(3) 
calculated from the following formula: 
T - 8 .7-1 
tc = _J_ HL (S + 1) 
0.6 L 1900 SG* b J 
where: 
HL = Hydraulic length of the largest stream. 
SG = Slope gradient index. 
S = Storage term. 
The slope gradient can be computed from the DEM created for any 
particular area. In this case an area of 3 x 3 pixel was used as a 
minimum size area. To save computing time, the slope gradient are 
classified into four catagories as shewn in Table 2. 
TABLE 2 : Slope Gradient Catagories 
Type 
Gradient 
Index Value 
SG1 
< 25 % 
0.25 
SG2 
25 - 50 % 
0.50 
SG3 
51 - 75 % 
0.75 
SG4 
> 75 % 
0.95 
A high value of tc translates that the probability of flood to occur 
on that particular area is very high. Therefore more weights will be 
assigned. 
Interception 
The conversion from forest to a rubber plantation, the actual volumes 
of interception loss may not be greatly different, but there could be a 
considerable change in the regulating effects of the different 
vegetations. Basically, the greater the interception loss the less water 
will arrive at the soil surface and be available for stream flow. This 
implies that the greater interception loss, the lesser the water yield 
will be. Via ter yeild will be at least, in dense multi-storied forest 
which have the greatest interception loss potential. Zinke (1967) 
reported that the annual interception loss for comiferous has been 
reported as being as high as 50 % of the annual rainfall and 25 to 35 % 
seem to commonly accepted value for vegetated surfaces. Therefore, an 
area which consisted of primary forest will be assigned less weights as 
compared to, say, urban area. Table 3 shows the weights assigned. 
As an example for every 3x3 window of forest detected, a factor of 5 
is added to the window. In this case an area which covered mostly by 
forest will recieved the least weight compared to urban area. 
672