C-l. General. There are two basic types of pump­ing tests: equilibrium (steady-state flow) and nonequi­librium (transient flow).

a. Equilibrium-type test. When a well is pumped, the water discharged initially comes from aquifer stor­age adjacent to the well. As pumping continues, water is drawn from an expanding zone until a state of equi­librium has been established between well discharge and aquifer recharge. A state of equilibrium is reached when the zone of influence has become sufficiently en­larged so that: natural flow into the aquifer equals the pumping rate; a stream or lake is intercepted that will supply the well (fig. C-l); or vertical recharge from precipitation on the area above the zone of influence equals the pumping rate. If a well is pumped at a con­stant rate until the zone of drawdown has become sta­bilized, the coefficient of permeability of the aquifer can be computed from equilibrium formulas subse­quently presented.