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Specific Yield :
Groundwater fills all of the interstices in the saturated zone, hence the porosity
is a direct measure of the water contained per unit volume. Not all of this water
may be removed from the ground by drainage or pumping from a well, however,
as molecular and surface tension forces will hold a portion of the water in place.
Thus, retained water is that held in place against gravity. The specific retention
of a rock or soil is the ratio expressed as a percentage of the volume of water it
will retain after saturation against the force of gravity to its own volume. If Sr is
the specific retention, then
Sr = 100Wr
V
Where Wr = volume occupied by retained water, and V = Gross volume of the
rock or soil.
The water on the other hand, which can be drained is expressed as the specific
yield, Sy. It can be defined as the ratio expressed as a percentage of the volume
of water which after being saturated, can be drained by gravity to its own
volume.
Therefore,
Sr = 100Wy
V
Where Wy is the volume of water drained. Because Wr + Wy = W, it is
apparent that :
I I = Sr + Sy.
Thus, specific yield is a fraction of the productivity of an aquifer. Values depend
upon grain size, shape and distribution of pores, and compaction of the
stratum.
