The total energy at a point in the aquifer. Sum effect of the elevation, the pressure, and the kinetic energy or velocity. Elevation and pressure are by far the most important factors. The water table and the head is depressed around a pumping well. The cone of depression expands until a dynamic equilibrium is reached.
Input equals output.
This may cause rivers to dry up, other wells or springs to go dry, etc. A major effect of pumping groundwater from aquifers can be land subsidence. As water is pumped from a aquifer, the pore pressure is reduced and the aquifer material can compress under its own weight. Leaning Tower of Pisa is a well known example. Mexico City is a much more drastic example.