Abstract
As climate changes in the arid and semi-arid regions of the western United States the hydrology of the region is changing also. Warmer temperatures are resulting in earlier runoff and winter snow cover retreating to higher elevations. Warmer temperatures are also allowing farmers to grow more variety of crops in higher elevation basins like the Snake River basin. There are also indications that climatic extremes may increase. Water conservation is an important tool to deal with drought. Historically each drought cycle has driven more water conservation. To plan for these changes water resource managers need to understand the role water conservation can have in exacerbating downstream water shortages by increasing consumptive demand and decreasing incidental recharge to the underlying aquifers. While water conservation will increase the reliability of surface water resources and profitability of agriculture, it will increase downstream water shortages as aquifer discharge will decline unless losses in incidental recharge are offset by managed recharge. Higher temperatures and longer growing seasons are also likely to increase the demand on both groundwater and surface water resources. To understand climate impacts on irrigation there is a need to define water shortages in irrigated agriculture based both on water supply parameters and climatic conditions. The Surface Water Demand Index (SWDI) presented here is the first index developed for the western United States to quantify irrigation shortages on an annual basis.
A review of aquifer levels in many arid and semi-arid regions indicate that aquifer levels are often closely tied to climatic conditions. Both the Big Lost River basin and the eastern Snake Plain Aquifer have clear climatic signals in the historical water level record. A more frequent occurrence of pluvial events are likely to lead to an easier path to aquifer recovery, but a future with less frequent pluvial events could result in more serve aquifer declines than historically observed.
Significant measures by the Idaho Water Resource Board (IWRB) and the Surface Water Coalition, and the Idaho Groundwater Users Association have led to a 2,000,000 ac-ft gain to the eastern Snake Plain Aquifer that provides the vital link to maintaining the two-river system that has governed water resource management for the last 100 years in Idaho. It is my sincere hope that the knowledge gained in this dissertation can be applied to a reservoir operations model linked with the Enhanced Snake Plain Aquifer Model to compare projected climate impacts against historical impacts of water resource management on the Snake River Basin to make sure that water management plans being utilized today account for projected impacts of a warming climate.