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U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4101
Version 1.20, August 2004

Estimating Ground-Water Recharge from Precipitation on Whidbey and Camano Islands, Island County, Washington, Water Years 1998 and 1999

Prepared in cooperation with
ISLAND COUNTY HEALTH DEPARTMENT

By S. S. Sumioka and H. H. Bauer

ABSTRACT

Ground-water recharge from precipitation to unconsolidated deposits on Whidbey and Camano Islands, Washington, was estimated for water years 1998-99 using a near-surface water-balance method and a chloride mass-balance method.


A daily near-surface water-balance method, the Deep Percolation Model (DPM), was used to simulate water budgets for October 1, 1997 through September 30, 1999 (water years 1998-99) for six small drainage basins—four on Whidbey Island and two on Camano Island. Adjusted parameters from the DPM for each small basin were then used in island-wide DPM simulations. A spatial distribution of annual recharge was simulated for each island, with island averages of 5.71 inches per year for Whidbey Island and 5.98 inches per year for Camano Island. The spatial distribution of simulated annual recharge for each island reflects variations in precipitation amounts and the distribution of surficial materials. DPM results indicate that recharge generally is higher in areas underlain by coarse-grained deposits (outwash) than in areas underlain by fine-grained deposits (till).


A chloride mass-balance method was used to estimate combined recharge to unconsolidated deposits on Whidbey and Camano Islands. The average combined recharge for Whidbey and Camano Islands estimated by this method was 2.00 inches per year. The range of chloride concentrations in ground-water samples from selected wells indicates that the average recharge to unconsolidated deposits ranges from 0.78 to 7.81 inches per year. Sources of chloride in ground water other than from the atmosphere would cause recharge estimated by the chloride mass-balance method to be less than the actual recharge, therefore, these estimates may represent lower limits
.

CONTENTS

Abstract
Introduction
Description of Study Area
Estimates of Ground-Water Recharge
Summary
References Cited
Appendix A -- Monthly computer water-budget summaries for the six study basins
Appendix B -- Monthly computer water-budget summaries for Whidbey and Camano Islands

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Send questions or comments about this report to the author, Steve Sumioka (253) 428-3600 ext. 2645.

For more information about USGS activities in Washington, visit the USGS Washington District home page.

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