12
(3)
The Isohyetal Method (Fig. 1.4)
Isohyets are the contours of equal rainfall. Values of annual rainfall are recorded
in a map of basin of respective rain gauge stations and then contours are drawn
based on the judgement of equal rainfall. The areas between two isohyets are
estimated either by planimeter or by tracing on a graph paper. Let the areas be
A
1
, A
2
, A
3
etc. Corresponding to the rain gauge stations of P
1
, P
2
, P
3
etc. Then
the average or mean rainfall is given by :
P = P
1
A
1
+ P
2
A
2
+ P
3
A
3
-------- P
n
A
n
A
1
+ A
2
+ A
3
------ A
n
Fig. 1.4 :
Isohyetal method for computing average rainfall
(After Punmia et al, 1992)
Rainfall - Run off
When rainfall occurs over an area, it falls on the earth's surface after being
intercepted by trees, habitations, buildings, etc. and flows overland, generally
towards the nearby rivers or streams. If during the passage it encounters small
depressions, the area is first filled up and hardly contributes to the direct run-off,
but very significant for groundwater recharge or the contribution of rainfall to the
groundwater body. Movement of overland flow is also influenced by the soil type
over which surface run-off is moving towards the streams. If the soil type is very
pervious or having high infiltration capacity water will move downwards to reach
