Processing Storm Rainfall
From the MIDUSS Version 2
Reference Manual - Chapter 7
(c) Copyright Alan A. Smith Inc.
The significance of
the components of rainfall loss is illustrated in Figures 7-14 and
7-15. For this comparison the storm used is a 2nd quartile Huff
distribution with a total depth of 50 mm over a period of 120
minutes. The rainfall abstractions have been modelled using the SCS
Curve Number method with
Figure 7-14 – Rainfall
loss components with Ia = 10 mm, Yd = 0.0
Fig 7-14 above shows
the normal application of the SCS method in which an initial
abstraction Ia = 10 mm has been applied.
It is clear that this is a first demand on the storm hyetograph. The
remaining 40 mm of rainfall is split into an infiltrated volume of
18.6 mm leaving 21.4 mm of direct runoff.
Figure 7-15 – Rainfall
loss components with Ia = 0.0 and Yd =
Fig 7-15 shows an
unusual application of the SCS method developed by means of some of
the options in MIDUSS. In this case the initial abstraction is zero
so that an infiltration volume of 20.46 mm is the first demand on the
storm hyetograph. The remaining 29.54 mm of rainfall is divided
between 19.54 mm of direct runoff and 10 mm which is detained as
surface depression storage.
Note the difference in
volume, peak intensity and shape of the direct runoff component. This
would certainly be reflected in the resulting overland flow
hydrograph. In this example the differences have been exaggerated by
using a relatively large depth for Ia or
yd. You will find it instructive to recreate this experiment using
the Horton method to model the infiltration process or with smaller
values of Ia and yd.