Topographic Factor (LS)
The (LS) factor reflects the combined effect of slope length and slope
steepness on erosion. The empirical equation developed by Wischmeier &
Smith (1978) as follow.
LS = (L/22.15) × (65.41 sin2S +4.56 sin S+0.065) (2)
For practical reasons of calculation under GIS, we have adopted the
following formula: LS = Pow ([flowacc] * resolution / 221.04) * Pow
(Sin [slope] * 0.01745 / 0.09,1.4) * 1.4 where ”flowacc” is the flow
accumulation, ”resolution” is the spatial resolution of the numerical
model of terrain used and ”slope” is the slope gradient in degree (°).
Cropping management factor
(C)
The C factor is a most important factor that controls soil erosion risk
(Knijff et al., 2000) and it reflects the effect of cropping and
management practices on the soil erosion rate (Jong, 1994; Biesemans et
al., 2000; Patil & Sharma, 2013). For our case, the land use classes
were digitized by photo-interpretation based on the very high spatial
resolution satellite image with provided by ESRI © servers.
Rainfall-runof f erosivity (R)
factor
Rainfall-runoff erosivity is defined as the intrinsic capacity of the
rain to cause soil erosion, and amount, intensity, terminal velocity,
drop size and drop size distribution of rain are the variables affecting
total erosivity of a rain event ( Blanco-Canqui and Lal , 2008).
According to Wischmeier and Smith (1978) and Renard et al. (1997),
estimation of R factor requires total rainfall energy (E) and maximum 30
min rainfall intensity (I30) of storm events for at least 20 years. With
the lack of sufficient return period climate data in the study area, the
R factor was calculated adopting the modified formula of Rango and
Arnoldus (1987). Which is defined as Eq (3 ).
\(logR=1.74.log\sum_{i=1}^{12}{\left(Pi\hat{}2/P\right)+1.29}\)(3)
Support practice (P)
factor
The P factor indicates the significance of support practices, which
reduce soil loss by modifying the flow pattern, grade or direction of
surface runoff and by reducing the rate of runoff (Wischmeier and Smith,
1978; Renard and Foster, 1983). It varies from 0 to 1, according to the
practice adopted and slope. In our case, the value assigned to the
factor P is 1 for the whole watershed, since the anti-erosion measures
are absent.
Soil Erodibility Factor (K)
The soil erodibility factor K represents the susceptibility of soil
particles to be detached (Renard and al., 1997). K is strongly related
to the physical properties of the soil and plays an important role in
soil conservation strategies (Shabani, Kumar, & Esmaeili, 2014). All of
the K factor equations are related to the soil texture, soil organic
matter, and percentage of sand, silt, and clay in the soil (Millward &
Mersey, 1999). It is expressed in
t.haMJ-1.mm-1, and determined
according to the following formula (Eq. 4).
K = 2.77M 1,14. 10-7(12 − OM) + 4.28 (10-3) (SS − 2) + 3.29 (10-3) (PP −
3) (4)
Where OM is the soil organic matter content, M is the indicator of the
soil textural class ((%fine sand+silt) × (100–%clay)), SS and PP
represent the soil structure and permeability classes.