Climate Change Impacts on Water for Agriculture in California: A Case Study in the Sacramento Valley

Global climate change has the potential to dramatically alter hydrologic conditions in California by changing the spatial and temporal patterns of snow accumulation and snow melt. The water management infrastructure in California has been designed and is operated in accordance with historic hydrologic patterns. Understanding if and how this infrastructure can be managed in the face of global climate change in order to meet the array of vital water management objectives for the system is a critical research question addressed by the impacts investigation conducted by the Climate Action Team in response to Executive Order S-3-05 issued by California Governor Arnold Schwarzenegger. This effort included the application of three different water models run under four different global climate model (GCM)/emission scenario combinations. The goal was to begin to understand the potential impacts of and adaptation to global climate change and to evaluate the utility of various tools in refining this understanding in the future. The current study presents an application of the Water Evaluation and Planning (WEAP) system, developed by the Stockholm Environment Institute, for California’s Sacramento River Basin. WEAP was used to evaluate the impact of four future climate scenarios on agricultural water management in the region, and to investigate whether water management adaptation could reduce potential impacts. The four climate scenarios were derived by downscaling the output from two GCMs (Parallel Climate Model and Geophysical Fluid Dynamics Laboratory) and two emission scenarios (A2 and B1) combinations to a 1/8 degree grid over California. The Sacramento Valley WEAP application sampled these climate fields to provide input to a model of the Sacramento River Basin that disaggregates the basin into 64 sub-watersheds. Each of these sub-watersheds is described by an internal rainfall runoff module that dynamically calculates runoff to streamflow, evaporative demand, and surface water/groundwater interactions. These climate-derived hydrologic conditions drive the simulated operation of a representation of the installed water management infrastructure in the region. As such, WEAP is an integrated hydrology/water resources systems model that allows for assessment of climate change impact and adaptation in the water sector based solely on future climate time series. The integrated nature of WEAP is unique among the tools that were used as part of the water resources portion of the impacts investigation. The model was applied under two formulations, one where cropping and irrigation management patterns remained fixed over the course of a 100-year simulation and one where copping and irrigation management patterns evolved over the course of the 21st century along with the climate. Model runs under all four scenarios showed the largest impacts at the end of the 100-year simulations. In particular, the GFDL/A2 combination produced a downscaled climate series that included a major drought during the final 15 years of the coming century. With no adaptation, a lack of sufficient surface water to meet elevated evaporative demand for irrigated crops led to a dramatic increase in groundwater pumping and a coincident decline in simulated groundwater levels. All simulations also resulted in much lower reservoir levels in