Hydrological Theory
Calculating Runoff
Rainfall Runoff Models 

From the MIDUSS Version 2
Reference Manual  Chapter 7
(c) Copyright Alan A. Smith Inc. 
Rainfall runoff models
may be grouped in two general classifications that are illustrated in
Figures 716 and 717. The first approach uses the concept of
effective rainfall in which a loss model is assumed which divides the
rainfall intensity into losses and an effective rainfall hyetograph.
The effective rainfall is then used as input to a catchment model to
produce the runoff hydrograph. It follows from this approach that the
infiltration process ceases at the end of the storm duration.
Figure 716  A
rainfallrunoff models using effective rainfall.
An alternative
approach that might be termed a surface water budget model,
incorporates the loss mechanism into the catchment model. In this
way, the incident rainfall hyetograph is used as input and the
estimation of infiltration and other losses is made as an integral
part of the calculation of runoff. This approach implies that
infiltration will continue to occur as long as the average depth of
excess water on the surface is finite. Clearly, this may continue
after the cessation of rainfall.
Figure 717 – A
rainfallrunoff model using a surface water budget
MIDUSS allows you the
option to use particular implementations of both these techniques.
The effective rainfall approach is employed in a convolution algorithm
that uses response functions of different shape. For the case of
triangular or rectangular response functions the time base is computed
by a kinematic wave equation which
involves the intensity of the effective rainfall.
The convolution
process is therefore nonlinear in that the response function changes
throughout the storm but the principle of superposition is retained.
These two approaches are embodied in the ‘Rectangular’ and ‘Triangular
SCS’ options of the Hydrology/Catchment command.
The third convolution
option uses a response function which is obtained by routing a
rectangular input of duration
Dt
and height A/Dt
through a linear reservoir with a lag or storage coefficient
KL=tc/2.
For this case, the time of concentration tc
is computed using the kinematic wave
equation [741] but for the maximum value of effective rainfall
intensity.
An example of a
surface water budget model is also made available in the form of the
SWMM/RUNOFF algorithm and can be implemented by using the ‘SWMM
method’ option. It is important to note that if the ‘SWMM method’
option is to be employed it is necessary to use the Horton or Green
and Ampt infiltration models to represent
the rainfall losses. The four options are described in the sections
that follow.
