Erosion On Rangelands PDF Download

Soil erosion presents a multi-faceted problem in rangelands, causing loss of fertile topsoil—often at the expense of forage production—and compromising downstream water quality. Removal of vegetative cover by grazing increases susceptibility to erosion. However, the diversity of California’s landscape complicates the relationship between grazing intensity and erosion, making the effectiveness of uniform grazing recommendations inconsistent. This study attempted to elucidate the relationship between grazing intensity and erosion by providing maps of erosion risk across California’s annual grass rangelands under three grazing scenarios using the Revised Universal Soil Loss Equation (RUSLE). Scenarios were defined by residual dry matter (RDM) ranges, and were heavily grazed (HG; 600 kg/ha−1 RDM), moderately grazed (MG; 800-1000 kg/ha−1 RDM), and lightly grazed (LG; 1100 kg/ha−1 RDM). We developed geospatial layers for each of the RUSLE factors—rainfall erosivity, soil erodibility, slope length and steepness, and vegetative cover—and overlaid them to estimate erosion potential under each scenario at a 30-m spatial resolution. Vegetative cover was varied by grazing scenario, and was based on relationships between residual dry matter (RDM) and vegetative cover established from field data. We found that erosion potential was extremely variable spatially. Erosion potential was low in most areas, but extremely high in some cases. Statewide mean erosion potential for HG was 0.27 Mg ha−1 yr−1, for MG was 0.19 Mg ha−1 yr−1, and for LG was 0.12 Mg ha−1 yr−1. Likewise, small land portions of areas modeled were in excess of soil loss tolerance values established by the Natural Resources Conservation Service (NRCS). Portion of rangelands in which erosion potential exceeded soil loss tolerance was 1.2% (70,700 ha) under HG, 0.5% (32,200 ha) under MG, and 0.2% (11,700 ha) under LG. To assess model performance, results were compared to measured soil loss values from the field, and to measured suspended sediment from two streams draining rangeland watersheds. Though there were discrepancies, we found that all modeled scenarios were within an order of magnitude of measured values. Findings suggested that managing grazing to mitigate erosion is most effective when targeted toward areas of high vulnerability. We hope that this study will promote management decisions that protect soils and sustain rangeland productivity.