The Adaptability and Sustainability of Surface Water Diversions Along the Main Stem of the Snake River in Southern Idaho

Agriculture in southern Idaho depends heavily on the conversion of snowpack into spring runoff. The Natural Resource Conservation Service (NRCS) has developed a Surface Water Supply Index (SWSI) as a tool to predict whether or not forecasted runoff and reservoir storage will be adequate to meet irrigator’s needs at a basin scale. This research by comparing SWSI to diversions for individual canals advances the use of SWSI to develop a Surface Water Supply Metric (SWSM) that can be used to estimate the reliability and sustainability of diversions under historic and projected time periods. An historic analysis of diversions during three time periods 1928-1957, 1960-1980, and 1980-2009 indicates how the construction of Palisades Reservoir in 1956 allowed some canals to increase diversions, while other canals where able to improve the reliability of diversions. The analysis also highlights how decreasing diversions by irrigators (10% and 13% in July and August, respectively) from the Twin Falls North Side Canal Compnay has increased diversion reliability in those months. The second section of the research uses the SWSM to assess the sustainably of diversions under three projected climate change scenarios. All projected flow scenarios were run using a system dynamics version of the Snake River Planning Model (SRPM) developed by the authors. SRPM is currently used by the Idaho Department of Water Resources (IDWR) to plan water resource management in the Snake River basin. The analysis indicates based on the projected climate scenarios analyzed that upstream irrigators may see a significant decline in reliability while downstream users may see improved irrigation reliability