Watsonville Sloughs Hydrology Study Prepared For: Santa Cruz Resource Conservation District Prepared by: In collaboration with: February 14, 2014 WATSONVILLE SLOUGHS HYDROLOGY STUDY TABLE OF CONTENTS SUMMARY ..................................................................................................................................................... 1 Data Collection Program ........................................................................................................................................... 1 Hydrologic modeling ................................................................................................................................................. 5 1 OBJECTIVES AND TECHNICAL APPROACH ........................................................................................ 8 1.1 Format of this Report ................................................................................................................................... 8 1.2 Project Funding ............................................................................................................................................ 8 1.3 Study Objectives ........................................................................................................................................... 8 1.4 Work Plan ................................................................................................................................................... 10 1.5 Acknowledgments ...................................................................................................................................... 12 2 PROJECT SETTING ............................................................................................................................... 13 2.1 Location and Study Limits .......................................................................................................................... 13 2.2 Climate ....................................................................................................................................................... 14 2.3 Soils ............................................................................................................................................................ 15 2.4 Drainage Features and Patterns ................................................................................................................. 17 3 DATA COLLECTION PROGRAM ......................................................................................................... 24 3.1 Topographic Survey .................................................................................................................................... 24 3.2 Hydrologic Data Collection ......................................................................................................................... 25 3.2.1 Precipitation Gaging .......................................................................................................................... 28 3.2.2 Water Surface Elevation Gaging Network ......................................................................................... 28 3.2.2.1 Previously Established Gaging Sites .............................................................................................................. 29 3.2.2.2 New Gage Installations ................................................................................................................................. 29 3.2.3 Stream Flow Gauging Network ......................................................................................................... 29 3.2.4 Groundwater Monitoring .................................................................................................................. 29 3.2.5 Salinity and Temperature .................................................................................................................. 30 3.2.6 Shell Road Pump Station Calibration ................................................................................................. 30 ©2014 BALANCE HYDROLOGICS, INC. i | Page WATSONVILLE SLOUGHS HYDROLOGY STUDY 3.3 Field Visits and Measurements .................................................................................................................. 30 3.4 Data Reduction and Post-Processing ......................................................................................................... 31 4 FIELD DATA DISCUSSION ................................................................................................................... 32 4.1 Topographic Survey Findings ..................................................................................................................... 32 4.1.1 Survey Control Points ......................................................................................................................... 32 4.1.2 Watsonville Slough Channel Long Profile .......................................................................................... 33 4.2 Hydrologic Data Findings ........................................................................................................................... 36 4.2.1 Precipitation Gaging Results ............................................................................................................. 36 4.2.2 Water Surface Elevations .................................................................................................................. 38 4.2.2.1 Tidal Conditions and Pajaro River Interactions ............................................................................................. 38 4.2.2.2 Barrier Beach Overtopping and Seawater Incursions ................................................................................... 41 4.2.2.3 Slough Interactions (a Complicated Confluence) .......................................................................................... 43 4.3 Stream Gaging Results ............................................................................................................................... 48 4.3.1 Harkins Slough at Buena Vista (HSBV)............................................................................................... 49 4.3.2 Gallighan Slough below Landfill (GSBL) ............................................................................................. 49 4.3.3 East Struve Slough at Main Street (SSES) .......................................................................................... 50 4.3.4 West Struve Slough at the High School (SSWS) ................................................................................. 51 4.3.5 Watsonville Slough at Highway 1 (WSHO) ........................................................................................ 53 4.3.6 Watsonville Slough at San Andreas (WS@SA) ................................................................................... 53 4.4 Groundwater Monitoring Findings............................................................................................................. 54 4.5 Additional Data Collected .......................................................................................................................... 57 4.5.1 Shell Road Pump Calibration ............................................................................................................. 57 5 HYDROLOGIC MODELING ................................................................................................................ 59 5.1 Model Structure ......................................................................................................................................... 59 5.2 Data Sources and Model Input ................................................................................................................... 62 5.2.1 Topography and Sub-watersheds ...................................................................................................... 63 5.2.2 Groundwater Recharge Areas ........................................................................................................... 65 ii | Page ©2014 BALANCE HYDROLOGICS, INC. WATSONVILLE SLOUGHS HYDROLOGY STUDY 5.2.3 Land Use Types .................................................................................................................................. 66 5.2.4 Soil Properties .................................................................................................................................... 67 5.2.5 Impervious Cover ............................................................................................................................... 68 5.2.6 Meteorological Data .......................................................................................................................... 69 5.2.6.1 Precipitation .................................................................................................................................................. 69 5.2.6.2 Evapotranspiration ........................................................................................................................................ 73 5.2.7 Irrigation Data ................................................................................................................................... 73 5.3 Model Calibration ...................................................................................................................................... 74 5.3.1 Calibration Parameters.....................................................................................................................