ABSTRACT

The Wood River Valley contains most of the population of Blaine County and the cities of Sun Valley, Ketchum, Haley, and Bellevue. This mountain valley is underlain by the alluvial Wood River Valley aquifer system which consists of a single unconfined aquifer that underlies the entire valley, an underlying confined aquifer that is present only in the southernmost valley, and the confining unit that separates them. The entire population of the area depends on ground water for domestic supply, either from domestic or municipal-supply wells, and rapid population growth since the 1970s has caused concern about the long-term sustainability of the ground-water resource. To help address these concerns this report describes a ground-water budget developed for the Wood River Valley aquifer system for three selected time periods: average conditions for the 10-year period 1995–2004, and the single years of 1995 and 2001. The 10-year period 1995–2004 represents a range of conditions in the recent past for which measured data exist. Water years 1995 and 2001 represent the wettest and driest years, respectively, within the 10-year period based on precipitation at the Ketchum Ranger Station.

Recharge or inflow to the Wood River Valley aquifer system occurs through seven main sources (from largest to smallest): infiltration from tributary canyons, streamflow loss from the Big Wood River, areal recharge from precipitation and applied irrigation water, seepage from canals and recharge pits, leakage from municipal pipes, percolation from septic systems, and subsurface inflow beneath the Big Wood River in the northern end of the valley. Total estimated mean annual inflow or recharge to the aquifer system for 1995–2004 is 270,000 acre-ft/yr (370 ft³/s). Total recharge for the wet year 1995 and the dry year 2001 is estimated to be 270,000 acre-ft/yr (370 ft³/s) and 220,000 acre-ft/yr (300 ft³/s), respectively.

Discharge or outflow from the Wood River Valley aquifer system occurs through five main sources (from largest to smallest): Silver Creek streamflow gain, ground-water pumpage, Big Wood River streamflow gain, direct evapotranspiration from riparian vegetation, and subsurface outflow (treated separately). Total estimated mean 1995–2004 annual outflow or discharge from the aquifer system is 250,000 acre-ft/yr (350 ft³/s). Estimated total discharge is 240,000 acre-ft/yr (330 ft³/s) for both the wet year 1995 and the dry year 2001.

The budget residual is the difference between estimated ground-water inflow and outflow and encompasses subsurface outflow, ground-water storage change, and budget error. For 1995–2004, mean annual inflow exceeded outflow by 20,000 acre-ft/yr (28 ft³/s); for the wet year 1995, mean annual inflow exceeded outflow by 30,000 acre-ft/yr (41 ft³/s); for the dry year 2001, mean annual outflow exceeded inflow by 20,000 acre-ft/yr (28 ft³/s). These values represent 8, 13, and 8 percent, respectively, of total outflows for the same periods. It is difficult to differentiate the relative contributions of the three residual components, although the estimated fluctuations between the wet and dry year budgets likely are primarily caused by changes in ground-water storage.

The individual components in the wet and dry year ground-water budgets responded in a consistent manner to changes in precipitation and temperature. Although the ground-water budgets for the three periods indicated that ground-water storage is replenished in wet years, statistical analyses by Skinner and others (2007) suggest that such replenishment is not complete and over the long term more water is removed from storage than is replaced. In other words, despite restoration of water to ground-water storage in wet years, changes have occurred in either recharge and (or) discharge to cause ground-water storage to decline over time. Such changes may include, but are not limited to: lining or abandoning canals and ditches, conversion of surface-water irrigation rights to ground-water rights, changes in location of diversion points, changes in irrigation method and efficiency, increased consumptive use by evaporation or evapotranspiration, and long- or short-term climatic change.

Estimates were made of evapotranspiration (consumptive use), simulated irrigation, and deep percolation for a 1-acre parcel for each of 14 land-use classifications in the Wood River Valley. The mean evapotranspiration rate for urban land uses generally is less than for agricultural land uses, mean simulated irrigation for urban land uses is less than for agricultural uses, and the volume of deep percolation (recharge) tends to be larger for urban land uses. Most urban land uses in the Wood River Valley generally are estimated to have slightly less consumptive water use than agricultural uses. However, many other factors influence the ultimate effects of the conversion of agricultural land to urban uses and may have greater effects on the aquifer system by the redistribution or reduction of recharge.