Technical Reports Document Library

Note:  This document is Part 1 of 3 parts.

The purpose of this study was to develop a new method for estimating recharge to the Edwards Aquifer.  The computer program Hydrologic Simulation Program – Fortran (HSPF) was used to develop a comprehensive recharge estimation model for the region.  A previously completed pilot study project modeled portions of two of the nine basins applicable to the Edwards Aquifer.  The pilot study indicated HSPF as a viable tool for recharge estimation and led to the initiation of this project.  This project developed recharge models for all nine basins relevant to the Edwards Aquifer, by modeling the upstream (drainage) area, as well as the recharge zone of the aquifer.  The project was completed in March 2005 by LBG Guyton Associates, Inc., Austin, Texas.

[From the Executive Summary]
Hydrologic models have been developed for the nine basins that recharge the San Antonio section of the Edwards Aquifer and recharge estimates from these models have been implemented into recharge data for the Edwards Aquifer MODFLOW model. The models have been calibrated for the basins of the (1) Nueces/West Nueces Rivers, (2) Frio/Dry Frio Rivers, (3) Sabinal River, (4) area between Sabinal and Medina River (Seco and Hondo Creeks), (5) Medina River, (6) area between Medina and Cibolo (San Geronimo, Helotes, and Salado Creeks), (7) Cibolo/Dry Comal Creeks, (8) Guadalupe River, and (9) Blanco River.

The main objective of the project was to develop hydrologic simulation models for the entire contributing and recharge zone of the Edwards Aquifer with the main focus being to estimate historical recharge to the Edwards Aquifer from 1950 through 2000. In addition to estimating

recharge, secondary objectives of the project were to ensure that the models were compatible with the future goals of assessing issues important to the Authority and other stakeholders, including water quality, land use changes, weather modification, brush control, recharge dams, and flood control. To achieve these goals, the entire contributing zone (above the Edwards aquifer recharge zone) of each of the nine basins was incorporated into the models….

The HSPF models are valuable tools for simulating the hydrology of the nine basins and assessing recharge under various hydrologic conditions. The models provide a new tool for assessing hourly hydrology, which could not be done with the traditional methods. They simulate each basin more discretely than the traditional methods do; which makes the models more appropriate for evaluating recharge enhancement, water quality and many other technical and regulatory issues.

To improve the accuracy of the models, more detailed field studies should be completed in selected areas of the recharge zone. These assessments should include field-scale rainfall-runoff and evaporation studies, tracer studies, channel loss studies, and other evaluations. Suggested model improvements include incorporating more detailed precipitation data from Authority rain gages that has been collected since this study was initiated; estimating model parameters by developing models for smaller watersheds; and enhancing the existing models to simulate water quality.