TABLE 3 : Weights Assigned Due to
Interception
Land Cover
Weights
Forest
5
Grass
10
Urban
15
Bare
20
Depression Storage
The irregular features of the land surface are filled with water
during rainfall and which does not run off but infiltrates or evaporates.
Land tillage, loading and housing construction, swamp drainage, the
construction of terraces, etc. tend to change the natural depression. The
greater the depression storage, the lesser the overland flow, and it is
assumed that the lower the flood potential. If the depressions are
sufficiently large, such as swamps in upland basin, it tend to regulate
the water yield and provide a supply for stream flow.
In this paper it is assumed that the natural depression storage
ranges from 1 mm to 50 mm. For sloping ground it is assumed that the
depression storage increase by 50 mm and 75 mm if on fairly flat ground.
Detention Storage
The amount of water detained on the land surface that provides the
head for overland flow is directly associated with the roughness and
condition of the land surface. Overland flow is, in general, more
pronounced in arrid conditions.
The equation used to study the relation between the surface detention
and overland flow is:
De = Klqc
(4)
where:
De is the detention volume under equilibrium conditions.
K is a coefficient.
1 is the length of flow.
qc is the discharge per unit width of equilibrium.
The value K depends on the rainfall intensity (I), the slope surface
(S) and a roughness factor (n), that is :
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