Agroforestry for Soil Conservation - World Agroforestry Centre

28 Agroforestry for Control of Soil Erosion
as a low-yielding maize or tobacco, may have a C factor as high as 0.8,
meaning that erosion is not much less than on bare soil.
On the other hand, a dense cover crop or perennial crop (e.g. well-maintained
tea) can have a C value of the order of 0.01 and natural rain forest
as low as 0.001, meaning that erosion is one hundredth and one thousandth
as fast, respectively, as on bare soil under the same climate, soil and slope.
P, the support practice factor, is defined as the ratio of soil loss with a
given conservation practice to that under crops in rows running up and
down the slope. It is only meaningful where such practices are standardized
and closely defined. For the examples given in the US handbook, practices
which leave the slope as it is, such as strip cropping, have P factors of 0.4
or more in most circumstances; that is, they may reduce erosion by about
half. Well-maintained terracing can produce P values in the region of 0.1
to 0.05.
The USLE should be used with caution in the tropics, where its predictions
do not always seem realistic. Results for humid climates and steep
slopes are extremely high; for example, cereal cultivation on a 20° slope
in the humid tropics leads to values of the order of R = 1000, K = 0.2,
LS = 16 and C = 0.4, giving a predicted erosion of 1280 t/ha/yr, or about
10 cm of soil thickness.
The most significant feature of this model is the very high potential for
reducing erosion by management practices which lead to greater soil cover.
The Soil-Loss Estimator for Southern Africa (SLEMSA) (Elwell, 1980,
1981; Stocking and Elwell, 1981; Stocking, 1981). The model has the same
objective as the USLE, to predict erosion at a specific farm site, as a basis
for land-use planning. It was designed and calibrated specifically for southern
Africa, and has been adapted to the mapping of erosion hazard over
large areas (Stocking, 1987).
Soil loss, Z, in t/ha/yr, is given by the equation:
Z = K x C x X.
K is the soil loss from bare soil on a standard plot 30 m long with a 4.5%
(2.6°) slope. It is derived from an equation in which the variables are E,
the rainfall energy in J/m 2 , and F, the soil erodibility index.
C is the crop ratio, which adjusts soil loss from a bare fallow to loss
under the crop grown. C is a function of i, the percentage of rainfall energy
intercepted by the crop cover. When even 20% of rainfall energy is intercepted,
the value of C is reduced to 0.3, whilst with 40% energy intercepted,
C becomes 0.1 and at 50%, about 0.05.
X, the topographic ratio, is a function of S, slope steepness, and L, slope
length. Its values are very similar to those of the LS factor in the USLE.
Thus there are 5 basic control variables: E, rainfall energy, F, soil erodibility,
i, energy interception by the crop, S, slope steepness, and L, slope
length. These give rise to three intermediate variables, K, soil loss from