Hydrology - PRecipitation, Summaries of Hydrology

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THIESSEN POLYGON METHOD

THIESSEN POLYGON METHOD (ii) Thiessen polygon method—This method attempts to allow for non-uniform distribu- tion of gauges by providing a weighting factor for each gauge. The stations are plotted on a base map and are connected by straight lines. Perpendicular bisectors are drawn to the straight lines, joining adjacent stations to form polygons, known as Thiessen polygons (Fig. 2.10). Each polygon area is assumed to be influenced by the raingauge station inside it, i.e., if P,, P,, P,, .... are the rainfalls at the individual stations, and A,, Ay, Ag, .... are the areas of the polygons surrounding these stations, (influence areas) respectively, the average depth of rainfall for the entire basin is given by ZAP, = —t1L Faw = SA, w-A2.2) where EA, = A = total area of the basin. The results obtained are usually more accurate than those obtained by simple arithme- tic averaging. The gauges should be properly located over the catchment to get regular shaped polygons. However, one of the serious limitations of the Thiessen method is its non-flexibility since a new Thiessen diagram has to be constructed every time if there is a change in the raingauge network. Fig. 2.10 Thiessen polygon method, Example 2.3