Nyarko, Benjamin Kofi 
  
  
Q= 0.28*C,*C*I*A (1) 
Where: 
Q^ runoff rate [m /sec] 
C= Runoff Coefficient 
Cs= Storage Coefficient 
I- Rainfall Intensity [mm/hr] 
A- Drainage area [Km?] 
Table: 1.1 Total discharge of section of the Study area 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Catchment Name Area Runoff Storage Rainfall Discharge 
(Km2) [coefficient coefficient (mm) M'/sec 
Kpeshie Catchment [62.6 0.7 0.2 140.2 344 
Lower Densu 79.4 0.4 0.6 140.2 748.1 
Lower Odaw 90.1 0.9 0.2 140.2 636.7 
Lower Sakumo 116 0.7 0.4 140.2 1271 
Middle High Odaw [18.7 0.7 0.2 140.2 102.8 
Middle Odaw 118 0.7 0.2 140.2 650.7 
Middle Sakumo 155 0.6 0.5 140.2 1825 
Mokwe catchment 13.9 0.7 0.3 140.2 114.6 
Songo catchment 16.8 0.8 0.2 140.2 105.5 
Upper Densu 24.9 0.6 0.7 140.2 410.5 
Upper Odaw 64.5 0.6 0.7 140.2 1063 
Upper Sakumo 9.4 0.6 0.7 140.2 155 
West Densu 17.3 0.6 0.7 140.2 285.2 
  
  
  
  
  
  
Source: Authors Context, 1999 
From Table 1.1 the total runoff discharge over the land surface of the study area determines the 
maximum flood that an area under consideration is likely to experience. Various segments 
covering the topography (Figure 1.4) produced varied runoff rates, for instance, the Odaw and 
the Sakumo being the biggest catchments produce a total discharge rates of 1825 m®/sec and 
1271 m’/sec respectively, that is if all the entire catchment contributes to runoff at the same time. 
However, segments within the catchments also produce varied runoff rates. For instance, the 
Sakumo catchment presents three different runoff discharge rates of 155 m/sec, 1825 m>/sec and 
1271 m?/sec for the Upper, Middle and the Lower catchment areas, respectively. 
As noted in the use of the modified rational model (Table 1.1) the rate of discharge is mostly 
dependent on rainfall intensity and area. For instance the lower and middle Sakumo with area 
measurements of 116 km? and 155 kn? presented calculated runoff discharge rates of 1271 
m?/sec and 1825 m”/sec, respectively. The Kpeshie and Songo catchments, having smaller area 
measurements of 62.6 km’, and 16.8 km? produce a discharge of 344 m? and 105.5 mj, 
respectively. However, with the given segment area that is constant over the topography within 
the catchments, an increase in rainfall will also lead to an increase in the total runoff discharge 
rate. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 1043