P —supporting practice factor, the ratio of soil lost with a support practice, 
like contouring, stripcropping, or terracing to that with straight row 
farming up and down the slop. This study established P value from many 
previous reports and references as shown on table 1. 
Economic 
Although there were many discussions about the affect on soil erosion caused by 
Betel nut farming activity. None of those papers evaluated the external cost and the 
degrading of social welfare which not been covered by most of the farmers. Still, 
there are lots of models relating to the evaluation of external cost for non market value 
from Environmental Economics and Environmental Impact Assessment theory. This 
study used produces materials and real market price for labor cost to count the change 
of goods price. Real market value from environmental protect facilities were 
evaluated to count the prevention, updating, and shadow cost for soil and water 
conservation facilities. The idea for counting the economic externality is strategically 
try to internalize the cost been paid by social welfare and eliminate the free rider for 
soil erosion. The result from these non market price evaluation methods are described 
as following: 
  
  
  
Table2 The internal and external cost of the Betel nut ( dollars / hectare ) 
items internal cost external cost 
benefit prevention | Recovering | Shadow engineer 
Average 221800 490.8 671.9 2279.5 
  
  
  
  
  
  
  
During the analytical of Shui-Li River watershed, the benefit goes to the 
farmer's part will be covered by the social “worse off” payment where the prevention, 
recovering, and shadow engineer costs. Still, in the study, some of the important 
externalities has not been input as the calculation for externalities, such as the micro 
climate changes due to the over development of natural forest land, the impact on 
water quality and quantity, the change of the diversity of animal. The summing up 
for all these cost should go much higher than the estimation from table 2 
  
  
  
  
  
below 700 
[ETE] 700-900 
[LT] 900- 1100 
1100-1300 
“= Vo [C] above 1300 
Figure 4. The Slope Length figure 
  
  
  
  
  
  
  
  
  
  
140 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996