Page 134 - Biosystems Engineering

Soil and W ater Conservation 115

worldwide in estimating annual soil loss from small fields. The USLE
was developed from more than 10,000 plot-years of basic runoff and
soil loss data. It was originally described in The Agriculture Handbook
(No. 537) published in 1965 and later revised in 1978 (Wischmeier and
Smith 1978). Since then, USLE has been widely used for major conser-
vation planning in the United States and around the world. Although
the core formulation of USLE has remained the same, continued
research and additional experiments have resulted in improvements of
the determining factors and this new version was renamed RUSLE.
In 1997, the U.S. Department of Agriculture published Handbook
No. 703, which describes the RUSLE model comprehensively (Renard
et al. 1997). A Windows-based version of the RUSLE model is available
through the Natural Resources Conservation Service (NRCS), called
RUSLE2 (Foster et al. 2000).
Both RUSLE and USLE provide estimation of average annual soil
loss based on the following simple equation:

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⋅
⋅
⋅
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A = R K L S C P (3.27)
where A is the average annual soil loss given in tons per acre. Descrip-
tions of each parameter are provided as follows.
R This is the rainfall–runoff erosivity factor It is derived from long-
term rain gauge data and is the kinetic energy of rainfall multiplied
by 30-minute maximum rainfall intensity. Storms having total vol-
umes less than 0.5 in (12.7 mm) often do not generate surface runoff.
Therefore, such small events are, in general, not considered in com-
puting R. The R factor shows great variations by location due to spa-
tial variation of rainfall. Renard et al. (1997) provide detailed maps of
the continental United States for the R factor. Lal (1994) also provided
worldwide estimates of the R factor.

K This factor is relevant to soil and is called the soil erodibility fac-
tor. This factor aggregates many features of soil that could affect
soil erodibility, such as soil runoff potential, susceptibility of soil to
erosion, and soil transport properties. Both fine- (e.g., clay) and
coarse-textured (e.g., sand) soils have relatively small K values
(0.05 to 0.20). Soils high in clay content show great resistance to soil
detachment, although they have high runoff generation potential.
On the contrary, sandy soils have very low runoff generation poten-
tial even though these soils can be easily detached. Soils rich in silt
content have the highest K values (>0.4). Soils high in organic mat-
ter content show more resistance to erosion. Renard et al. (1997)
tabulated values of K values for various soils. They also provided a
regression-based equation to estimate K values as a function of
organic matter content, soil structure, soil permeability, particle
size diameter, and so on.