The Universal Soil-Loss Equation (USLE) is a widely used method for ca
lculating annual soil erosion losses, based on rainfall, runoff, slope
, runoff length, soil type, and landuse parameters. The equation was o
riginally developed on small agricultural plots, but has since been ad
opted for evaluating erosion from large watersheds under a wide range
of landuses. This study was designed to compare USLE calculations in a
GIS environment at grid sizes ranging from 30 x 30 m up to 6 x 6 km.
The analysis of two watersheds in Mississippi shows that large grid si
zes tend to underestimate soil losses. At grid sizes exceeding 100 x 1
00 m, a correction factor must be included in the calculations. With t
he use of the correction factor, the USLE can be applied directly at m
acroscales, providing an important tool for studies of basin-wide upla
nd erosion rates. The proposed method is shown to be applicable for la
rge watersheds under different climates. (C) 1998 Elsevier Science Ltd
. All rights reserved.