A comparison of the R-factor in the universal soil loss equation and revised universal soil loss equation

The R-factor in the Universal Soil Loss Equation/Revised Universal Soil Los s 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 t o determine the discrepancy in the calculated R-factor as a result of using different unit energy equations and different rainfall thresholds. The mea n annual rainfall varies from 261 to 4030 mm for the 41 sites. The calculat ed R-factor using the unit energy equation far the USLE is greater than tha t using the unit energy equation recommended for RUSLE. The typical differe nce is about 10% for the tropical region of Australia. The difference tends to increase as peak rainfall intensity decreases. The percentage differenc e in the R-factor due to different unit energy equations was found to be si gnificantly correlated with the ratio of the R-factor to mean annual rainfa ll. The discrepancy in the calculated R-factor due to different rainfall th resholds increases as mean annual rainfall decreases because the relative c ontribution to the R-factor from small storm events increases in low rainfa ll areas. Lowering the rainfall threshold from 12.7 mm to 0.0 mm would on a verage increase the calculated R-factor by 5% far the same region. Relation ships 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 differe nt unit energy equations and different rainfall thresholds can be readily a ssessed.