WMOD00546

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

1. Introduct ion

The statistical characteristics of rainfall are important considerations

in many climatological, agricultural, hydrological, and weather modification
applications. Of the various statistical properties, the variability of
rainfall is, perhaps, the most important, especially in the evaluation of
precipitation augmentation experiments in which rainfall is the primary
response variable. Estimates of the natural variability of rainfall are,
however, compounded by vari ance components that are introduced by the

method of measurement, namely those due to sampl ing and measurement errors.

It is the purpose of this papE~r to examine the sampling variance of raingage

networks, the most commonly used precipitation measurement system, and

quantitatively relate this variance to both the density of raingd.ges and the
characteristics of the isohyetal pattern.

The variance due to sampl ing t",esults from the necessity of estimclting areal

rainfall values from point measurements of a storm footprint that contains
spatial gradients. This is especially true for convective storms which

are of primary interest in this study. The estimation procedure is usually

based on the assumption that a point measurement by a precipitatjion gage is
represent at ive of the area that surrounds it. Th us, depend i ng on just how
the storm passes over the gagE! network, the precipitation is usually either

under or overestimated.

A number of investigators (e.g., Light 2, Linsley and Kohler, 1951;
McGuinness, 1963; Huff, 1970; Woodley et al., 1975; and Eddy, 1976) have

2L ight, P., 1947: Hydrol ogi c aspects of thunderstorm rainf all. Hydrometeor
Report No.5, U.S. Weather BurE~au-Corps of Engineers, 260-268.