Geoscience Reference
In-Depth Information
P
1
P
2
d
1
d
2
Figure 14.3
Calculating
area-average precipitation
using the reciprocal-
distance-squared method.
(Precipitation
P
1
,
P
2
, and
P
3
in mm, distance
d
1
,
d
2
, and
d
3
in m.)
d
3
P
3
Triangle method
This method involves:
(1)
constructing a network of triangles as near equilateral as possible, with one
of the available gauges at the apex of each triangle;
(2)
measuring or computing the area of each triangle or portion of each trian-
gle that is within the basin to provide the weights to be used when calculat-
ing the average value;
(3)
if needed, making a 'best guess' of the precipitation for any area of the
catchment not included in a triangle; and
(4)
calculating the weighted average by adding the products of the area of each
triangle with the mean precipitation for the three gauges at the apex of
each triangle.
Figure 14.4 illustrates the approach used in the Triangle Method for an example
catchment. Using this method with these data and this example catchment,
Sumner (1988) calculated a catchment average precipitation of 17.8 mm.
Theissen method
The Theissen method is often the preferred method for estimating area-average
precipitation. The method involves:
(1)
constructing a network of polygons by drawing the perpendicular bisector
of the line joining each pair of gauges;
