Determining Yield of a Well: 2 Methods | Ground Water | Water Engineering

The measurement of an open well yield can be determined by the following method: 1. Actual Pumping Method 2. By Theoretical Calculations.

1. Actual Pumping Method:

The yield of well is determined by pumping out water from it. In this method first the water is withdrawn from the well at very high rate, then the rate of pumping is reduced and adjusted by the trail and error such that the water level in the well remains constant.

At this point the rate of percolation inside the well and the rate of pumping, both become equal. The rate of pumping is determined, which is equal to the yield of the well. Generally this test is done during summer to determine the yield of the well in dry weather.

Sometimes the yield of the well is determined by determining the rate of percolation inside the well. In this method first of all the water level inside the well is depressed considerable by pumping at very high rate. Now the pumping is stopped and the rate of rise in water level inside the well is actually determined by calculating the quantity of water percolating inside the well. This method is generally known as Recuperating Test Method.

The specific yield or specific capacity of a well in recuperating test can be determined by the following formula:

Where C’/A = Specific yield

S₁ = Depression head in the well at the time immediate after the pumping was stopped.

S₂ = Depression head in the well at time T after the pumping was stopped.

T = Time after which the measurement S¹₂ was taken.

Knowing the value of C’/A the discharge Q of the well can be determined by the following formula:

Where, A = cross-sectional area of the well

S = depression or yield of the well

Q = discharge or yield of the well.

2. Theoretical Calculation:

The approximate quantity of water entering or percolating in the well can be calculated theoretically as under:

If, A = Area of aquifer opening into well

v = Velocity of the water percolating in the well

β= Permeability constant

Q = Yield of the well.

Then Q = A.v.β ………………(4.9)