WMOD00546

• Project Name: High Plains Cooperative Program
• Title: On the Sampling Variance of Raingage Networks
• Weather Modification - Doc Type: Report

tion, gage inclination,splash, reading, exposure, and wind effects) and how
much is due to local, storm-related random variability is not known.

Of all the measurement-related errors, exposure and wind have the greatest
effect on gage catch, the others being estimated by Kurtyka3 to be an
average of 1.5 percent. Larson and Peck (1974) have shown that the effect of
wind speed on gage catch increases with increasing wind speed. The average
gage catch deficiency for rain is 12 percent at 4 ms-1, increasing to 18
percent at 8 ms-1. These errors can be reduced, but not eliminated, by the

use of gage shields and proper site selection. Woodley et at. (1975) examined

the accuracy of point rainfall measurements by comparing the measurements

of collocated gages and found that the percentage uncertainty (maximum rain
difference divided by maximum point rainfall) is about 5 percent for maximum

rainfalls near 25.4 mm, increasing to 12 percent for maximum rainfalls of

2.54 mm. He felt that this uncertainty was random variability probably due
to subtle differences in gage exposure. Huff (1955) compared the measurements
of gages 6 feet apart and found a relative variability (average difference
divided by areal mean rainfall) of 4 percent for mean rainfalls of 0.25 to 5

mm, decreasing to 1 percent for rainfalls greater than 12.70 mm. Since great

care was devoted to gage exposure, gage maintenance, and observation techniques,

Huff concluded that these differences were the minimum to be expected in

shower-type rainfall. Both Woodley et al. (1975) and Huff (1955) found that

the variability tends to decrease with increasing rainfall.

In view of the potential magn'itude of measurement-related errors" it is
difficult to conceive of an approach to estimate the magnitude of local,

3 Kurtyka, J. C., 1953: Precipitation measurements study, Repoy't of Investi-
gation No. 20, Illinois State Water Survey Division, 178 pp.

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