In an aquifer with a water table (unconfined aquifer), the volume of water released from groundwater storage per unit surface area of aquifer per unit decline in the water table is known as the specific yield, Sy. also known as the drainable porosity. Hydrologists divide water in storage in the ground into the part that will drain under the influence of gravity (called specific yield) and the part that is retained as a film on rock surfaces and in very small openings (called specific retention). The physical forces that control specific retention are the same forces involved in the thickness and moisture content of the capillary, fringe.
Specific yield tells how much water is available for man’s, use, and specific retention tells how much water remains in the rock after it is drained by gravity.
n =Sy +Sr
Sy = Vd / Vt , Sr = Vr / Vt
where n is porosity, Sy is specific yield, Sr is specific retention, Vd is the volume of water than drains from a total volume of Vt, Vr is the volume of water retained in a total volume of Vt, and Vt is total volume of a soil or rock sample.
The following table gives some values of total porosity and specific yield for various reservoirs:
In unconfined aquifers, the storativity is almost exactly equal to the specific yield. The most reliable method of calculating the storage coefficient (S) and/or the specific yield (Sy) is to carry out test pumping.
Reference and further reading:
ACTION CONTRE LA FAIM, 2005, Water, sanitation and hygiene for populations at risk, HERMANN ÉDITEURS DES SCIENCES ET DES ARTS, P 78.
Heath, Ralph C., 1983, Basic ground-water hydrology: U.S . Geological Survey Water-Supply Paper 2220, p 8-9.