LS = (H/22) *(65.41*sin? N + 4.56*sin N + 0.065) 
where LS = Topographic factor 
H = Slope length 
m = Slope length exponent 
N = Slope 
The slope length exponent (m) depends on the mean 
slope within the watershed area (Hensel, 1991). 
m = 0.2 for slope « 1? 
m = 0.3 for 1° < slope < 3° 
m = 0.4 for 3? « slope « 5? 
m = 0.5 for slope > 5° 
For an assessment of the risk of erosion, the above 
derived parameters were combined, according to 
decision rules provided by NARF (Table 3). 
Instead of using soil depth for the measure of water 
storage capacity, as was proposed by NARF, the 
maximum green vegetation cover for the dry period 
between 1984 and 1993 was used to estimate this factor 
(see section 4.1). Similarly, the slope parameter 
proposed by NARF was replaced by the more 
comprehensive topographic factor (LS), as described 
above. 
As for the estimate of the PNR, 13 of the 14 validation 
sites correspond with the field based estimates within + 1 
category for the estimate of the Risk of Soil Erosion. 
Table 3: Risk of Soil Erosion Decision Rules. 
4.3. Risk of Desertification 
According to NARF, the additive combination of the two 
factors for the PNR and the ROE gives the relative risk of 
desertification, which, for the four test areas, has been 
divided into five classes, with values ranging from a 
minimum of 2 (without any risk) for class 1, to a 
maximum of 10 (very high risk) for class 5. 
Table 4: Class Definitions for Risk of Desertification 
  
  
Class | Values |Desertification Risk 
1 2 Sites without any risk 
2 3,4 Sites with a low risk 
3 5,6 Sites with a moderate risk 
4 7,8 Sites with a high risk 
5 9,10 Sites with a very high risk 
  
  
  
  
  
  
The accuracy assessment gave a correspondence of t1 
category for all 14 validation sites compared with the field 
based estimates. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Permeability | Water Storage | Topographic Factor Risk of Erosion 
Capacity LS Category Characterisation 
bare rocks - 1 no to slight risk 
low < 1:5 2 slight risk 
permeable 4.5 3 moderate risk 
high «4.5 1 no to slight risk 
>1.5 2 slight risk 
bare rocks -- 1 no to slight risk 
low « 1.5 4 high risk 
impermeable > 1.5 5 very high risk 
«1.5 2 slight risk 
high 1.5103 3 moderate risk 
>3 4 high risk vend 
  
  
  
  
244 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996