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#### Hydrological Theory

##### Calculating Runoff

Components of Rainfall

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

• the SCS method

• the 'moving curve' Horton equation

• the Green & Ampt model.

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.

(c) Copyright 1984-2010 Alan A. Smith Inc.
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