Agricultural productivity is strongly dependent on the availability of water. As the world's food supply continues to be taxed by population growth, increasing demand, and rapidly developing economies, a greater percentage of arable land will need to be utilized and land currently producing food must become more efficient. However, our ability to predict future water resources and the allocation of those water resources across competing sectors remains limited. The proposed research will comprehensively explore current and future water availability for irrigation at both state and regional scales in collaboration with the U.S. Bureau of Reclamation. This will be done within the context of California's Central Valley, a heavily managed, tightly constrained, and agriculturally important area. The Decision Support System for Agrotechnology Transfer (DSSAT) Cropping Systems Model will be coupled to the Water Evaluation and Planning (WEAP) System to substantially improve the representation of plant processes and agricultural water demand. The coupled model will be thoroughly evaluated with remotely sensed data to improve the prediction of climate change impacts on water resources and crop yields. The Water Balance/Transport Model, which unites remote sensing and hydrographic data sources, will then be used to determine whether drought conditions in the Central Valley coincide with droughts in neighboring basins, as well as identify sources of water that could potentially alleviate drought in the Central Valley. The primary research tasks are: (1) couple DSSAT to WEAP, and evaluate both models over California's Central Valley; (2) simulate current and future available water throughout the Central Valley and distributions of water among competing uses, including agricultural, municipal, industrial, environmental, and hydropower; (3) assess the impacts of future water availability on crop yields in irrigated agricultural areas; and (4) create a regional framework of water resources for the broader western United States.