The basic limitation of the above equation is that it neglects the subsurface inflow and outflow and assumes that every inflow and outflow is uniformly distributed over the area. This may be approximately true for the rainfall and even for the return flow from irrigation but it is rarely true for the abstraction from the aquifer. When pumping is reduced or ceases during the rainy seasons, a redistribution of ground water heads occurs so that part of the observed increase in water level may be due to normal well recovery. Moreover, the above equation is dependent on the value of the specific yield, which is difficult to determine since the water table fluctuation occurs in the partially saturated zone.