Definition: Water in storage is the volume of water, expressed in acre-feet, that underlies a given area of the land surface.  It is the product of multiplying the saturated thickness (ST) and specific yield (SY), and represents the volume of water that could be recovered if that area of the aquifer were pumped dry.  For example, a saturated thickness of 100' below an acre of ground in an aquifer with a specific yield of 10% represents 10 acre-feet (AF) of water.

Volume = Area x ST x SY

Map of predevelopment water storage         Map of change in water storage

Discussion: Water in storage is calculated using saturated thickness (which is determined from measurements of water-table and bedrock elevation), specific yield (which is estimated based on a limited number of aquifer tests), and surface area.  Of these factors, only the saturated thickness is likely to change over time. The saturated thickness determined for any given time can be converted to volume in storage if the specific yield is known. The map of the predevelopment water volume in storage is based on the saturated thickness values already determined and the specific yield (see appendix on groundwater storage and flow).  The darker blue areas of the map represent the greater original reserves of groundwater, and the light-colored areas show the regions least able to support use without careful management. The map of change in water storage reflects withdrawals over time; comparing the maps shows that the regions with lower original storage (light blue on the predevelopment map) and greater change (darker shades on the change map) generally coincide with the areas showing the shortest Estimated Usable Lifetime.  Such areas are particularly noticeable along the Arkansas River corridor and in Wallace, Greeley, Wichita, Scott, and Finney counties.

Sources and methods: Saturated thickness values were obtained as described in those sections of the atlas and the appendices cited. The US Geological Survey has prepared databases and maps showing estimates of the distribution of specific yield and hydraulic conductivity in the High Plains aquifer.  The saturated thickness values at section centers were multiplied by the section-center specific yield values obtained from the USGS data, and then multiplied by the surface area for each section, giving results in units of acre-feet.  These results were grouped into ranges of values, and plotted in the accompanying maps.

Qualifications: These values should not be considered highly accurate estimates of available water, and should not be used for developing detailed assessments or management policies. The uncertainties involved in determination of saturated thickness are discussed in those sections of the atlas; the specific yield estimates are discussed in the information appendix on groundwater storage and flow.  Because the specific yield estimates are based on rather widely spaced data points and treat the aquifer as a single homogeneous layer, the volumes calculated should be considered regional estimates.

See also: Estimated Annual Groundwater Recharge, Current Maximum Authorized Use, Predevelopment Saturated Thickness, Change in Saturated Thickness

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