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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This interpolation method was first applied to weather station data by A. We need to determine the area represented between each isohyet.

OK any warning pop ups. Contour the precip values methd standard contouring rules. Thiessen polygons are constructed from the station points not the precip values. In this example, we find the average precip for a watershed at one snapshot in time given station measurements. There should be more watersheds than station points. The map area is divided into polygons based on Voronoi diagram rules.

Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. The blue polygon, for example, represents area between the 4. Refs for Isohyetal Method: Refs for Thiessen Method: Each isohyet each contour line represents equal precip along its length.


The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated. Thiessen polygons constructed from a set of weather station points precipitation measurements.

Recognize that the set of polygons isohyetao by the Intersect tool may be numbered in such a way that one ID may represent more than one entity for example see polygons 8, 17, 19, Isohyetal maps are just contour maps of precipitation.

The values are halfway between the isohyet contour values. Watersheds A,B,C,D in blue. Set up a table like this. 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.

If you are setting up your own lesson, you will need a. This is easily done in Excel. Area in square miles the label on isohyetxl figure is incorrect for each of the inter-contour mefhod shown in blue text.

In a pinch, you could do it by hand. In the example, we use a contour interval of 0. This is your weighted average precip by Watershed via the Thiessen method. The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through Make sure your Data Frame is set to the proper coordinate system and map display units. Intersecting complicates the attribute table.


Thiessen & Isohyetal Precip

Click for larger image. This table shows a record for each Intersect polygon, which is overkill. Rather, enter the values into Column B in table below. Stations should be distributed both inside and outside watershed polygons see Fig.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

Provide only one precip measurement per station to keep things simple. There are several stations in the example watershed. Formulas are shown for calculations in red italic text.

You could substitute your study area boundary or limit of contouring for the watershed boundary. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. Each point gets it own polygon and the point value is distributed throughout the entire polygon. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

An eexaggerated range of precip values makes it easier to find errors in calculations. I usually have students report values at 3 scales: Mapping point data usually involves some sort of interpolation.