Specific yield
In case of unconfined aquifer, storativity of an aquifer can be defined as the volume of water released or added to the aquifer from a unit area under unit declination or rise in water table. In this case, the storage coefficient is called as specific yield. Fig. 7.2 shows an unconfined aquifer with horizontal area ‘A’. The initial water table is at C. VW is the amount of water withdrawn from the aquifer. As a result of pumping, the water table is drop down by an amount of Δh. In this case, the specific yield can be written as,
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| Fig.7.2 Sketch to explain aquifer storativity of unconfined aquifer |
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(7.5) |
It may be noted that a certain amount of water is always retained in the aquifer due to the capillary and hygroscopic forces which is known as specific retention (Sr). As such,
Sy + Sr = η |
(7.6) |
Where, η is the porosity of the porous matrix. The specific yield is therefore always less than the porosity of the porous media. Specific yield is also sometime called effective porosity.