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

There are several stations in the example watershed. Make sure your Data Frame is set to the proper coordinate system and map display units. There should be more watersheds than station points. Provide only one precip measurement per station to isonyetal things simple.

The map area is divided into polygons based on Voronoi diagram rules. Each point gets it own polygon and the point value is distributed throughout the entire polygon.


We need isohyyetal determine the area represented between each isohyet. Watersheds A,B,C,D in blue. In a pinch, you could do it by hand. Intersecting complicates the attribute table.

OK any warning pop ups. To do this, draw polygons.

Area in square miles the label on this figure metgod incorrect for each of the inter-contour polygons shown in blue text. This is easily done in Excel. An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

In this example, every watershed has one station and one precip value. Isohyetal maps are just contour maps of precipitation. So the mefhod value, initially measured at the station point, expands to become the value for the entire polygon. The values are halfway between the isohyet contour values.

Set up a table like this.

Refs for Isohyetal Method: Stations should be distributed both inside and outside watershed polygons see Fig. Rather, enter the values into Column B in table below.


Thiessen & Isohyetal Precip

In the example, we use a contour interval of 0. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets.

Formulas are shown for calculations in red italic text. Recognize that the set of polygons generated 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, The blue polygon, for example, represents area between the 4.

Often one map will represent a snapshot of precip in time. Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. Discuss results with respect to scale and measurement density. Thiessen polygons constructed from a set of weather station points precipitation measurements. This interpolation method was first applied to weather station data by A.

The next step is to calculate the area of each intersect polygon.