ASSIGNING WEIGHTS TO FACTORS 
Runoff Volume 
The volume of runoff 0 can be estimated from the formula: 
2 
Q = ( I - 0.2 S) 
I + 0.8 S 
(1) 
where: 
I is the rainfall intensity. 
S is the storage detention. 
The storage term S, is defined in accordance with: 
S = 
1000 _ 10 
CN 
(2) 
where: 
CN is the curve number. 
The value of CN can be obtain from the Soil Conservation Service (SCS) 
model as shown in Table 1. 
Table 1 : Runoff Curve Numbers Definable from Land sat 
Land 
Cover 
A 
Soil Type 
B 
C 
D 
Forest 
25 
55 
70 
77 
Grass 
36 
60 
73 
78 
Urban 
90 
93 
94 
95 
Bare 
72 
82 
88 
90 
For every 3x3 pixel window, the value of g will be calculated. The 
highest and lowest value will be noted. The interval between the two 
extreme g's are then interpolated so that each differenr g's will be 
assigned with different weights. In this case, a high g will recieved a 
higher weight and vice versa. 
Timing of Runoff Events 
The timing of the runoff events plays a major role in the 
determination of the flood prone area. This factor will determine the rate 
of a particular area recieving water flow from a stream. The timing can be 
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