Abstract
Delayed release of pore water from a pumped, unconfined aquifer is treated as a situation of restricted air movement in the vadose zone due to layers of high water content. This restricted movement produces below‐atmospheric pressures in the pore air above the water table, which cause the water table initially to drop faster than the dewatered zone. The initial yield thus is less than the full specific yield, which develops after a certain water table drop. Equations are derived that relate delayed yield to air entry value and height above water table of saturated layers in the vadose zone. These equations agreed with results from a delayed yield experiment on a vertical soil column and produced ratios between initial and final yield that resembled those calculated by others from actual pumping tests. An axisymmetric flow system, simulated by interconnected jars with narrow standpipes, produced delayed yield drawdown curves that were amenable to Boulton's analysis.
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