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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An eexaggerated range of precip values makes it easier to find errors in calculations. Discuss results with respect to scale and measurement density.

Often one map will represent a snapshot of precip in time. Each point gets it own polygon and the point value is distributed throughout the entire polygon.

Set up a table like this.

Thiessen & Isohyetal Precip

Intersecting complicates the attribute table. The next step is to calculate the area of each intersect polygon. Make sure your Data Frame is set to the proper coordinate system and map display units.

To do this, draw polygons. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin. You could substitute your study area boundary or limit of contouring for the watershed boundary. This interpolation method was first applied to weather station data by A.


mehtod Click for larger image. Stations should be distributed both inside and outside watershed polygons see Fig. Each isohyet each contour line represents equal precip along its length.

Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. This is your weighted average precip by Watershed via the Thiessen method. Thiessen polygons constructed from a set of weather station points precipitation measurements.

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. Area in square miles mdthod label on this figure is incorrect for each of the inter-contour polygons shown in blue text. Contour the precip values using standard contouring rules.

In this example, every watershed has one station and one precip value. Watersheds A,B,C,D in blue. Rather, enter the values into Column B in table below. In a pinch, you could do it by hand. The values are halfway between the isohyet contour values. There are several stations in the example watershed.

This table shows a record for each Intersect polygon, which is overkill.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

Isohyetal maps are just contour maps of precipitation. If you are setting up your own lesson, you will mehod a. Mapping point data usually involves some sort of interpolation. The blue polygon, for example, represents area between the 4.


There should be more watersheds than station points. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point isoyetal. We need to determine the area represented between each isohyet. The map area is divided into polygons based on Voronoi diagram rules.

This is easily done in Excel. The reason for doing this simply for visualization. OK any warning pop ups. Recognize that the siohyetal 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, In this example, we find the average precip for a watershed at one snapshot in time given station measurements.

Do not add the new table to the map; open it in Excel and work with it there. In the example, we use a contour interval of 0. Thiessen polygons are constructed from the station points not the precip values.