ON METHODS OF DETERMINING SPECIFIC YIELD

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. A water-balance method promulgated by these workers and used earlier by others is invalid because it overlooks a major component of the water budget. This method suggests erroneously that specific yield increase with time during a pumping test. It is shown that these specific yield values are consistent with water-balance considerations when all the components of the water budget are properly taken into account. Whereas the larger specific yields usually obtained from laborator 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. Additional results are discussed.