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Hydrological Theory

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 components:-

  1.  interception by vegetation or tree canopy

  2.  infiltration into the soil

  3.  storage in 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 method.

The descriptions of the infiltration models given in the sections that follow apply equally to both impervious and pervious fractions of the catchment.

 

 


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