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The following tests may be performed to get an idea of the probable yield of the well:
1. Pumping Test:
The water level is depressed to an amount equal to the safe working head for the subsoil. Then the water level is kept constant by making the pumping rate equal to the percolation into the well. The quantity of water pumped in a known time gives an idea of the probable yield of the well of a given diameter. The test may be carried out on an existing open well. In hard rock areas if D = diameter of the well; d – depth of water column; Q = pumping rate; and t = time required for emptying the well; then the volume of water stored in the well.
2. Recuperation Test:
The water level in the well is depressed by an amount less than the safe working head for the subsoil. The pumping is stopped and the water level is allowed to rise or recuperate. The depth of recuperation in a known time is noted from which the specific yield of the well is calculated as follows.
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If the water level inside the rises from s1 to s2 (measured below S.W.L.) in time t and if s is the drawdown at any time t, Fig. 5.55, from Darcy’s law-
Q = k A i = KA (s/L) = C A s
Where s is the head loss in a length of flow path L and C is a constant = K/L which can be determined after integrating-
Q dt = – A ds
The negative sing indicates a decrease in drawdown or depression head-
C is called the specific yield of the well per unit cross-sectional area per unit depression head. If A is the cross-sectional area of the well, the specific capacity of the well (Slitcher)
This equation assumes vertical impervious well staining and flow into the well only from the previous bottom. C has a dimension of 1/T and has values of 0.25 for clay, 0.50 for fine sand and 1.00 for coarse sand, all per hour.