The methods of computing average rainfall over the basin are:-

1. Arithmatic average method.
2. Thiessen Polygon method.
3. Isohyetal method.
4. Grid point method.

Arithmatic average method.

If the rainfall is uniformly distributed on its areal pattern, the simplest method of estimating average rainfall is to compute the arithmetic average of the recorded rainfall values at various stations. Thus, if P1,P2, P3,…….Pn etc., are the precipitation or rainfall values measured at n gauge stations, we have

$$P_{av} = \frac{P1+P2.....+Pn}{n} = \frac{1}{n} \sum_{i=1}^n Pi$$