Evaluation of Methods Used to Estimate Recharge in the Edwards Aquifer

The Balcones Fault Zone Edwards Aquifer (Edwards Aquifer) located in south central Texas USA is a prolific source of groundwater for industry and agriculture, and a primary source of water for regional municipalities including the City of San Antonio and surrounding communities.   Modeling in support of resource planning requires reliable estimates of recharge to the aquifer, which occurs over a 280 km long area comprising nine significant watersheds.  For the past 35 years, recharge to the aquifer has been estimated using a relatively simple water-balance equation based on data collected from a network of stream-gage stations and assumptions that runoff characteristics of gaged areas can be applied to ungaged areas.  More recently, process models for each of the nine watersheds were developed using Hydrologic Simulation Program Fortran (HSPF) to more explicitly account for variability of land characteristics, evapotranspiration, and precipitation patterns.  Recharge estimates can also be obtained from the regional MODFLOW groundwater model, in which initial estimates of recharge are calibrated to match water-level observations and achieve water balance with modeled outflows.  This presentation will provide an overview and comparison of the various recharge estimation methods applied to the Edwards Aquifer and highlight challenges and uncertainties involved in using these estimates for regional groundwater management.  Lastly, using results of the HSPF models as “truth” targets, the potential for predicting recharge using calibrated NEXRAD rainfall data as input to a simple lumped-parameter model is evaluated.  The goal of this lumped-parameter model is to develop a viable method to quickly estimate the magnitude of recharge from each contributing watershed for input to groundwater management models without the need to rerun nine HSPF models following each period of significant rainfall.