USGS Groundwater Information
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Water-Table Fluctuation (WTF) Method
Estimating Specific Yield (Sy)
“The specific yield (Sy) of a rock or soil, with respect to water, is the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume” ( Meinzer 1923). The following formula is generally used as an operational definition of Sy:
Sy = f - Sr (1)
where f is porosity and Sr is specific retention (the volume of water retained by the rock per unit volume of rock). Specific yield is treated as a storage term, independent of time that in theory accounts for the instantaneous release of water from storage. In reality, the release of water is not instantaneous. Rather, the release can take an exceptionally long time, especially for fine-grained sediments. The fact that Sy is not constant but varies as a function of depth to the water table is described from the perspective of soil physics by Childs (1960).
Estimates of Sy for application in the WTF method typically are derived from laboratory or field approaches. Laboratory approaches usually involve column-drainage experiments or determination of the moisture-retention curve for a soil. Field approaches include aquifer tests, water budgets, geophysical surveys, and field-capacity tests. The reader is referred to Healy and Cook (2002) for a detailed discussion on methods for estimating Sy.