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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I usually have students report values at 3 scales: If you are setting up your own lesson, you will need a. Refs for Thiessen Method: This is easily done isohyetl Excel. Rather, enter the values into Column B in table below. Each isohyet each contour line represents equal precip along its length.

Thiessen & Isohyetal Precip

The reason for doing this simply for visualization. Intersecting complicates the attribute table.

Thiessen Polygons are Voronoi Cells, a geometric means of dividing up an area given a set of known metho at a relatively small number of points.

We need to determine the area represented between each isohyet.


Thiessen polygons constructed from a set of weather station points precipitation measurements. There should be more watersheds than station points.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

Stations should be distributed both lsohyetal and outside watershed polygons see Fig. There are several stations in the example watershed. This table shows a record for each Intersect polygon, which is overkill. Recognize that the set of polygons generated by the Intersect tool may be numbered in such a way that one ID isohyftal represent more than one entity for example see polygons 8, 17, 19, Thiessen polygons are constructed from the station points not the precip values.

Provide only one precip measurement per station to keep things simple. This is your weighted average precip by Watershed via the Thiessen method.

Start with precip measurements rainfall collected at several points weather stations. In a pinch, you could do it by hand.

In this example, every watershed has one station and one precip value. Click for larger image. Mapping point data usually involves some sort of interpolation. 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

Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. Contour the precip values using standard contouring rules.


Discuss results with respect to scale and measurement density. Watersheds A,B,C,D in blue. Each point gets it own polygon and the point value is distributed throughout the entire polygon.

The values are halfway between the isohyet contour values. To do this, draw polygons. You could substitute your study area boundary or limit of contouring for the watershed boundary.

Isohyetal maps are just contour maps of precipitation. The Thiessen Polygon method is an interpolation isohyefal commonly used for precipitation, but can be used on other point datasets.

Formulas are shown for calculations in red italic text. The map area is divided into polygons based on Voronoi diagram rules. OK any warning pop ups. In this example, we find the average precip for a watershed at one snapshot in time given station measurements.

Set up a table like this.

An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin.