A Comparison of the R-factor in the USLE and RUSLE

Abstract

The R-factor in the Universal Soil Loss Equation /Revised Universal Soil Loss Equation (USLE/RUSLE) characterizes the climatic influence on the average rate of soil loss. The way in which the R-factor was calculated for RUSLE differs from that for the USLE. Rainfall intensity data at 6-min intervals from 41 long-term sites in the tropical region of Australia were analyzed to determine the discrepancy in the calculated R-factor as a result of using different unit energy equations and different rainfall thresholds. The mean annual rainfall varies from 261 to 4030 mm for the 41 sites. The calculated R-factor using the unit energy equation for the USLE is greater than that using the unit energy equation recommended for RUSLE. The typical difference is about 10% for the tropical region of Australia. The difference tends to increase as peak rainfall intensity decreases. The percentage difference in the R-factor due to different unit energy equations was found to be significantly correlated with the ratio of the R-factor to mean annual rainfall. The discrepancy in the calculated R-factor due to different rainfall thresholds increases as mean annual rainfall decreases because the relative contribution to the R-factor from small storm events increases in low rainfall areas. Lowering the rainfall threshold from 12.7 mm to 0.0 mm would on average increase the calculated R-factor by 5% for the same region. Relationships based on mean annual rainfall and the R-factor were developed so that the magnitude of the discrepancy in the calculated R-factor due to different unit energy equations and different rainfall thresholds can be readily assessed.