Rainfall is expressed in terms of the depth to which
rainwater would stand on an area, if all the rain were
collected on it.
Thus, 1cm of rainfall over a catchment area of 1km2
represents a volume of water equal to 104m3.
The precipitation is collected and measured in a Raingauge.
In India, the raingauges commonly used are Symon’s gauge.
Average Rainfall over an area
Arithmetic Mean Method
Thiessen Polygon Method
To convert the point rainfall values measured by various raingauge
stations into an average value over a catchment, following methods
If there are small variations in the rainfall values measured by the
stations, this method can be used.
If P1, P2, . . . , Pn are the rainfall values obtained from n raingauge
stations within a catchment, then the average precipitation is given
1 2 1.....
P P P
In this method, the rainfall recorded at each station is given a
weightage on the basis of an area closest to that station.
1. Draw the catchment area to a scale and mark the raingauge
stations on it.
2. Join each station by straight line to create a triangulated
3. Draw perpendicular bisectors on each sides of each triangles.
Extend the bisectors to meet the other bisectors and the catchment
4. The polygons formed by the perpendicular bisectors (and part
of catchment boundary) are the influence areas of each stations.
If there are n number of raingauge stations in and around the
catchment and if A1,A2, . . . ,An are the respective influence areas of
Thiessen Polygon, then the average rainfall is given by
Four raingauge stations are located in a catchment area,
approximated by a circle of radius 100km, as shown in the figure.
Annual precipitation recorded by these stations in the year 2003 are:
A(85cm), B(130cm), C(105cm), D(98cm).
Draw the Thiessen polygon and calculate the average annual
precipitation on the catchment in 2003.
Isohyet: An isohyet is a line joining points of equal rainfall
If P1, P2, . . . , Pn are the values of isohyets and if A1,A2, . . . ,An−1 are
the inter-isohyet area respectively, then the mean precipitation over
the catchment is given by
The isohyets due to a storm in a catchment were drawn and the
area of the catchment bounded by isohyets were tabulated as
below. Estimate the mean precipitation due to the storm.