AN APPROACH FOR SIMULATING AGRICULTURAL WATER MANAGEMENT IN THE SAN JOAQUIN VALLEY, CALIFORNIA

The Central Valley Project (CVP) annually delivers about 7 million acre-feet of water for agricultural, urban and wildlife uses including supplies for approximately 3 million acres of irrigated agricultural lands, 1 million urban households and 19 wildlife refuges. Starting in February, Reclamation issues a series of potential water supply announcements indicating to its contractors what percentage of their maximum contracted water supplies may be available for delivery during the remainder of the water year. These water supply announcements are based on a combination of measured snow pack, current reservoir storages and 90 and 50 percentile exceedance estimates of additional reservoir inflows. The Central Valley Operations Office uses these estimated supplies as inputs to a Demand Model. The computed demand estimates are subsequently used as inputs to an Operations Model which computes various important metrics of system performance. If the performance metrics are unacceptable, the preliminary announcement is adjusted and the process is repeated until acceptable results are obtained. These potential water supplies can be adjusted either up or down as the fall-spring precipitation season evolves. However, agricultural water users must make decisions about the types and acreages of crops they will plant in the late winter or early spring when the uncertainty in the final water supply is still significant. Consequently, methods that reduce uncertainty in either supply or demand earlier in the year would have significant benefits to water users. To reduce demand uncertainty, our focus has been on improving estimates of agricultural water requirements in the San Joaquin Valley by forecasting potential evapotranspiration (PET) out to 90 days in the future (Yates and Tansey, 2010) and using these PET forecasts in a physically based simulation model designed for the management of large-scale multi-organizational water supply systems. This model (Land, Atmosphere, Water Simulator, LAWS) simulates daily, field-scale land, crop, and water management practices and allows for the consideration of alternative methods for managing soil moisture on a daily basis during the irrigation season based on soil properties, crop type and vegetative growth stage using the FAO 56 dual crop coefficient method. Using water supply estimates generated by the Operations Forecast Model, a LAWS simulation is run to compute a current estimate of future water demands and to determine whether there will be an imbalance between forecasted supplies and demands. In the event of shortages, the LAWS model is used to develop alternative management strategies (such as increased groundwater pumping, recycling, water purchases, changes in crops types, acreages irrigation methods, deficit irrigation, land fallowing among others) to achieve a balance between supply and demand. These updated demand estimates are input into the Operations Forecast Model which would be re-run to determine whether the CVP system metrics are still acceptable. If unacceptable, the process can be repeated until an acceptable operation is achieved. In this way, the LAWS model is used to provide a physically based estimate of irrigation requirements and to include in the process an accounting of the various management practices that can be used in times of water scarcity.