Geoscience Reference
In-Depth Information
14.9
27.2
15
20
25
12.7
10
7.1
26.1
Basin margin
Contours
Isohyets (mm)
Point values (mm)
5.6
10
19.0
5
10.8
9.2
27.3
26.5
4.7
7.1
5
15.0
24.2
10
25.8
15
25
20
18.5
Figure 14.2
Calculation of area-average precipitation using the Isohyetal Method. (From Sumner, 1988, published with
permission.)
(2)
measuring or computing the area between isohyets to provide the weights
to be used when calculating the average value; and
(3)
creating the weighted average by adding the products of the area between
isohyets in the catchment with the mean precipitation between isohyets.
Figure 14.2 illustrates the approach used in the Isohyetal method for an example
catchment. Using this method with these data and this example catchment,
Sumner (1988) calculated a catchment average precipitation of 17.3 mm.
Computer calculations of mean rainfall over selected catchments based on
mapping can also be made using kriging methods or the simpler (but arguably as
effective) Reciprocal-Distance-Squared method, both of which are similar in
concept to the Isohyetal method. In the Reciprocal-Distance-Squared method, a
value of precipitation,
P
i
(in mm), is assigned to each area element in the catchment
into which the catchment is subdivided by the computer program. This value is
the average of the nearest three gauges weighted by the square of the inverse
distances,
d
1
,
d
2
, and
d
3
(in m) between the element and the three gauges (Fig. 14.3).
Thus
P
i
is calculated from:
−
2
−
2
−
2
Pd Pd Pd
()
+
()
+
()
(14.1)
11
2 2
3 3
P
=
i
−
2
−
2
−
2
()
d
+
()
d
+
()
d
1
2
3
