Abstract
In the Upper Snake River Basin (USRB) (southern Idaho, USA), irrigation for an agricultural sector increasingly dominated by dairy comprises 97% of total human water use. The USRB is a semi-arid, snow melt-dominated basin where agricultural growth was spurred by a managed network of reservoirs. Increasingly, irrigation is dependent on groundwater abstractions from the expansive Eastern Snake Plain Aquifer (ESPA), with recharge from irrigation excess a major component of aquifer recharge. Discharge from the ESPA enters back into the Snake River supporting critical hydropower, aquaculture, and other instream uses. Shifts towards increasingly efficient irrigation technologies reduce ESPA recharge making decisions to invest in efficiency complex when considering the system as a whole. Quantifying the reliance of the ESPA on irrigation water re-use, and the Snake River on discharge from the ESPA, is necessary to better understand the resiliency of this hydrologic-social landscape. A suite of model simulations using the UNH Water Balance Model test the coupled sensitivity of irrigated agriculture and hydrologic states to changes in irrigation technology, agricultural expansion, and climate stressors. To assess these linkages explicitly, new modules were developed to represent a lumped aquifer and irrigation technology. Investment in improved irrigation efficiency reduces gross irrigation from surface water sources, leading to greater surface water stores. Coupling investment with expanded agriculture increases groundwater abstractions only marginally. Representing drought conditions by simulating only the 8 driest years since 1990, has the largest impact across all tested metrics and dramatically decreases surface water stores and discharge from the ESPA. Both drought and reduced snowpack increase aquifer abstractions. In all scenarios reduced aquifer recharge exceeds changes in abstractions resulting in decreased aquifer discharge, which has adverse implications for downstream users. Moreover, reduced head at upgradient sections of the ESPA shift the composition of discharge to the Snake River towards more local agricultural inputs presenting additional water quality concerns. These simulations are informing stakeholder perspectives regarding possible adaptations for the USRB.