The withdrawal of water from a well necessarily produces a drop in water‐level in the well. The ground‐water level in the vicinity of the well from which the water is withdrawn likewise declines, but the amount of decline decreases with increasing distance from the well, so that a cone of depression of the water‐surface in the vicinity of the well is produced. The cone of depression is an actual water‐surface if the ground‐water is not confined under pressure. If the ground‐water is under artesian pressure, the cone of depression is a depression in the piezometric surface. If the cones of depression of two or more wells ending in the same formation overlap, interference of the wells occurs. In this case (the combined yield of the wells when pumped simultaneously will be less than the sum of the individual yields if the wells are pumped separately. In choosing the proper spacing of wells from the operator's point of view, it is important to know the lateral extent of the cones of depression of supply‐wells ending in a given formation. In many installations two or more wells are so closely spaced that their mutual interference is excessive.
