Evaluating the reliability of specific-yield determinations

The specific yield of the alluvial aquifer in the Gila River flood plain in southeastern Arizona has been determined using two methods of analysis - the time-drawdown method and the soil-moisture-content method. Time-drawdown data measured at 17 observation wells during a 3.5-day aquifer test define an average apparent specific yield of 0.13. Soil-moisture-content data measured at nine access holes during the aquifer test indicate that complete gravity drainage had not been attained in the cone of depression by the end of the drawdown period. The moisture-content data were therefore extrapolated with time to define an average specific yield in the range 0.13 to 0.15. The results obtained by the two methods arc in close agreement. However, the significantly lower standard deviation of the results from the moisture-content analysis indicates that extrapolation of the apparent values derived by this method may provide a more reliable estimate of the true specific yield than the apparent values derived by the time-drawdown method. Reliable estimates of specific yield in the zone of seasonal water-level fluctuations are also possible from an evaluation of the soil-moisture change in the spring and summer recession period.