Date of this Version
Ground Water, Vol. 45, No. 3, May-June 2007; doi: 10.1111/j.1745-6584.2006.00274.x
Considerable attention has been given in recent years to debunking the water budget myth that equates the safe or sustainable yield of ground water with recharge. There is another water budget myth that occasionally appears in water resources planning—that the volume of recoverable ground water in storage for a particular area or aquifer (i.e., the product of the area, saturated thickness, and specific yield) is by itself meaningful in analyses of water availability.
At the global scale, consider the commonly cited statistic that ground water comprises more than 95% of the nonfrozen fresh water on earth. Although this statistic illustrates the value of ground water as a reservoir, it also is misleading in that it implies much more water is available in the global ground water pool than is realistically recoverable, and it overlooks the large spatial variations in storage and transmissive properties and in water quality from location to location.
At the local or regional scale, estimates of the useful life of an aquifer are sometimes derived by dividing an estimate of recoverable water in storage by an estimate of annual ground water consumptive use. Aside from uncertainties in the numerator and denominator, the resultant estimate is only potentially useful in a ground water mining situation where recharge is minimal and no other effects beyond depletion of aquifer storage are of concern. Where the recharge is significant, this estimate grossly underestimates the useful life of the aquifer.
As a practical matter, it is impossible to remove all water from storage with pumping wells. The use of specific yield in calculations of the recoverable ground water in storage takes account of water retained in the rock matrix by capillary forces, but many other factors limit the amount of water actually ‘‘recoverable.’’ The aquifer’s permeability, water quality, the cost of drilling wells, the cost of lifting water, and the design of the well and pump all limit the volume of water that is usable in practice. Slow leakage from confining units and water quality changes make it particularly difficult to relate estimates of the volume of ground water in storage to the usable volume of ground water in confined aquifers.