Geoscience Reference
In-Depth Information
14.9
27.2
B
12.7
D
A
7.1
26.1
H
C
J
G
F
19.0
I
5.6
E
R
10.8
9.2
N
L
P
27.3
K
Q
26.5
X
4.7
7.1
M
O
S
W
Y
15.0
T
24.2
Figure 14.4
Calculation of
area-average precipitation
using the Triangle Method.
(From Sumner, 1988,
published with permission.)
U
V
25.8
18.5
(2)
measuring or computing the area of each polygon or portion of each poly-
gon that is within the basin to provide the weights to be used when calcu-
lating the average value; and
(3)
calculating the weighted average by adding the products of the within-
basin area of each polygon with the precipitation measured by each gauge.
Figure 14.5 illustrates the approach used in the Theissen method for an example
catchment. Using this method with these data and this example catchment,
Sumner (1988) calculated a catchment average precipitation of 17.5 mm.
Spatial organization of precipitation
Analysis of individual storm events as measured using dense networks of gauges
suggests that storms generally comprise one or more cells of high intensity
precipitation embedded in a surrounding field of lower intensity precipitation. The
precipitation pattern generated by individual storms measured by gauges differ
significantly from one another but, based on analyses made by many observers and
at many sites, opinion is that on average storms tend to be (a) elliptical in shape,
(b) more elongated when larger (i.e., the ratio of the major axes to the minor axis
of the ellipse is greater for bigger storms), and (c) organized into groups which
themselves are either larger pseudo-elliptical areas or linear bands (a linear band
