VARIABILITY IN GREEN-AMPT EFFECTIVE HYDRAULIC CONDUCTIVITY UNDER FALLOW CONDITIONS

Hydraulic conductivity of the soil matrix dynamically responds to chan ges in the surrounding environment. Therefore, infiltration parameters for the Green-Ampt equation should change for each storm event in con tinuous simulation models. This study focused on improving Water Erosi on Prediction Project (WEPP) model estimates of runoff using over 220 plot-years of natural runoff plot data from 11 locations. By optimizin g the effective Green-Ampt hydraulic conductivity, K-e, for each event within the simulation, a method of correlating hydraulic conductivity on any given day to many other parameters was established. Factors wi th significant correlation to optimized values of K-e fell into three distinct categories; (1) factors related to soil crusting and tillage; (2) factors related to event size; (3) factors related to antecedent moisture conditions. Equations were developed to represent the tempora l variability of hydraulic conductivity for each group. The equation d escribing the decrease in hydraulic conductivity owing to crusting use d an exponential decay function based primarily on cumulative rainfall kinetic energy since last tillage, a soil stability factor, and a cru st factor. The relationship between hydraulic conductivity and event s ize was characterized using an exponential relationship with total rai nfall kinetic energy. The final adjustment used the moisture content i mmediately below the infiltration zone to account for the influence of antecedent moisture conditions on optimized hydraulic conductivities. All three adjustments were incrementally incorporated into WEPP and e ach improved the average model efficiency.