Page 247 - Soil and water contamination, 2nd edition

234 Soil and Water Contamination

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-1
where E = the splash erosion (g s m ), A = the aggregate stability (median number of drops
s s
required to decrease the aggregate by 50 percent), K = rainfall or throughfall kinetic energy
e
-2
(J m ), H = the depth of the water layer on the soil surface (mm), P = the amount of rainfall
(mm). The kinetic energy of free rainfall and leaf drainage from the plant canopy can be
estimated from:
K . 8 95 . 8 44 log(I )
e ,r (12.24)
K e ,l 15 8 . h . 5 87
-2
-2
where K = rainfall kinetic energy (J m ), K = kinetic energy of leaf drainage (J m ), I = the
e,r e,l
-1
rainfall intensity (mm h ), and h = the height of the plants (m).
12.6 LONG-TERM SOIL EROSION AND DEPOSITION
The physically-based soil erosion models mentioned above predict the sediment losses and
gains at the time scale of a single runoff event. These models are less adequate at predicting
sediment transport in the long term because upscaling from event-based predictions to long-
term, e.g. annual, predictions is time-consuming and prone to uncertainties. A more effective
way to predict long-term soil erosion on slopes is to use empirical models that predict soil
erosion and deposition on the basis of topography and soil properties.
One of the most popular simple, empirical soil erosion models is the Universal Soil Loss
Equation (USLE ), which is based on statistical analysis of soil erosion data collected from
small soil erosion plots in the USA (Wischmeier and Smith, 1978). The USLE predicts
soil loss on a slope by multiplying a series of numbers, each representing a key factor
contributing to soil erosion. The USLE in formula form is as follows:

E = R · K · L · S · C · P (12.25)

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-2
where E = the mean annual soil loss [M L T ], R = the rainfall erosivity factor, K = the
soil erodibility factor, L = the slope length factor, S = the slope steepness factor, C = the
crop management, and P = the erosion control practice factor. The slope length L and slope
steepness factor S are combined to produce a single index LS that represents the ratio of soil
loss on a given slope to the soil loss from a standard erosion plot 22 m long and with a slope
of 5°, for which LS = 1.0. The value of LS can be estimated from:
n
l 2 ) (12.26)
LS ( 065.0 + . 0 045s + . 0 0065s
22 . 13
where l = the slope length (i.e. the horizontal distance from the divide or field boundary)
(m) and s = the slope gradient (%). The value of n is varied according to the slope gradient.
Morgan (1995) provides the details for the other factors in the USLE . A well-known
disadvantage of the USLE is that the model is less appropriate for sites in Europe because
of the differences in climate and soil between the USA and Europe. In particular, the model
should not be used to determine the soil erodibility factor K of European loess soils without
first carrying out some fundamental modifications.
Another model to predict annual soil loss from field-sized areas on slopes is the Revised
Morgan, Morgan and Finney method (Morgan et al., 2001). This model separates the soil
erosion process into a water phase and a sediment phase. The water phase is considered in
order to calculate the kinetic energy of the rainfall and the volume of overland flow ; these
are required in order to be able to predict the detachment of soil particles by raindrop impact

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