nbul 2004 
|] 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
  
3.1.3 Topographic factor (LS): 
Four 1:50,000 topographic map sheets were 
digitized. A DEM was generated using TIN and 
GRID Modules. The resulting layer was exported to 
ERDAS Imagine to generate a slope layer. The slope 
image was classified into six classes using Model 
Maker. Slope length was calculated by combining the 
slope angle thematic layer with a hydrological layer 
containing the wadi network for the study area. 
The LS factor was calculated using the tables 
produced by Renard et al. (1994) for the RUSLE in 
conjunction with the thematic slope layer. The 
results of these operations are presented in Table 2. 
  
  
  
  
  
  
  
  
  
  
  
Slope 0-3 3.1-6 6.1- 10.1- | 151-1 725. 
class (9a) 10 15 25 | 
Mean L 177. 71.6 50.0 51.6 56.7 59.0 
(m) 16 2 | 7 5 3 
Mean LS, | 0.29 0.78 1.28 2.30 4.39 10.0 
rangeland 0 
Mean LS, | 0.37 0.93 1.48 2.69 5.04 11.8 
cropland 6 
  
  
Table 2: The estimation of LS values for each class 
present in the study area, for each type of land use. 
3.1.4 Vegetation/Cropping and Management 
Practices Factors (C and P): The 
vegetation/cropping — C — factor was parameterized 
from remotely sensed data. The fractional vegetation 
cover map for the 1992 TM imagery produced by 
linear spectral unmixing was used to estimate the 
values of C for rangelands and croplands in the 1992 
dry season. Calibrated NDVI images were used to 
estimate C from similar areas in 1972. 
In these estimations three cultivation systems (rain- 
fed fields, irrigated fields and rangeland) were 
considered. It was considered that the residual plant 
remains (stubble) would be very low for rain-fed 
fields due to low yields and stubble grazing. At 30% 
cover of mulch the estimated C value is 0.4. For the 
irrigated fields and rangeland the C values are 
presented in Table 3. 
  
Type of < < < < 
land use 20% 40% 60% 80% 100% 
Irrigated | 0.48 0.37 0.22 0.12 0.04 
fields 
Rangela 0.35 0.20 0.12 0.062 | 0.027 
nd 
  
  
  
  
  
  
  
  
  
  
Table 3: Estimation of C values for vegetated fields 
and rangelands by vegetation cover percentage classes.’ 
A close inspection of the NDVI statistics revealed 
that for the dry season 1972 the NDVI values never 
exceeded 0.087. According to the calibration data, 
this indicates that the vegetation cover percentages 
never exceeded 20% (Edwards et al. 1996). 
Moreover, as 20% is the lowest threshold value for 
the estimation of C, a constant value for C (= 0.35) 
was applied to the rangelands and bare fields in the 
1972 image, and this value was used in the soil loss 
model (Wischmeier 1978). 
The management factor (P factor) was assigned a 
value of one for the entire study area as no specific 
507 
management measures are used for either rangelands 
or croplands in the study area. 
3.2 Maps of Soil Losses 
The GIS input layers discussed are listed in Table 4. 
They were combined, as described by the RUSLE, to 
estimate annual soil losses on a pixel-by-pixel basis. 
A low pass (7 * 7 Kernel) filter was applied to all 
input layers before running the model. 
  
  
  
  
  
  
  
  
  
  
  
  
Layer Description 
number 
1 Rainfall erosivity layer (R). 
2 Soil erodibility layer (K). 
3 Topographic layer (LSc) for 
cropland. 
4 Topographic layer (LSg) for 
rangeland. 
3 Vegetation cover layer (Co?) 
for bare fields, for 1992. 
6 Vegetation cover layer (Cs) 
for vegetated fields, for 
1992. 
7 Vegetation cover layer (Cop) 
for rangeland, for 1992. 
8 Vegetation cover layer (C7) 
for bare fields and 
rangeland, for 1972. 
  
Table 4: Thematic input layers to the GIS-based 
erosion model. 
Change in vegetation cover proportions is the key 
dynamic variable in predicting soil losses over time in 
the study area. It was assumed that all other variables 
were constant over the 20 year time period of the 
study. To capture this change, the soil loss model was 
run separately for the years (1972 and 1992). These 
runs were based on the following combinations of 
GIS layers (layers refer to Table 4): 
Soil loss maps for 1972: 
layer 1 x layer 2 x layer 3 x layer 8 
layer 1 x layer 2 x layer 4 x layer 8 
Soil loss map for 1992: 
-. layer 1 x layer 2 x layer 3 x layer 5 
layer 1 x layer 2 x layer 3 x layer 6 
layer 1 x layer 2 x layer 4 x layer 7 
The results of the model runs, as soil loss maps for 
each year are shown in Figures 2 and 3. These maps 
were classified using Model Maker, and 11 soil loss 
classes were produced.