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11 000711 Great Plains Aquifer The specific yield of the sandstones in the Great Plains Aquifer have been estimated to be between 10 and 20 percent (Keene and Bayne, 1977 and the UoSo Geological Survey, 1966)0 An average value for specific yield of 0015 was chosen for the sandstones of the Great Plains Aquifero However, not all of the Great Plains Aquifer is made up of sandstoneo Some of the aquifer includes large amounts of non-aquifer materials (siltstones and shales). In order to estimate a specific yield that would be representative of the entire thickness of the Great Plains Aquifer, a map displaying the percent sandstone of the aquifer was preparedo This map was based on the thickness map of the Great Plains Aquifer and the sandstone thickness map of Keene and Bayne (1975)0 Three zones of percent sandstone were identified on the map (100 percent, 50 to 99 percent and less than 50 percent) and an average percent sandstone was assigned to each zone (100, 66 and 33 percent, respectively) 0 The specific yield of the sandstone was then multiplied by the average percent sandstone in each zone, in order to estimate the specific yield for the entire aquifer thickness in each zoneo It should be noted that there are many areas in central and western Kansas in which Great plains Aquifer wells provide low yieldso Hence if a high volume of water is needed~ the Great Plains Aquifer may not be a viable water source for these specific areas of the stateo Ozark Plateau Aquifer For carbonate rocks such as those that make up the Ozark Plateau Aquifer, much of the porosity, and therefore the specific yield, is dependent on unpredictable fractures and solution openingso In these cases porosity is the best approximation available of specific yieldo The average porosity of the rocks of the Ozark Plateau Aquifer in southeastern Kansas is estimated at five percent by the investigators of the Central Midwest Regional Aquifer System Analysiso Therefore, a value for specific yield of 0005 (equal to the average porosity) was chosen for the Ozark Plateau Aquifero Term (c): Economic Availability It is not possible to pump out all the water that would theoretically drain from rocks by gravity, nor is it economically feasibleo Estimates of the part of the volume of water in storage that is economically available range from less than 50 percent to as much as 85 percent 0 An intermediate value of two- thirds was chosen as the value for term "c" in equation (1), because it was used in the 1984 Kansas Water Supply and Demand Reporto