsurface of these multiple aquifer systems. These different aquifer systems dip towards the Gulf of Mexico and outcrop in segments that are nearly parallel to the present day coastline. History-matching of the model through estimation of best-fit parameter values of the model is being supplemented with linear and non-linear uncertainty analysis. Streamflow capture response maps are also being created for the two predictive scenarios (one map for each scenario) to quantify spatial variations in streamflow depletion response within the basin.