A QUASI-DYNAMIC WETNESS INDEX FOR CHARACTERIZING THE SPATIAL-DISTRIBUTION OF ZONES OF SURFACE SATURATION AND SOIL-WATER CONTENT

A quasi-dynamic wetness index that accounts for variable drainage time s since a prior rainfall event is derived from simple subsurface flow theory. The method is tested through a series of field observations an d numerical experiments using a spatially distributed, dynamic hydrolo gic model. The quasi-dynamic wetness index is shown to be a useful ext ension of previously developed static indices for predicting the locat ion of zones of soil saturation and the distribution of soil water (i. e., the soil water content overlying a shallow impermeable or semiimpe rmeable layer). The new index is not constrained by the steady state a ssumption that forms the basis of existing indices.