Precipitation Over an Area

We know that, we install rain gauge station as per terrain type and form a Rain Gauge Network. In a rain gauge network, there are so many rain gauge which show rainfall data. As the rainfall data given by rain gauge is point sample, but we need to find out the rainfall over the complete area. 

Methods to find out precipitation over an area: 

1. Arithmetic Mean Method
2. Thiessen Polygon Method
3. Isohyetal Method

Arithmetic Mean Method :

This is very simple method to convert precipitation over an area. This method is used, when the rainfall measured at various stations in a catchment are show little variation or we can say the rain is uniformly distributed. This method does not take into account the rain gauges located outside the catchment area.

It is calculated by simply taking the arithmetic mean of rainfall reading shown by rain gauges in particular catchment, 

Where,

P1, P2, P3,................Pn are the rainfall shown rain gauges in a catchment, in a given period.

Thiessen Polygon Method :

In this method, the rainfall recorded at each station is given a weightage on the basis of area closest to the station. This method is reliable only fro plain areas, because it doesn’t take into account elevation difference. In this method the whole catchment area is divided into smaller areas, on which we assume the uniform rainfall.   

Where, 

1.The polygons are formed by the perpendicular bisectors of the lines joining nearby stations. 

2.The area of each polygon is used to weight the rainfall amount of the station in the centre of the polygon. 

3.If the amount for any station is missing, the polygon must be changed. 

Precipitation calculated by, 

Isohyetal Method :

Isohyet is a line joining the points of equal rainfall magnitude or isohyets are the contours of equal rainfall. In the Isohyetal method, the catchment is drawn on paper with suitable scale and the rain gauge stations are marked on paper at appropriate place. With the help of planimeter we find the area between the two isohyets. We assume that the area between two isohyets having the rainfall equal to the arithmetic mean of isohyets. 

This is most accurate method, because this method gives most accurate results as compare two remaining two methods. precipitation calculated by,