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.
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