The influence of delayed drainage on data from pumping tests in unconfined aquifers

Equations are derived for the flow to a pumped well in an aquifer having uniform anisotropy and overlain by a low-permeability aquitard. The water-table is assumed to be located in the aquitard. Drainage from the capillary zone above the water-table is taken into account.The differential equation for the flow in the aquifer is identical with that derived in a previous paper. The formation constants may therefore be evaluated by using type curves as described in that paper.A well-known pumping test is reanalysed, using the equations in the present paper. It is shown that the time-drawdown curves can be explained only by the existence of a low-permeability stratum in the vicinity of the water-table. In this example the slow draining of the unsaturated zone above the water-table seems to be a significant factor in determining the shape of the time-drawdown curves.