On Methods of Determining Specific Yield

Abstract

A comparative discussion of several methods for the determination of specific yield is presented. The discussion relies on a pumping test conducted recently by Nwankwor and coworkers from the University of Waterloo in an unconfined aquifer at Borden, Ontario, Canada. We show that a water-balance method promulgated by these workers and used earlier by others, including Wenzel in 1942 and 1946, is invalid because it overlooks a major component of the water budget. This method suggests erroneously that specific yield increases with time during a pumping test. The method can therefore lead to arbitrarily large specific yield values which may lack physical significance. By relying on this water-balance method and on laboratory drainage experiments, Nwankwor and coworkers conclude incorrectly that type-curve methods, such as those proposed by Prickett in 1965 and Neuman in 1975, give unreasonably low specific yield values. We show instead that these specific yield values are consistent with water-balance considerations when all the components of the water budget are properly taken into account. We further point out that whereas the larger specific yields usually obtained from laboratory drainage experiments may be well suited for the evaluation of ground-water reserves that can potentially be recovered from an unconfined aquifer over long periods of time, they are not directly relevant to the problem of relating ground-water level variations to pumpage which is characterized by a shorter time scale. This is especially true when specific yield is taken to be the difference between water content at saturation and residual water content at high suctions as done by Nwankwor and coworkers. The rate at which ground-water levels drop or fluctuate in response to pumpage is controlled by the smaller specific yield that one obtains from time-drawdown analyses.