Estimation of green-Ampt effective hydraulic conductivity for rangelands.

Abstract

Effective hydraulic conductivity (Ke) is an important parameter for the prediction of infiltration and runoff volume from storms. The Water Erosion Prediction Project (WEPP) model, which uses a modified Green-Ampt equation, is sensitive to the hydraulic conductivity parameter in the prediction of runoff volume and peak discharge. Two sets of algorithms developed from cropland data to predict Ke have previously been used in the WEPP model. When tested with data collected on rangelands, these equations resulted in low predictions of Ke which significantly over-estimated runoff volume. The errors in runoff prediction were propagated through the model and resulted in poor predictions of peak discharge and sediment yield. The objective of this research was to develop a new predictive equation to calculate Ke specifically for use on rangelands using field data collected in 8 western states on 15 different soil/vegetation complexes. A distinction was made between ground cover parameters located outside and underneath plant canopy in an effort to account for the significant spatial variability that occurs on most rangelands. Optimized Ke values were determined using the WEPP model and observed runoff data. A regression model (r2=0.60) was then developed to predict Ke using measured soil, canopy cover, and spatially distributed ground cover data from 44 plots. Independent rangeland data sets are now required to test the new equation to determine how well the relationships developed from the data used in this study extend to other rangeland areas.