Thomas Kwader explains why rarely does the piezometric map reflect that all the ground water in a particular area is being drawn to and captured by the pumping wells.
Rarely does the piezometric surface reflect this phenomena for a number of reasons. The water level surface often has a very small slope toward the pumping well which may not be evident if the contour interval is not fine enough, i.e. measured to 0.1 foot or less. The more transmissive the aquifer, the smaller the slope will be at a given pumping rate, i.e. a lake (high transmissivity) will have a very small, probably immeasurable, sloping toward the spill way or culvert if present. On the other hand, a tight clay aquifer will have a steep slope in order to induce movement of ground water toward the pumping well. The steep slope is needed to overcome the friction and capillary forces impeding the horizontal flow.