Assessing the Impact of Climate Change on Columbia River Basin Agriculture through Integrated Crop Systems, Hydrologic, and Water Management Modeling

The Columbia River Basin (CRB) in the Pacific Northwest covers parts of US and Canada with a total drainage area of about 670,000 square kilometers. The water resources of the CRB are managed to satisfy multiple objectives including agricultural withdrawal, which is the largest consumptive user of Columbia River water with 14,000 square kilometers of irrigated area in the CRB. Agriculture is an important component of the economy in the region, with an annual value over $5 billion in Washington State alone. The availability of surface water for irrigation in the basin is expected to be negatively impacted by climate change. Previous climate change studies in the CRB region suggest a likelihood of increasing temperatures and a shift in precipitation patterns, with precipitation higher in the winter and lower in the summer. Warming further exacerbates summer water availability in many CRB tributaries as they shift from snowmelt-dominant towards rain-dominant hydrologic regimes. The goal of this research is to study the impacts of climate change on CRB water availability and agricultural production in the expectation that curtailment will occur more frequently in an altered climate. Towards this goal it is essential that we understand the interactions between crop-growth dynamics, climate dynamics, the hydrologic cycle, water management, and agricultural economy. To study these interactions at the regional scale, we use the newly developed crop-hydrology model VIC-CropSyst, which integrates a crop growth model CropSyst with the hydrologic model, Variable Infiltration Capacity (VIC). Simulation of future climate by VIC-CropSyst captures the socio-economic aspects of this system through economic analysis of the impacts of climate change on crop patterns. This integrated framework (submitted as a separate paper) is linked to a reservoir operations simulations model, Colsim. ColSim is modified to explicitly account for agricultural withdrawals. Washington State water rights data as well as instream flow requirements are incorporated to inform water allocation and curtailment decisions. This modeling framework is applied over the 1976-2006 period and compared to a future 30-year period centered on the 2030s. Impacts of climate change on irrigation water availability, crop irrigation demand, frequency of curtailment, and crop yields are quantified and presented. Development of this modeling framework is part of a larger effort to develop a regional-scale earth system model, "BioEarth". The goal of BioEarth is to understand the interactions between land use and water and nutrient cycling under decadal-scale climate variability to inform decisions related to agricultural and natural resources management.