%STIMATING'ROUND 7ATER2ECHARGEFROM 0RECIPITATIONON7HIDBEYAND#AMANO)SLANDS )SLAND#OUNTY 7ASHINGTON 7ATER9EARSAND 0REPAREDINCOOPERATIONWITHTHE )3,!.$#/5.49(%!,4($%0!24-%.4 53'%/,/')#!,3526%9 7ATER 2ESOURCES)NVESTIGATIONS2EPORT 6ERSION !UGUST Estimating Ground-Water Recharge from Precipitation on Whidbey and Camano Islands, Island County, Washington, Water Years 1998 and 1999 By S. S. Sumioka and H. H. Bauer U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 03-4101 Version Number 1.20, August 2004 Prepared in cooperation with the ISLAND COUNTY HEALTH DEPARTMENT Tacoma, Washington 2003 U.S. DEPARTMENT OF THE INTERIOR GALE A. NORTON, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: Director, Washington Water Science Center U.S. Geological Survey U.S. Geological Survey Information Services 1201 Pacific Avenue – Suite 600 Building 810 Tacoma, Washington 98402 Box 25286, Federal Center http://wa.water.usgs.gov Denver, CO 80225-0286 CONTENTS Abstract ................................................................................................................................................................ 1 Introduction ......................................................................................................................................................... 1 Purpose and Scope ...................................................................................................................................... 3 Previous Studies.......................................................................................................................................... 3 Well-Numbering System............................................................................................................................. 3 Acknowledgments....................................................................................................................................... 3 Description of Study Area.................................................................................................................................... 5 Description of Study Basins........................................................................................................................ 5 Climate and Precipitation............................................................................................................................ 6 Hydrogeology.............................................................................................................................................. 6 Recharge, Evapotranspiration, and Discharge ............................................................................................ 9 Estimates of Ground-Water Recharge.................................................................................................................. 9 Near-Surface Water-Balance Method......................................................................................................... 10 Data Collection and Processing ......................................................................................................... 11 Precipitation .............................................................................................................................. 13 Precipitation Throughfall.......................................................................................................... 13 Streamflow................................................................................................................................ 13 Shortwave Solar Radiation and Temperature ........................................................................... 18 Soil and Subsoil Properties ....................................................................................................... 18 Land Cover and Vegetation ...................................................................................................... 20 Land Surface ............................................................................................................................. 20 Other Data................................................................................................................................. 22 Model Adjustment.............................................................................................................................. 22 Island-Wide Recharge Estimates for Whidbey and Camano Islands Using the Deep Percolation Model........................................................................................................ 23 Sources of Uncertainty in DPM Recharge Estimates ........................................................................ 26 Chloride Mass-Balance Method.................................................................................................................. 27 Data Collection .................................................................................................................................. 28 Recharge Estimates and Sources of Uncertainty Using Chloride Mass-Balance Method................. 29 Summary ......................................................................................................................................................... 31 References Cited .................................................................................................................................................. 32 Appendix A.Monthly computer water-budget summaries for the six study basins............................................. 34 Appendix B. Monthly computer water-budget summaries for Whidbey and Camano Islands............................ 46 Contents iii FIGURES Figure 1. Map showing locations of six study basins on Whidbey and Camano Islands, Island County, Washington................................................................................................................ 2 Figure 2. Diagram showing well-numbering system used in Washington ....................................................... 4 Figure 3. Map showing average annual precipitation on Whidbey and Camano Islands, Island County, Washington................................................................................................................ 7 Figure 4. Map showing generalized surficial geology of Whidbey and Camano Islands, Island County, Washington................................................................................................................ 8 Figure 5. Graphs showing precipitation for the six study basins on Whidbey and Camano Islands, Island County, Washington, water years 1998-99............................................................................. 14 Figure 6. Graphs showing daily mean stream discharges for the six study basins on Whidbey and Camano Islands, Island County, Washington, water years 1998-99........................... 16 Figure 7. Map showing generalized soil groupings of Island County soil series for Whidbey and Camano Islands, Island County, Washington ............................................................................. 19 Figure 8. Map showing distribution of land cover reclassified for the Deep Percolation Model for Whidbey and Camano Islands, Island County, Washington........................................................ 21 Figure 9. Map showing distribution of average annual recharge simulated by the Deep Percolation Model, Island County, Washington ...................................................................... 25 iv Figures TABLES Table 1. Selected basin characteristics of the six study basins on Whidbey and Camano Islands, Island County, Washington, water years 1998-99............................................................................. 5 Table 2. Predominant soil types used in the Deep Percolation Model to determine recharge from precipitation for the six study basins on Whidbey and Camano Islands, Island County, Washington................................................................................................................ 12 Table 3. Land-cover categories used in the Deep Percolation Model for the six study basins on Whidbey and Camano Islands, Island County, Washington ............................................................. 12 Table 4. Grid dimensions for modeled basins and Whidbey and Camano Islands, Island County, Washington................................................................................................................ 13 Table 5. Summary of streamflow data for the six study basins on Whidbey and Camano Islands, Island County, Washington, water years 1998-99............................................................................. 18 Table 6. Reclassification of land-cover data to categories used by the Deep Percolation Model for Whidbey and Camano Islands, Island County, Washington........................................................ 20 Table 7. Land-cover categories used in the Deep Percolation Model (DPM) for Whidbey and Camano Islands, Island County, Washington.................................................................................... 23 Table 8. Soil groups composited for the Deep Percolation Model using Island County soil series for Whidbey and Camano Islands, Island County, Washington........................................................ 24 Table 9. Summary of chloride in