"Impact of Climate Change on Snake River Plain Water Supply"

October 16th 
Russell Qualls - Biological and Agricultural Engineering

Abstract: Climate change would affect southern Idaho irrigation water supplies. While the effects might be climate change-driven, the actual irrigation impacts would be mediated by basin hydrology, water storage and control infrastructure, institutional arrangements governing water use and allocation, changes to competing demands for power generation and in-stream flows, and farmer reactions to changing water supplies. The basin hydrology is simulated by means of a snowmelt runoff model, SRM, to estimate changes to the timing and magnitude of the inflow from watersheds into the river system. MODSIM is used to simulate reservoir operation and prior-appropriation doctrine water allocation given the hydrologic inputs. Modeled inflow changes are based on simulated snowmelt runoff changes resulting from climate change scenarios referenced to the years 2030 and 2080. For each of these reference time frames, results from three different General Circulation Models (GCMs) were implemented, representing the range of "Wet", "Middle", and "Dry" conditions simulated by all the models included in the IPCC Fourth Assessment Report (AR4) for the mid-range (A1B) emissions scenario. Modsim model results include both the average and time-distribution of expected irrigation water shortages by river reach and tributary within the Snake River Basin.

Presently water supplies in the basin are abundant, and storage and distribution systems are robust, so water shortage is significant only in occasional dry years. Our results indicate that climate change may have either a beneficial or deleterious effect on the supply of agricultural irrigation water. The models corresponding to the "Wet" and "Middle" (i.e., average or median) climate change scenarios both indicate that there will be a surplus of water relative to current climate conditions. Even though the simulated runoff occurs somewhat earlier in the melt season, the existing system of reservoirs is sufficient to provide a surplus of water even in the dry summer months. The "Dry" model scenarios for 2030 and 2080 both produced a small deficit in irrigation water supply, which basin-wide amount to 1.7% and 2.7%, respectively, of the delivery provided by the current climate.

These shortages are not uniformly distributed across the basin. Instead, they are increased or decreased at specific locations or nodes corresponding to end-users throughout the basin as a result of complex interactions among the amount and timing of runoff, water rights seniority of the end-users, and the spatial distribution of storage and transport infrastructure throughout the system. Despite the deficit to water supply resulting from the most extreme climate change scenario, the supply functions which provide the price-quantity relationships for water in the system were virtually identical between the baseline current climate, and the worst-case 2080-Dry scenario. Increased water demands for downstream and in-stream uses, such as hydropower and endangered salmon passage, could make the impact much greater, which would also be true in the present climate.