directions. The downhill pointers were then erased. After completing the whole image, the process 
started over again to find new pixels, now without flow directions pointing towards them; adding the 
accumulated water to their downhill neighbors; erasing the downhill pointers; and starting from the 
top again... Eventually, all pixels were assigned an amount of water passing by on its way to the 
sea. This amount of water can also be seen as the catchment area to the pixel. 
4. A variation of the procedure above was used to determine the length and height of drop of the slope 
above each pixel. In this case only one downhill pointer was used in each pixel. Instead of the 
groundwater flow, the length of the slope and height of drop were accumulated as the water runoff 
simulation, described above, proceeded downhill. 
Estimating the site variables 
The site variables were obtained for each pixel as follows: 
The latitude and altitude were directly available through the location in the DEM. 
The soil texture and soil depth were directly derived from the soil map. 
The occurence of sub-surface waterflow is defined in the existing SCS in three classes: Seldom, during 
Shorter periods and during Longer periods according to FIGURE 1. Given the slope length and height of 
drop above each pixel as described above, it was possible to classify the sub-surface flow for each 
pixel (FIGURE 5). 
FIGURE 1 
Classification of sub-surface waterflow. S stands for Seldom, 
K for Shorter periods, L for Longer periods. 
SLCFF >3 2o i>i5v.i 
SLOPS 3 : 20 I 5 -15%) 
The ground moisture is in the existing SCS divided into four classes: 
Dry : ground water surface > 2 meters below the ground 
+ sub-surface flow must be of class Seldom 
Mesic : ground water surface 1-2 meters below the ground 
Moist : ground water surface < 1 meters below the ground 
Wet : ground water at surface 
A chain of rules had to be defined to determine the ground moisture class for a pixel: 
First, the ground was considered Dry if the soil cover was both morain and thin or if the soil cover was 
coarse and consisted of thick fluvial or littoral sediments. In both cases the sub-surface flow by 
definition had to be of class Seldom. 
873