33
Landset TM+
Geometric correction
Georeferencing Radiometric
correction Haze removal
Supervised Classification
Land Cover Map Information erosion
management practice Land
Conservation Value
Land conservation value map
Cover Management
Factor C
Rainfall Intensity
distribution
Calculate rainfall erosivity R Rainfall Intensity
Map
Soil Classification
Map
Calculate Soil erodibility k Soil Erodibility Map
K factor
Rainfall Intensity
distribution
DEM Raster
Contour Line Map 12.5 interva
TIN Grid Coverage
Interpolation Slope in
Slope Aspect Topographic Map
LS factor Calculate with Map
Calculator A= R K LS C P
Erosion Potential
Fig 3.4: The flow chart of prediction erosion process by RUSLE model
3.6.1. Rainfall-runoff Erosivity Factor R
The R factor represents the erosivity of the rainfall at a particular location. Rainfall Erosivity R is originally calculated as a product of storm kinetic energy
E and the maximum 30-minute storm depth I 30 summed for all storms in a year. An average annual value of R is determined from historical weather records
and is the average annual sum of the erosivity of individual storms Weischmeier and Smith 1978
. Regarding the study area, available dataset are records of monthly rainfall amount mmmonth and raindays daysmonth, as well as an
isohyets map representing the general distribution of annual rainfall. As common situation found in developing countries, there was no rainfall intensity data
collected in the rainfall station. Therefore, the following equation, which is
34 derived for tropical areas El-Swaify et al. 1985, was used to calculate R-Factor
based on annual rainfall amount. R = 38.5 + 0.35P
Where R: annual rainfall-runoff erosivity factor MJ.mm.ha-1.h-1.yr-1 P: annual rainfall amount mm summed from monthly-recorded
rainfall amount mm P value was derived from averaged annual rainfall record collected from
two rain station. Considering the spatial variation of the rainfall amount, the isohyets map, with additional data from the rain stations, was interpolated to grid
map with 300 meter cell size to come up with spatial distribution of annual rainfall. The grid map of annual rainfall was resample to 30 meters to allow
spatial overlay with grid map of other factors of RUSLE. All climatic data were collected from BAHL Agro-climate and Hydrology Laboratory.
3.6.2. Soil Erodibility Factor K
The soil erodibility factor is the average long-term soil and soil profile response to the erosive power of rainfall and runoff. Soil erodibility factor
represents the effect of soil properties and soil profile characteristics on soil loss. To determine the K-value for each soil type, the following equation was used
Weischmeier and Smith 1978, given the silt fraction does not exceed 70.
Where: K : Soil Erodibility Factor t.ha-1.MJ-1.ha.mm-1.h
OM : Organic matter content M : Product of primary particle size fractions
M = Silt + Very Fine Sand 100 - Clay 3.2
3.3
35 S : Code of Soil Structure
P : Code of Soil Permeability
3.6.3. Slope Length and Slope Steepness Factor LS
