A quasi-dynamic wetness index that accounts for variable drainage times since a prior rainfall event is derived from simple subsurface flow theory. The method is tested through a series of field observations and numerical experiments using a spatially distributed, dynamic hydrologic model. The quasi-dynamic wetness index is shown to be a useful extension of previously developed static indices for predicting the location of zones of soil saturation and the distribution of soil water (i.e., the soil water content overlying a shallow impermeable or semiimpermeable layer). The new index is not constrained by the steady state assumption that forms the basis of existing indices.