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United States Department of Agriculture

Agricultural Research Service

Title: Estimation of Green-Ampt Effective Hydraulic Conductivity for Rangelands

Authors
item Kidwell, M. - UNIV. OF ARIZ.
item Weltz, Mark
item Guertin, D. - UNIV. OF ARIZ.

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 27, 1996
Publication Date: N/A

Interpretive Summary: The average rate at which water infiltrates into the ground is called effective hydraulic conductivity (Ke). This is an important value for the prediction of infiltration and runoff volume from rain storms in the Water Erosion Prediction Project (WEPP) model. Two equations, developed from cropland data, have previously been used to calculate Ke in WEPP. Using rangeland data, both equations resulted in under-prediction of Ke and over through the model and resulted in poor estimates of peak discharge and sediment yield. The objective of this study was to develop a new predictive equation to calculate Ke specifically for use on rangelands. A distinction was made between the amount of ground cover located outside and underneath plant canopy to account for the tremendous spatial variability that exists on most rangelands. Field data collected in 8 western states on 15 different soil/vegetation complexes including measured soil, canopy and ground cover data were used to develop the equation. Predicted values of K from the new equation compared well with optimized values that were determined using the WEPP model and observed runoff data. The new equation contains three terms representing the distribution of ground cover, indicating the importance of considering spatial variability in modeling hydraulic conductivity.

Technical 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 is sensitive to the hydraulic conductivity parameter in the prediction of runoff. Two sets of algorithms developed from cropland data to predict Ke have previously been used in the WEPP model. When tested with rangeland data, 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 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 located outside and underneath plant canopy to account for the tremendous spatial variability that occurs on most rangelands. Optimized K values were determined using the WEPP model and observed runoff data. Regression models were then developed to predict Ke using measured soil, canopy cover, and spatially distributed ground cover data. The coefficient of determination of the predicted to optimized Ke was 0.60. New data sets are now required to test the new equation to determine how well the relationships developed in this study extend to other rangeland areas.

Last Modified: 12/18/2014
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