In a well, drilled to an unconfined aquifer, the water will always stand in it at the level of the water table when the pump is not operating. As soon as the pump starts, the water level in the well starts dropping. The distance by which the level in the well is lowered below the water table by pumping is called the ”drawdown”. Around the well, the water table assumes a form, which is comparable to an inverted cone. This cone-shaped water table around the pumping well is known as the ”cone of depression”, as shown in the following diagram.
Frequently, the cone is found elliptical rather than circular in cross-section, because the material of the aquifer is seldom homogeneous, and the water table is usually inclined. The distance from the pumped well up to the outer limit of the cone of depression on the ground surface is termed as ”radius of influence”.
As pumping is continued at a constant rate, the drawdown increases and the radius of influence expands until the flow of water from the aquifer into the well becomes equal to the withdrawal by the pump. An increase or decrease in the rate of pumping will have a corresponding increase or decrease in the amount of drawdown. The relation of drawdown to the yield of a formation both for artesian and non-artesian wells is shown under;
The figure shows that the maximum yield (about 76%) can be obtained with a drawdown of about 50 % in the non-artesian wells.
When the pumping is stopped, the water from the surrounding formation rushes to the well, and after sometimes the water table restores to its normal level.
The permeability of an aquifer affects:
- the rate of recovery of the water table after pumping.
- the shape of the cone of depression.
In an aquifer having low permeability, the pumping will cause a great drawdown with a smaller radius of influence. On the other hand, a well pumping at the same rate in a permeable aquifer would cause a cone of depression that has a small drawdown and a relatively large radius of influence.