| The accurate estimation of soil erosion by the Revised Universal Soil Loss Equation version 2 (RUSLE2) is critical for several conservation assessments, least of which is its use in the Phosphorus Index (PI) to identify and rank the vulnerability of agricultural fields to phosphorus (P) runoff. Earlier versions of RUSLE reported a soil loss overestimation, which were revised to give RUSLE2, where biomass production in different climatic regions was more accurately represented. RUSLE version 2.0, which contains the new vegetative biomass production routine, was evaluated using two performance indices, the Nash Sutcliffe Efficiency Index (NSE) and Index of Agreement (D) across 27 cattle grazed fields in Southeastern U.S. An overall NSE of -0.164 and D of 0.242, indicated RUSLE2 poorly predicts soil loss for this region. Further investigation was needed to understand the reason for these poor soil loss estimates by RUSLE2. RUSLE2 estimates of soil loss are based on Hortion overflow sediment delivery from daily storm events accrued to an annual soil loss along a given field slope. Compared with measured sediment delivery from seven tall fescues (Festuca arundinacea) fields in northwest Arkansas over five years, with various manure and grazing management, sediment delivery estimated by RUSLE2 was acceptable, with log NSE (1.4). However, RUSLE2 over-predicted the number of storm events between 2009 - 2013 for all seven fields, from field collected rainfall- intensity data which created the localized 5- years erosivity values. Over-prediction on the number of storm events would lead to an increase in annual soil loss estimate. A need for a lower restrictive rainfall threshold value that does not initiate field runoff, and in turn, sediment delivery, particularly in grassland system, needs to be incorporated into RUSLE2 soil loss estimates. |
