This proposal addresses NASA's priority application areas in water resources and climate. We intend to improve California?s major water management and planning system, CalSim-II, to support statewide assessments of climate change?s impact on water resources. To prepare for and adapt to climate change?s potential effects on the state?s already-scarce water resources, state government and agencies and district water managers need a decision support system that will help them develop mitigation strategies. With guidance from the state government, California?s major water resource management agencies: Department of Water Resources (DWR) and Bureau of Reclamation recently developed one such tool, CalSim-II, designed to help manage California?s key water resources, in particular, operations of the massive state and federal water storage and transportation projects and to control potential water quality and water level risks caused by seawater intrusion i nto the Sacramento-San Joaquin Delta. This proposal represents a collaboration with the CalSim-II developer?the California DWR and NASA's Jet Propulsion Laboratory scientists to apply Earth science research results to strengthen CalSim-II?s decision support capabilities. The proposed improvements include 1) hybrid dynamical and statistical downscaling; 2) the use of satellite remote sensing products in conjunction with ground observations; 3) the modification of CalSim-II to produce uncertainty analysis-based probabilistic solutions; and 4) the development of a GIS-enabled Graphical User Interface (GUI) to support decision-making. The proposal?s three partners will implement these applications to address NASA?s goal of the NRA ?to help provide an integrated management approach for sustainable use of water and focus on Earth science applications to water resources management.? The involvement of the state?s major water management agency not only ensures that the applications will be critically useful to decis ion-making but also guarantees their prompt availability to the end-user. Strong support letters from California DWR and Metropolitan Water District of Southern California are included.