CHAPTER 5: RAINFALL DATA ANALYSIS

Computation of Average Rainfall over a Basin:

To compute the average rainfall over a catchment area or basin, rainfall is
measured at a number of gauges by suitable type of measuring devices. A
rough idea of the number of the needed rain gauges to be installed in a
practical area is depending on experience of the hydrologist although this
was determined by the regulation of the World Meteorological
Organization (WMO).
In areas where more than one rain gauge is established, following
methods may be employed to compute the average rainfall:
- Arithmetic average method
- Weighing mean method or Thiessen polygon method
- Isohyetal method.
5.3.1 Arithmetic Average Method:You
This is the simplest method of computing the average rainfall over a
basin. As the name suggests, the result is obtained by the division of the
sum of rain depths recorded at different rain gauge stations of the basin
by the number of the stations.
If the rain gauges are uniformly distributed over the area and the rainfall
varies in a very regular manner, the results obtained by this method will
be quite satisfactory and will not differ much than those obtained by other
methods. This method can be used for the storm rainfall, monthly or
annual rainfall average computations.


 isoyetal Method:
An isohyetal is a line joining places where the rainfall amounts are equal
on a rainfall map of a basin. An isohyetal map showing contours of equal
rainfall is more accurate picture of the rainfall over the basin. This
method is more suited under the following conditions:

Thiessen Polygon Method
This is the weighted mean method. The rainfall is never uniform over the
entire area of the basin or catchment, but varies in intensity and duration
from place to place. Thus the rainfall recorded by each rain gauge station
should be weighted according to the area, it represents. This method is
more suitable under the following conditions:
- For areas of moderate size.
- When rainfall stations are few compared to the size of
the basin.
- In moderate rugged areas.
For the construction of the polygon, the following procedure is to be
followed:

Summary for Basin Average Rainfall Analysis Techniques
The Isohyetal method allows the use of judgment and
experience in drawing the contour map. The accuracy is largely
dependent on the skill of the person performing the analysis and
the number of gauges. If simple linear interpolation between
stations is used for drawing the contours, the results will be
essentially the same as those obtained by the Thiessen method.
The advantages of both the Thiessen and Isohyetal methods can
be combined where the area closes to the gauge is defined by
the polygons but the rainfall over that area is defined by the
contours from the Isohyetal method. This combination also
eliminates the disadvantage of having to draw different polygon
patterns when analyzing several different storm events with a
variety of reporting gauges. Regardless of the technique
selected for analysis of basin average rainfall, a regional map of
areal distribution for the total storm event is also produced.


    















































































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