Estimation of a Water Budget for 1972-2000 for the Grasslands Area, Central Part of the Western San Joaquin Valley, California

By Charles F. Brush, Kenneth Belitz, and Steven P. Phillips



Scientific Investigations Report 2004-5180 _Version 1.1

Sacramento, California 2004

In cooperation with the
U.S. Bureau of Reclamation

Errata Sheet 10-27-04

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     Equitable implementation of regulations restricting discharges from agricultural drains into the San Joaquin River requires a greater understanding of the influence of extreme precipitation events on the ground-water flow system. As part of a larger investigation, this study estimated ground-water recharge and ground-water pumpage, two important components of the water budget in the Grasslands drainage area in the central part of the western San Joaquin Valley, California, for the water years 1972 through 2000. These estimates will be used as inputs to a numerical simulation model of the regional ground-water flow system in the continuing investigation. Crop-acreage and surface-water delivery data were compiled for 14 water districts and 6 other areas comprising approximately 97 percent of the 600-square-mile study area. Little ground-water pumpage data exists for the study area. A climate-based approach was employed to estimate annual water-table recharge flux and ground-water pumpage for 11 water-budget areas. Ground-water pumpage was estimated from the residual irrigation demand after crop consumption of surface water. Estimated recharge flux to the water table for the entire study area averaged 0.8 ft/yr, and estimated ground-water pumpage per unit area for the entire study area averaged 0.5 ft/yr. Increased discharges from agricultural drains in the late 1990s may have been due partly to 4 years of high recharge from precipitation over the 6-year period from 1993 to 1998. Knowledge of the ratio of annual crop water demand to annual potential evapotranspiration, expressed as an aggregate crop coefficient, Kd, will facilitate estimation of annual water-budget components in future studies. Annual aggregate crop coefficients, calculated each year for the entire study area, were nearly constant at 0.59 from 1983 to 2000, and reasonably constant at 0.53 prior to 1983. The overall trend suggests continuous reductions in recharge from irrigation over time. This reduction is most likely due to gradual improvements in irrigation management. The recharge and pumpage estimates are both sensitive to the consumption-distribution ratio, and the pumpage estimate is sensitive to the cropped acreage.




Location of the Study Area

Hydrogeologic and Climatologic Setting

Ground-Water Pumpage

Reuse of Irrigation and Drainage Water

Previous Studies


Data Acquisition

Climate Data

Data-Collection Areas

Crop Acreages

Water Deliveries

Surface-Water Deliveries

Ground-Water Pumpage

Reuse of Irrigation and Drain Water

Consumption-Distribution Ratio

Canal Seepage

Daily Crop Soil-Moisture Budgets

Daily Soil Surface Process Model

Daily Crop Water Demand Model

Daily Soil-Moisture Budget

Annual Water Budgets

Water-Budget Model

Supply-Based Consumptive Use in Broadview

Results and Discussion

Data Acquisition

Water-Budget Inputs

Climate-Based versus Supply-Based Consumptive Use in Broadview

Annual Water-Budget Calculations

Estimated Annual Ground-Water Pumpage

Estimated Annual Ground-Water Recharge

Comparison with Other Studies

Aggregate Crop Coefficient

Surface-Water Availability Indices

Estimation Sensitivity and Uncertainty

Limitations of this Methodology


References Cited

Appendix A. Agroclimatic Data

Appendix B. Water Deliveries and District-Level Pumpage

Appendix C. Soil-Moisture Budget Results

Appendix D. Water-Budget Results

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Water Resources of California

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