ABSTRACT
The average rainfall over a watershed can be evaluated using the data from a number of rain-gauge stations within or near the watershed. Depending on the situation, either the arithmetic mean or a weighted average is calculated. The Thiessen polygon method produces a weighted average by attributing an influence zone to each station, so that the value of precipitation measured at the station is assigned to the entire zone around it. The zones are determined by a geometric division of the watershed on a topographic map.
