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Statistical Analyses of Hydrologic System Components and Simulation of Edwards Aquifer Water-Level Response to Rainfall Using Transfer-Function Models, San Antonio Region, Texas

By Lisa D. Miller and Andrew J. Long

U.S. Geological Survey
Scientific Investigations Report 20065131

In cooperation with the San Antonio Water System


Abstract

In 2003 the U.S. Geological Survey, in cooperation with the San Antonio Water System, did a study using historical data to statistically analyze hydrologic system components in the San Antonio region of Texas and to develop transfer-function models to simulate water levels at selected sites (wells) in the Edwards aquifer on the basis of rainfall. Water levels for two wells in the confined zone in Medina County and one well in the confined zone in Bexar County were highly correlated and showed little or no lag time between water-level responses. Water levels in these wells also were highly correlated with springflow at Comal Springs. Water-level hydrographs for 35 storms showed that an individual well can respond differently to similar amounts of rainfall. Fourteen water-level-recession hydrographs for a Medina County well showed that recession rates were variable. Transfer-function models were developed to simulate water levels at one confined-zone well and two recharge-zone wells in response to rainfall. For the confined-zone well, 50 percent of the simulated water levels are within 10 feet of the measured water levels, and 80 percent of the simulated water levels are within 15 feet of the measured water levels. For one recharge-zone well, 50 percent of the simulated water levels are within 5 feet of the measured water levels, and 90 percent of the simulated water levels are within 14 feet of the measured water levels. For the other recharge-zone well, 50 percent of the simulated water levels are within 14 feet of the measured water levels, and 90 percent of the simulated water levels are within 27 feet of the measured water levels. The transfer-function models showed that (1) the Edwards aquifer in the San Antonio region responds differently to recharge (effective rainfall) at different wells; and (2) multiple flow components are present in the aquifer. If simulated long-term system response results from a change in the hydrologic budget, then water levels would be difficult to simulate accurately.

Table of Contents

Abstract

Introduction

Statistical Analyses of Hydrologic System Components

Correlation Coefficients

Hydrographic Analyses

Linear Regression

Simulation of Water-Level Response to Rainfall Using Transfer-Function Models

Effective Rainfall Estimation

Convolution

Parameter Estimation

Simulated Water Levels at Three Wells

Bexar County Index Well (AY–68–37–203)

Hill Country Village Well (AY–68–29–103)

Medina County FM 1796 Well (TD–69–38–601)

Discussion of Transfer-Function Model Results

Summary

References Cited

Appendix 1—Daily Water-Level Data for Seven Wells Used in Analysis (online version only)

Figures

1.   Map showing Trinity and Edwards aquifers and Edwards aquifer recharge zone, confined zone, and site locations in the San Antonio region, Texas
2.   Diagrammatic north-northwest-to-south-southeast section showing hydrogeologic framework and generalized ground-water-flow directions, Edwards Plateau to Gulf Coastal Plain, San Antonio region, Texas
3–5.   Graphs showing:
  3.   Daily maximum water levels at selected wells and daily mean springflow at Comal Springs relative to daily maximum water levels at Bexar County index well (AY–68–37–203), Edwards aquifer, San Antonio region, Texas, September 1986–December 2002
  4.   Daily maximum water levels at selected wells and daily mean springflow at Comal Springs, Edwards aquifer, San Antonio region, Texas, September 1986–December 2002
  5.   Water-level response to rainfall events for Bexar County index well (AY–68–37–203), Edwards aquifer, San Antonio region, Texas
6.   Map showing flow units of the Edwards aquifer, San Antonio region, Texas, and well catchment areas for Bexar County index well, Hill Country Village well, and Medina County FM 1796 well
7.   Graph showing calibrated transfer-function models for Bexar County index well (AY–68–37–203), Edwards aquifer, San Antonio region, Texas
8.   Hydrograph showing measured and simulated water level for Bexar County index well (AY–68–37–203), Edwards aquifer, San Antonio region, Texas
9.   Graph showing calibrated transfer-function models for Hill Country Village well (AY–68–29–103), Edwards aquifer, San Antonio region, Texas
10.   Hydrograph showing measured and simulated water level for Hill Country Village well (AY–68–29–103), Edwards aquifer, San Antonio region, Texas
11.   Graph showing calibrated transfer-function models for Medina County FM 1796 well (TD–69–38–601), Edwards aquifer, San Antonio region, Texas
12.   Hydrograph showing measured and simulated water level for Medina County FM 1796 well (TD–69–38–601), Edwards aquifer, San Antonio region, Texas

Tables

1.   Site name, site identifier, and period of record for selected well, spring, rainfall, and streamsites in or associated with the Edwards aquifer, San Antonio region, Texas
2.   Spearman's rho correlation coefficients for daily maximum water levels at Bexar County index well (AY–68–37–203) lagged from 0 to 16 days and daily maximum water levels at selected wells and daily mean springflow at Comal Springs, Edwards aquifer, San Antonio region, Texas
3.   Estimated parameters for transfer-function models developed for selected wells in the Edwards aquifer San Antonio region, Texas

This document is available in Portable Document Format (PDF):  (4.12 MB)

Appendix

1–1. Bexar County Index pdf file (490 KB)
1–2. Hill Country Village pdf file (577 KB)
1–3. City of Castroville pdf file (604 KB)
1–4. Medina County FM 1796 pdf file (536 KB)
1–5. Medina County Index pdf file (224 KB)
1–6. Quihi pdf file (119 KB)
1–7. Uvalde County Index pdf file (593 KB)


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