Hydrological Theory
Calculating Runoff
Components of Rainfall |
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From the MIDUSS Version 2
Reference Manual - Chapter 7
(c) Copyright Alan A. Smith Inc. |
For each fraction of
the catchment (pervious and impervious) the rainfall loss is the
difference between the rainfall depth and the depth of runoff. This
is made up of various components as illustrated in Figure 7-13. Not
all methods of modelling rainfall losses use all of these components.
Figure 7-13 – Rainfall
abstraction models use different components
The initial
abstraction Ia may be defined explicitly
as an average depth over the area (in mm or inches) or implicitly as a
fraction of the potential storage depth in the soil (e.g.
Ia
= 0.1 S). The notion of initial
abstraction is used in the SCS infiltration method, but not by either
the Horton equation or the Green & Ampt
methods. The initial abstraction depth is treated as a first demand
on the storm rainfall; surface depression storage is a first demand on
the surface water excess leading to runoff.
The infiltration
capacity is assumed to decrease continuously throughout the storm as
the storage potential in the soil is progressively reduced by the
volume of infiltration. The reduction in infiltration capacity is a
function of the infiltrated volume and not of the elapsed time from
the start of rainfall. In release 1 of MIDUSS no provision is made
for 'recovery' of infiltration potential during periods of zero or
very low rainfall. For single event modelling this is not likely to
be significant. MIDUSS models the infiltration process by
Surface depression
storage is represented by an average depth distributed uniformly over
the surface area. The usual assumption made is that when rainfall
intensity exceeds the infiltration capacity the depth of excess water
on the surface must attain a value greater than the surface depression
storage depth before runoff can occur. The concept of surface
depression storage depth is not used in the SCS method but plays a
significant role in the implementation of the Horton or Green &
Ampt methods.
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