Modeling the effects of climate change on the hydrology and water rights of a semi-arid irrigated watershed

The hydrology of irrigated watersheds in semi-arid regions is a dynamic interaction of streamflow, crop consumptive use (CU), diversions, recharge, and return flow. The Prior Appropriation water rights system used in the Western U.S. depends on stream flow and a complex allocation system based on seniority. Tools to analyze the potential impacts of climate change on watersheds operating under Prior Appropriation rules are limited. Modright, a comprehensive stream-aquifer modeling tool, was developed to simulate irrigation diversions under the Prior Appropriation system. Modright is coupled to the USGS Modflow model and is generalized for use in any setting. Modright uses Modflow's aquifer and stream information plus water rights and CU data to dynamically model diversions, stream flow and recharge.; A case study of a semi-arid region irrigated watershed was modeled with Modright to evaluate climate change effects. General Circulation Model scenarios were used as inputs to models that are sensitive to stream flow and CU. Results showed annual strewn flows were 131 and 75 percent of baseline flows and CU was 117 and 129 percent of baseline values for the wet and dry climate change scenarios, respectively. Peak stream flows arrived one to two months earlier than in the baseline case. These results were inputs to Modright and used to estimate the impacts on water diversions and streamflow. The analyses suggest that, under the dry scenario, senior water rights have the largest annual declines in diversion amounts and the more junior rights have the largest declines during the middle of the growing season. A water supply:demand ratio was developed to evaluate crop impacts. The annual ratios averaged 107 and 88 percent of baseline ratios and, for the corn growing season (weeks 16 to 36) averaged 102 and 67 percent of baseline ratios for the wet and dry scenarios, respectively. The supply:demand ratios fell below the critical threshold for a longer period of time during the middle of the growing season under both climate change scenarios than under the baseline scenario. The findings indicate that water rights and crop impacts will be more pronounced seasonally for both the wet and dry end-member climate change scenarios and could be particularly severe if future conditions are closer to the dry climate change projections.