GeoSelim

Aquifers Properties: Specific Yield and Specific Retention 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, S y . 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. fig.1

Aquifers Properties: Porosity (n) The porosity of a soil or rock is that fraction of a given volume of material that is occupied by void space, or interstices. Porosity, indicated by the symbol ( n ), is usually expressed as the ratio of the volume of voids. Most rocks naturally contain a certain percentage of voids that can be occupied by water. Fig. 1 Porosity (n)