Calculating Effective Rainfall
From the MIDUSS Version 2
Reference Manual - Chapter 7
(c) Copyright Alan A. Smith Inc.
Effective rainfall -
sometimes called excess rainfall - is the component of the storm
hyetograph which is neither retained on the land surface nor which
infiltrates into the soil. The effective rainfall produces overland
flow that results in the direct runoff hydrograph from a sub-area of a
catchment. The difference between the storm and the effective
rainfall hyetographs is termed the abstractions or rainfall losses.
Abstractions are made up of one or more of the following three main
by vegetation or tree canopy
into the soil
surface depressions and hollows
In the absence of
field observations it is usually necessary to employ some form of
mathematical model to represent the abstractions. This must be done
for the pervious and the impervious fractions of the catchment.
MIDUSS currently lets
you choose from three methods to define the infiltration model.
These options are
available for both the pervious and impervious fractions of the
catchment. However the choice of infiltration model must be made on
the Pervious tab of the Catchment form. When the Impervious tab is
displayed there is a note to remind you which option is in use. Refer
to the Catchment command; Data for the Pervious Area for details and a
view of the relevant forms.
The choice of
infiltration method is made after the selection of the overland
routing option. It is important to note that if you intend to use the
SWMM/Runoff option for the generation of the overland flow hydrograph,
the SCS infiltration option is not available and you must choose
between the Horton equation and the Green & Ampt
The descriptions of
the infiltration models given in the sections that follow apply
equally to both impervious and pervious fractions of the catchment.