In this article we will discuss about the calculation of ground water yield with formula.
When the water enters in the ground, it starts filling its pores. When all the voids of pores are filled with water, the soil is known as saturated soil or saturated formation. When the ground water soaked by the soil is drained or taken out by gravity-drainage, it is found that the quantity of water so obtained is less than the volume of the voids inside the soil.
As some quantity of water is retained by the soil particles due to their molecular attraction, and it is not possible to drain out the whole quantity of water, merely by gravitation force. The water so retained in the soil particles is known as pellicular water.
Ground water yield is the quantity of water extracted by gravity-drainage from a saturated water bearing stratum. The ratio of the volume of water obtained by gravity-drainage to the total volume of the sub-soil is known as specific-yield.
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Mathematically,
The ratio of the quantity of water retained by the material against gravity drainage to the total volume of the material or sub-soil is known as the specific retention or field capacity of the soil.
Mathematically,
Now mathematically
Porosity = Specific yield + Specific retention
The amount of the retention water depends upon the total interstitial surface of the particles. If the surface of the particles is more, more will be the retention power, and if it less, less will be the retention capacity.
The specific retention capacity of clay having fine soil particles is more, but specific yield is less. When the size of the soil particles increases the specific retention power decreases, whereas the specific yield increases.
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From the above it is true that for good water yield from the soil, its formation should be of coarse gravel. Hence the yield of the wells mainly depends on the formation of the nearby soil formation.