1. Introduction  Rising temperature and changes in the frequency and magnitude of precipitation due to climate change (IPCC-AR4 report) events are anticipated to affect crop production, water availability and quality, and flood risk in the PNW (Stockle et al 2009, Elsner et al 2009, Hamlet and Lettenmaier 2007).  Agriculture is a vital part of the economy in the Pacific Northwest (PNW), and particularly in the Yakima Basin. In 2008, PNW wheat production accounted for $1.7 billion, the third largest value in the United States (NASS, 2009).  The eastern side of the Cascade Mountains, which receives only 5-25” of rain annually, is particularly vulnerable to drought. In the last decade, there have been 10-20% yield losses during severe drought years, with an average of $90 million/year (NASS, 2009).  The challenge is to anticipate the probable effects of climate change on the hydrological cycle and make sound land use, water use, and agricultural management decisions that will best serve the needs of agricultural production while protecting our freshwater resources. Results Water Supply (below left). Historical (‘77-’06) and future surface water supply for dry, average, and wet (20 th, 50 th, and 80 th percentiles, respectively) year conditions. Supply as shown here is the unregulated supply and are reported prior to accounting for demands, and thus should not be compared to observed flows. Climate change causes a shift in water availability away from the summer season into the winter season for all flow conditions. Conclusions  Although future annual surface water supply in the Yakima is associated with larger uncertainty, the seasonality of supply is projected with higher confidence to shift away from the summer irrigation season and into the winter season of low demand. This is due to some loss of the snowpack as seasonal storage.  Irrigation demand in the Yakima is projected to increase, with the largest increases in July, one of the months with the largest reductions in supply, although the reservoirs will be able to buffer some of this shift. However, even with reservoir buffering, unmet irrigation demand in the basin will increase by an average of 50% by 2030 (results not shown), causing increased water stress in an already stressed system.  The Yakima reservoirs are modeled as a lumped system at the monthly time-step. (Details shown below.) Assessing the Impact of Climate Change on Irrigation Demand and Water Availability in the Yakima River Basin Kirti Rajagopalan a, Kiran J. Chinnayakanahalli a, Jennifer Adam a, Keyvan Malek b, Roger Nelson b, Claudio Stockle b, Michael Brady c, Shifa T. Dinesh a, Michael Barber a, Georgine Yorgey d, Chad Kruger d a Civil and Env.. Eng.; WSU, b BioSys. Eng., WSU; c Economics, WSU; d CSANR, WSU Modeling Framework  We applied our newly-developed system of linked models, including VIC hydrology model (Liang et al. 1994), a dynamic crop systems model (CropSyst: Stockle et al. 2003), reservoir model, and an economics model.  Irrigation demand and crop yield for each crop type in the basin as well as supply are simulated using VIC-CropSyst, while water management (reservoirs and curtailment) are simulated as a separate process. If curtailment occurs, VIC-CropSyst simulations with reduced irrigation are repeated to examine the effects of curtailment. (Details shown below.) Basin Description http://wa.water.usgs.gov Objective  To apply an integrated modeling framework (coupled hydrology and cropping systems model, reservoir model and economics model) to study future (2030s decade) water supply and irrigation water demand over the Yakima River Basin for improved water resources management.  This biophysical modeling system interacts with economic modeling for short and long-run producer response. This interactive model is run under for three sets of scenarios. (Details shown below.)  Five USBR reservoirs maintained provide irrigation supply. Snowpack also used as a natural reservoir, but is sensitive to even modest warming.  Water allocations are a little over 2 MAF/year of which about half are junior water rights that are pro- ratable. Water allocation for these water holders have been curtailed in 13 out of the 36 years between 1970 and 2005. This situation is expected to be exacerbated by climate change. Water Supply and Demand (below right). Comparison of surface water supply and demand for historical and the future using the baseline economic scenarios Water Demand (below). Historical (‘77-’06) and future demands for average conditions and the three economic scenarios. Irrigation demand increases the most in July.  Irrigated agriculture is the economic base. This includes high-value crops such as apples, cherries, and hops. Historical Future