U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 99-4197

Hydrogeologic Investigations of the Sierra Vista Subwatershed of the Upper San Pedro Basin, Cochise County, Southeast Arizona

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Prepared in cooperation with the
ARIZONA DEPARTMENT OF WATER RESOURCES
and COCHISE COUNTY

By D.R. Pool and Alissa L. Coes

ABSTRACT

The hydrogeologic system in the Sierra Vista subwatershed of the Upper San Pedro Basin in southeastern Arizona was investigated for the purpose of developing a better understanding of stream-aquifer interactions. The San Pedro River is an intermittent stream that supports a narrow corridor of riparian vegetation. Withdrawal of ground water will result in reduced discharge from the basin through reduced base flow and evapotranspiration; however, the rate and location of reduced discharge are uncertain.

The investigation resulted in better definition of distributions of silt and clay in the regional aquifer; changes in seasonal precipitation, runoff, and base flow in the San Pedro River; sources of base flow; and regional water-level changes. Regional ground-water flow is separated into deep-confined and shallow-unconfined systems by silt and clay. Precipitation, runoff, and base flow declined at the Charleston streamflow-gaging station from 1936 through 1997 for the months of June through October. Base flow at the Charleston station during 1996 and 1997 was primarily supplied by ground water recharged near the San Pedro River during recent major runoff and by minor contributions from the regional aquifer. The decline in base flow, about 2 cubic feet per second, has several probable causes including declining runoff and recharge near the river during June through October and increased interception of ground-water flow to the river by wells and phreatophytes. Water levels in wells throughout the regional aquifer generally declined at rates of 0.2 to 0.5 feet per year between 1940 and the mid-1980's, which corresponded with a period of below-average winter precipitation. Water levels in wells in the Fort Huachuca and Sierra Vista areas declined at rates that were faster than regional rates of decline through 1998 and caused diversion of ground-water flow that would have discharged along perennial stream reaches.

CONTENTS

Abstract
Introduction
Methods
Precipitation
Surface water
Ground water
Hydrochemistry
Future data needs
Summary and conclusions
References cited

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