Estimation of Missing Precipitation

a) Arithmetic Average Method:

b) Normal Ratio Method:

c) Thiessen’s Polygon Method:

• In the Thiessen polygon method, the rainfall recorded at each rain gauge station is given a weightage of the basis of the area which it represents. This method is better than the arithmetic mean method which gives equal weightage to all the stations.
• In this method, adjacent stations are joined by straight lines, thus dividing the entire area into series of triangles.
• Perpendicular bisects are erected on each line, thus, forming a series of polygons-each containing only one rainfall station.
• The value of precipitation measured at a station is assigned to the whole area covered by the enclosing polygon.

Where, P1, P2; P3….Pn represents rainfall at the respective stations, whose surrounding polygons have the area A1, A2, A3……An respectively.

d) Isohyetal method:

• Isohyets are the contour of the equal rainfall. They are drawn on the map using common sense, after the rainfall at each station is plotted. The area between the adjacent isohyets are either estimated on the graph or measured by planimeter

Then,