What are the downstream water availability consequences of switching to more efficient irrigation systems?

Climate change is projected to increase the magnitude and frequency of droughts in many parts of the world; to minimize negative consequences of droughts, irrigators may need to adapt their irrigation practices. One of these adaptations is switching to more efficient technologies. Efficient irrigation systems usually reduce irrigation losses to baseflow and runoff which diminishes return flows, a significant component of downstream water availability in many irrigated river basins. The purpose of this study is to understand how farmers' investment in more efficient irrigation systems (as a climate change adaptation strategy) impact downstream water availability and, therefore, regional agricultural productivity. Our case study area is Washington State's Yakima River Basin (YRB), a heavily irrigated agricultural basin with relatively low irrigation efficiency and significant contribution of return flow to summer water availability. Similar to many other snow-dominant basins in the western U.S., the YRB has already experienced major droughts in 20% of the years between 1980 and 2010, and droughts are projected to be doubled by 2070. To simulate farmers' investment decisions, we apply an integrated hydrologic-agricultural-economic platform. Components include a tightly coupled agricultural-hydrologic model (VIC-CropSyst), a river system model specifically developed for the YRB (Yakima RiverWare) and an economic model that simulates farmers' investment decisions. The modeling platform also simulates return flows, thereby capturing downstream water availability throughout the irrigation season. It can also simulate the impacts of changes in downstream water availability on crop yield. The results demonstrate that a modified return flow regime negatively impacts downstream water availability for irrigation during the late irrigation season, when water supply is already limiting. The results can be used to inform sustainable agriculture that take into account both intended and unintended consequences of farmer climate change adaption decisions.