Estimating WEPP cropland erodibility values from soil properties

Authors: ELLIOT, WILLIAM - Forest Service (FS); Flanagan, Dennis

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2022
Publication Date: 5/4/2023
Citation: Elliot, W.J., Flanagan, D.C. 2023. Estimating WEPP cropland erodibility values from soil properties. Journal of the ASABE. 66(2):329-351. https://doi.org/10.13031/ja.15218.
DOI: https://doi.org/10.13031/ja.15218

Interpretive Summary: Soil erosion is a serious land degradation process, that can negatively affect a soil’s ability to produce crops. Soil properties such as sand and clay fractions and amounts of organic matter can all affect the ability of a particular soil to resist detachment by raindrops and by flowing water. In this research results from an extensive set of field experiments at 36 locations with rainfall simulation and inflows of water to erosion plots were used to determine how erodible each soil was under standard conditions, and also if there were relationships between the measured erodibility parameters and other more common soil properties that are typically measured and available in soil databases. Numerous equations, with different levels of accuracy were developed, with the most complex equations having more required variables to produce the best erodibility predictions. A simplified set of equations was also developed in this report that provide better results than default parameterization functions currently in use with the Water Erosion Prediction Project (WEPP) model. These results impact other scientists, students, soil conservation personnel, and others involved with applications of the WEPP model. The improved equations should result in more accurate soil erosion predictions with WEPP, and better assessments of soil loss and sediment delivery from agricultural lands.

Technical Abstract: In the late 1980s, the USDA Agricultural Research Service along with other federal agencies and multiple universities collaborated to develop a new physically-based soil erosion model, the Water Erosion Prediction Project (WEPP) Model. The WEPP model was intended to replace the Universal Soil Loss Equation and was to include estimates for upland runoff and erosion, sediment delivery to first order channels, and runoff and sediment routing through a downstream channel network. The WEPP technology estimated erosion from raindrop splash and sheet flow (interrill erosion) and concentrated channel flow (rill erosion). To make these erosion estimates, WEPP required new soil erodibility values for interrill erodibility (Ki), and rill erodibility (Kr) and critical shear (TAUc) for concentrated flow erosion. The WEPP Core team determined that they needed to estimate these three erodibility values from soil properties for a wide range of soil conditions. To develop relationships between WEPP soil erodibility parameters and other measurable soil properties, an experimental field study was carried out using rainfall and runoff simulation to measure the three erodibility values for 36 soils. Sites were identified on croplands from Washington to Georgia and Maine to California for erodibility measurement. Concurrently, the USDA Soil Conservation Service (SCS) carried out detailed soil surveys and laboratory analyses for all sites to provide a large database of soil physical, chemical and engineering properties. Correlation and regression analyses were carried out to develop relationships between SCS measurable soil properties and WEPP soil erodibility values. This article provides a summary of those analyses, and subsequent predictive equations that were developed. The predictive equations that were finalized in the WEPP User Summary used sand, very fine sand, clay, and organic carbon contents to predict cropland soil erodibility, but the Coefficient of Determination (r^2) values were 0.34 or less. More complex predictive equations were developed with soil physical, chemical, mineralogical and geomorphic properties with r^2 values up to 0.79. Most of the better predictive equations included terms for soil texture, and clay mineralogy, often with additional chemical properties. A set of simplified erodibility equations using only the readily available properties of soil texture, organic carbon, cation exchange capacity, slope steepness, and taxonomic order were derived for use within the WEPP Model with r^2 values greater than 0.5 for all three equations for estimating soil erodibility from measurable soil properties.