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4.0 RESOURCES
4.1 INTRODUCTION
This chapter presents the physical characteristics (quantity and quality) of Wyoming’s surface
water and groundwater. Regarding water quantity, the intent of the chapter is to characterize Wyoming’s
total hydrologically available water supply irrespective of existing uses, institutional constraints, and
visions for how water resources might be put to future beneficial use. In other words, this chapter
provides estimates for the volume of water Wyoming must draw upon to sustain itself and future
generations.
The results described herein pertain to physical availability, which is to be distinguished from
legal or permitted availability. Physical availability is important in assessing the viability of any future
project, and lack of physical availability of water is an obvious fatal flaw for any water development.
4.2 GENERAL
Wyoming is a headwaters state, and
precipitation is the source of most water in the
state. Figure 4-1 presents mean precipitation
rates. Approximately 68 million acre-feet per
year 1 of precipitation joins 1.5 million acrefeet
of surface inflows to create the total water
resource. The basic hydrologic cycle is
depicted in the margin graphic for a typical
Wyoming river basin. Most of the
precipitation will evaporate from the surface
or be taken up and transpired by local
vegetation. Figure 4-2 presents mean rates of
potential evapotranspiration. Precipitation
and snowmelt in excess of immediate
demands will either run off (feeding rivers,
streams, and lakes) or infiltrate to become
groundwater. Approximately 15 million acrefeet
per year of water become either surface or
Hydrologic Cycle
groundwater and thus available for use. This
estimate is made up of the water that flows into the state plus the precipitation that runs off as streamflow
or infiltrates as groundwater. Groundwater recharge will eventually return to the surface. Surface water
will eventually evaporate to the atmosphere, completing the cycle. These two elements of the total water
resource, surface water and groundwater, are the subject of the present report.
1 13.07 inches per year mean annual statewide precipitation rate (Wyoming Climate Atlas, 2004).
4-1