Thiessen polygon Method In order to achieve accurate estimation of the spatial distribution of rainfall, it is necessary to use interpolation methods, for this, the Thiessen* method is considered as the most important in engineering praxis. This method assigns weight at each gauge station in proportion to the catchment area that is closest to that gauge. The method of constructing the polygons implies the following steps: 1. Gauge network is plotted on map of the catchment area of interest. 2. Adjacent stations are connected with lines. 3. Perpendicular bisectors of each line are constructed (perpendicular line at the midpoint of each line connecting two stations) 4. The bisectors are extended and used to form the polygon around each gauge station. 5. Rainfall value for each gauge station is multiplied by the area of each polygon. 6. All values from step 5 are summed and divided by total basin area. An example of spatial precipitation distribution according to Thiessen method can be appreciated in the following figure.

Figure 2.3 Construction of Thiessen polygon

Although it is widely used in engineering praxis, this method has its shortcomings. For example, in mountainous areas, an irregular spatial distribution of precipitation can be formed over small distances, and for such circumstances the Thiessen method can yield erroneous results. In order to overcome these problems, more accurate methods are used, in which the whole area is “rastered” and each of those rasters are calculated according to the quadrant method.