How to calculation ?
   There are there methods to calculate the  averaging precipitation depth over an area..

1 

 The simplest way of doing this is to take the  arithmetical mean of the amounts known
 for all points in the area. 
   
    If the  distribution of such points over the area is  not uniform and the variation 
in the  individual gauge's amounts are larg,  this method gives unreasonably  
results. To deal with this, Thiessen's Methods is introducted to acquire a accurate results.

2

Another Methods is Thiessen's Method. In this section, this methods will be presented.

Step 1: we defines the aone of influence of each station first by draing line between pairs of gauges. 

Step 2 :then, write a drop line to bisect the lines with perpendiculars 

Step 3 :finally, to assume all the area enclosed within the  boundary formed these intersecting 
perpendiculars has had  rainfall of the same  amount as enclosed gauge.By the following equation we have 

The result:

-------(1)

 Instructions and Quizzes

3 
Apart form Thiessen's Method and arithmetical mean, isohyet is another methods to achieve a best results.
However, the methods requirs a numerous rain gauge, so that the contours of equal rainfall depths can be obtained. 
Then, the areas between successive isohyets are measured and considered as   an average  value of rainfall. 
finally, the equation (1) is used again to obtain final result again.

One of the advantages of this methods is that the methods take account of local effects like prevailing wind and
 uneven topography.  

Quizzes