GROUNDWATER BUDGETS FOR THE WOOD RIVER VALLEY AQUIFER SYSTEM, 1995-2004, SOUTH-CENTRAL IDAHO

The Wood River Valley of south-central Idaho contains the cities of Sun Valley, Ketchum, Hailey, and Bellevue. This mountain valley is underlain by the Wood River Valley aquifer system which consists of a single unconfined aquifer and, in the southern part of the area, an underlying confined aquifer. The entire population of the area depends on groundwater for domestic supply, either from domestic or municipal-supply wells. Rapid population growth since the 1970s has caused concern about the long-term sustainability of the aquifer system. In order to help address these concerns groundwater budgets were developed for the Wood River Valley aquifer system for three time periods: the 10-year period 1995-2004, and the single years of 1995 and 2001, the wettest and driest years, respectively, within the 10-year period.

Recharge or inflow into the Wood River Valley aquifer system occurs through seven main sources: tributary canyons, streamflow loss from the Big Wood River, areal recharge from precipitation and excess irrigation water, seepage from canals and recharge pits, municipal pipe leakage, percolation from septic systems, and subsurface inflow beneath the Big Wood River in the northern end of the valley. Discharge or outflow from the Wood River Valley aquifer system occurs through four main sources: Silver Creek streamflow gain, combined pumpage, Big Wood River streamflow gain, and direct evapotranspiration from riparian vegetation. The budget residual encompasses subsurface outflow, groundwater-storage changes, and budget error; the estimated fluctuations between the wet- and dry-year budgets likely are primarily due to changes in groundwater storage.