COMPUTE AVERAGE PRECIPITATION BY VARIOUS METHODS

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COMPUTE AVERAGE PRECIPITATION BY VARIOUS METHODS

METHODS TO CALCULATE AVERAGE PRECIPITATION ARE
1. Arithmetic average method
2. Thiessen Polygon Method
3. Isohyets Method

1. ARITHMETIC AVERAGE METHOD
• in this method the average depth of rainfall over an area is obtained by dividing sum of
depths of rainfall recorded at the rain gauge stations located in the area by number of
station.
• When the area of the basin is less than 500 km2 this method implies summing up of all
the rainfall values from all the rain gauging stations and then dividing it by the number of
stations in that basin.
• This method gives accurate results if the stations are uniformly distributed over the area.
There should not be much variation in the rainfall values of the stations under
consideration
• Drawback of this method is the stations just outside of the basin are not considered
although these stations might have some influence on the basin under consideration.

2. THIESSEN POLYGON METHOD
• This method is very accurate. It is used for basins having areas between 500 – 5000 km2.
• In thiessen polygon method, the rainfall recorded at each each rain gauge station is given a
weightage on the basis of the area which it represents.
• This method is better than the arithmetic mean method which gives equal weightage to all
the station.
• Results are accurate than arithmetic method.
• This method eliminates error due to non-uniform distribution of rain gauge.
• If the gauging station are few compared to the size of area, Thiessen polygon method
should be used.
• A new thiessen polygon diagram is to be drawn every time for the catchment if there is a
change in gauge station.

3. ISOHYETS METHOD
• Isohyets are the contours of equal rainfall. An isohyets map showing contour of equal
rainfall represent a more accurate picture of the rainfall distribution over the basin.
• The properties of isohyets are similar to that of contours.
• This method is most suitable for hilly and rugged basin.
• If the analyst ha the knowledge of orographic effect, he can use it in constructing the
isohyet map for better results.
• This method permits the use of all available data.
• Overall this method is superior to the other two method.

EXAMPLE-IT IS GIVEN THAT THE AREAS OF THE POLYGONS
AND THE RAINFALL STATION WITH ITS VALUE IS AS
FOLLOWS:
Calculate average depth of precipitation over the catchment
by arithmetic mean, Thiessen polygon and Isohyet methods
and compare the results.
Rainfall(cm) 0.65 1.46 1.92 2.69 1.54 2.98 5 4.5
Area of polygon(km
2
) 7 120 109 120 20 92 82 76
Isohyets 9 10 11 12 13 14
Area of polygon (cm2) 22 80 105 98 78 16

Solution:
Step 1: Arithmetic Average Method:
Referring Fig. 9(a). There are 6 stations within the catchment.
Average depth of ppt. = (1+0.65+1.46 + 1.92 + 2.69 +1.54+4.50 + 2.98 + 5.00)/9 = 2.41 cm
Step 2: Thiessen Polygon Method:
Referring to Fig. 9(b) the precipitation and areas can be tabulated as in Table 2.3.
Average depth of PPT = (1770.31/626)
= 2.82cm.
Observed
PPT (cm)
Polygon
Area P X A
1 2 2
0.65 7 4.55
1.46 120 175
1.92 109 209
2.69 120 322.8
1.54 20 30.8
2.98 92 274.16
5.00 82 410
4.50 76 342
Total A=626 =1770.31

STEP 3: ISOHYET METHOD:
REFERRING TO FIG. 9(C) THE ISOHYET INTERVAL AND AREA
ENCLOSED CAN BE TABULATED AS FOLLOWS TO WORK
OUT THE AVERAGE DEPTH AS ON IN TABLE 2.4.
Isohyet
Interval
(cm)
Area of
polygon
(cm2)
Average
rainfall
(cm)
Product
A x (p1+p2/2)
9 22 9.5 209
10 80 10.5 840
11 105 11.5 1208
12 98 12.5 1225
13 78 13.5 1053
14 16 14.5 232
TOTAL A=399 =4767

THANK YOU
MADE BY :- BHANUSHALI HARSH

COMPUTE AVERAGE PRECIPITATION BY VARIOUS METHODS

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