[1] Climate change forced by anthropogenic activities has been ongoing since at least the beginning of the industrial revolution. Part of the recent warming in the western United States has been attributed to anthropogenic climate change. This research seeks to answer the basic question of how declining streamflow, increasing temperatures, and fluctuation in precipitation have impacted water resource allocation in the Snake River Plain over the past 35 years . Understanding how changes in climatic attributes have historically impacted water allocation should help water managers better understand how projected climate change may influence allocation. Annual and monthly diversion trends from 62 locations in the Snake River Plain were compared to temperature and precipitation trends at 10 climate stations across the basin. We found a strong trend of declining annual surface water diversions across the study area. Of the 62 diversion points examined, 45 have highly significant decreasing annual diversion trends, while an additional 8 have significant decreasing trends. Despite the annual decline in surface water diversions, April diversions have increased at more than half of the diversion points, with 15 locations showing highly significant trends and an additional 17 showing significant increasing diversion trends. A comparison of diversions to the Surface Water Supply Index indicates that the decline in midseason and late season diversions is mostly caused by decreasing supply in the study period, while a comparison of diversions to Palmer's Z index and the Standardized Precipitation Index indicates that early season diversions are highly correlated to early season moisture anomalies.Citation: Hoekema, D. J., and V. Sridhar (2011), Relating climatic attributes and water resources allocation: A study using surface water supply and soil moisture indices in the Snake River basin, Idaho, Water Resour. Res., 47, W07536,

The Pacific Northwest is expected to witness changes in temperature and precipitation due to climate change. In this study, we enhance the Snake River Planning Model (SRPM) by modeling the feedback loop between incidental recharge and surface water supply resulting from surface water and groundwater extraction for irrigation and provide a case study involving climate change impacts and management scenarios. The new System Dynamics-Snake River Planning Model (SD-SRPM) is calibrated to flow at Box Canyon Springs located along a major outlet of the East Snake Plain Aquifer. A calibration of the model to flow at Box Canyon Springs, based on historic diversions (1950-1995) resulted in an r2 value of 0.74 and a validation (1996-2005) r2 value of 0.60. After adding irrigation entities to the model an r2 value of 0.91, 0.88, and 0.87 were maintained for modeled vs. observed (1991-2005) end-of-month reservoir content in Jackson Lake, Palisades, and American Falls, the three largest irrigation reservoirs in the system. The scenarios that compared the impacts of climate change were based on ensemble mean precipitation change scenarios and estimated changes to crop evapotranspiration (ET). Increased ET, despite increased precipitation, generally increased surface water shortages and discharge of springs. This study highlights the need to develop and implement models that integrate the human-natural system to understand the impacts of climate change

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This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online 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.

Quantifying surface water shortages in arid and semiarid agricultural regions is challenging because limited water supplies are distributed over long distances based on complex water management systems constrained by legal, economic, and social frameworks that evolve with time. In such regions, the water supply is often derived in a climate dramatically different from where the water is diverted to meet agricultural demand. The existing drought indices which rely on local climate do not portray the complexities of the economic and legal constraints on water delivery. Nor do these indices quantify the shortages that occur in drought. Therefore, this research proposes a methodological approach to define surface water shortages in irrigated agricultural systems using a newly developed index termed the Surface Water Delivery Index (SWDI). The SWDI can be used to uniformly quantify surface water deficits/shortages at the end of the irrigation season. Results from the SWDI clearly illustrate how water shortages in droughts identified by the existing indices (e.g., SPI and PDSI) vary strongly both within and between basins. Some surface water entities are much more prone to water shortages than other entities based both on their source of water supply and water right portfolios.