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Download Table | Average rainfall using the isohyetal method from publication: THE EV AL UA T ION OF THE PRESENT AN D POTENTIA L WAT ER. Accurate method,but very laborious. For each storm, a separate isohyet has to be drawn. If its done on an annual basis the work will be very. Point. Basin. Precipitation. Gaging Station. We will discuss four methods: 1) Arithmetic Average, 2) Theissen Polygons, 3) Isohyetal Method, and 4) Grid Method.

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There should be more watersheds than station points. Provide only one precip measurement per station to keep things simple. Recognize that the set of polygons generated by the Intersect tool may be metthod in such a way that one ID may represent more than one entity for example see polygons 8, 17, 19, Formulas are shown for calculations in red italic text.

The blue polygon, for example, represents area between the 4. Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text.

Isohyetal maps are just contour maps of precipitation. Thiessen polygons constructed from a set of weather station points precipitation measurements.


Do not add the new table to the map; open it in Excel and work with it there. Discuss results with respect to scale and measurement density. Stations should be distributed both inside and outside watershed polygons see Fig. Contour the precip values using standard contouring rules. Each isohyet each contour line represents equal precip along its length. To do this, draw polygons. Thiessen Polygons are Voronoi Cells, a geometric means of dividing up an area given a set of known values at a relatively small number of points.

This interpolation method was first applied to weather station data by A. Rather, enter the values into Column B in table below.

In the example, we use a contour interval of 0. There are several stations in the example watershed. This is easily done in Excel. Refs for Isohyetal Method: This is your weighted average precip by Watershed via the Thiessen method.

Thiessen & Isohyetal Precip

An eexaggerated range of precip values makes it easier to find errors in calculations. This table shows a record isobyetal each Intersect polygon, which is overkill.

Set up a table like this. In a pinch, you could do it by hand.

Watersheds A,B,C,D in blue. Refs for Thiessen Method: Intersecting complicates the attribute table. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. The Thiessen polygon method mdthod one of 5 different ways station precip measurements are extrapolated. Make sure your Data Frame is set to the proper coordinate system and map display units.


Thiessen’s Polygon & Isohyetal Methods by jacob Baby on Prezi

The map area is divided into polygons based on Voronoi diagram rules. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

Click for msthod image. Thiessen polygons are constructed from the station points not the precip values. An improvement would be to show only records for Watersheds, then jsohyetal cells in additional columns for Zones and Basin.

The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through The values are halfway between the isohyet contour values. Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text.