Abstract: The unsteady flow pumping tests of completely penetrating wells in a confined aquifer with infinite areal extent were studied.The relationship between the drawdown and the time variations defined by the Theis formula was treated as the theoretical true values of the water level changes outside the observation well.The water level changes which were measured inside the observation well were derived theoretically,considering the time lag factor induced by the equalization of hydrostatic differences inside and outside the observation well.The relative errors between the values measured and the theoretical true values were analyzed.The observation time required for those values approaching the true values was also examined.