Wyoming Framework Water Plan - Living Rivers Home Page

Executive Summary
in most areas, to the depth at which there is no significant porosity to contain ground water, e.g.
the crystalline basement rocks underlying the entire Snake/Salt River basin and forming the visible
core of the highest mountains. The volume of pore space in this material represents the volume
of ground water and is likely on the order of 100's of millions of acre-ft. Much of this water
is of unusable quality (e.g. due to great depth) or is contained in formations from which ground
water cannot be extracted at useful rates (e.g. thick shale units), so the useable ground water
resource is vastly smaller than the total ground water in storage.
Considering only the alluvial aquifer (covering approximately 400 mi 2 ) in the Snake/Salt River
basin, and assuming an average saturated thickness of 200 feet and an effective porosity of 20%,
a volume of 10 million acre-ft of useful ground water in storage is calculated. Were ground water
a static, nonrenewable resource, like coal, this volume might approximate the developable
resource.
Ground water is a very dynamic resource, particularly ground water of high quality occurring at
depths feasible for development. Data suggest an average annual recharge rate of approximately
4 inches = 1 million ac-ft/yr spread across the 4700 mi 2 of the Snake/Salt River basin. The base
flows of the Salt and Greys Rivers (i.e. the streamflow that is sustained by ground water input
through the period of the year without significant precipitation input) suggest average ground
water output of 250 and 350 ac-ft/mi 2 /yr, respectively. (The Snake River basin below Jackson
Lake is not considered due to the impact of reservoir modulation on base flows.) Applying a value
of 300 ac-ft/mi 2 /yr to the entire basin suggests a total ground water output of 1.5 million ac-ft/yr,
roughly comparable to the recharge-based estimate. Of course, development and consumption of
this "available" ground water would leave the streams of the Snake/Salt River basin dry through
much of the year.
A more detailed, ground water-model based mass balance for the alluvial aquifer between Jackson
Lake and Hoback Junction was developed by San Juan and Kolm (1996). They estimated total
recharge of approximately 50,000 acre-ft per year, with 25,000 acre-ft per year of ground water
discharge through evapotranspiration and 25,000 acre-ft per year of discharge to streams.
In any case, the inescapable requirements of mass balance mean that additional ground water consumption
causes either a decrease in ground water storage (declining ground water level/pressure),
a decrease in consumption elsewhere in the hydrologic system, or depletion of surface
water. There is little indication of widespread reductions in ground water levels in the study area
(although long-term data are sparse).
WATER DEMAND PROJECTIONS
Water demand projections were developed utilizing an economics based approach. Using a planning
period of 30 years, the various aspects of water use were projected for high, mid, and low
Snake/ Salt River Basin Plan
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