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. Click for larger image. Rather, enter the values into Column B in table below. This table shows a record for each Intersect polygon, which is overkill. Area in square miles the label on this figure is incorrect for each of the inter-contour isohyetak shown in blue text.

We need to determine the area represented between each isohyet. If you are setting up your own lesson, you will need a. In this example, we find the average precip for a watershed at one snapshot in time given station measurements. The values are halfway between the isohyet contour isohyetaal. Watersheds A,B,C,D in blue. The Thiessen polygon method is one of 5 different ways isohyyetal precip measurements are extrapolated.

Set up a siohyetal like this. This is your weighted average precip by Watershed via the Thiessen method. 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, Thiessen polygons are constructed from the station points not the precip values.

## Thiessen & Isohyetal Precip

The reason for doing this simply for visualization. An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin.

In this example, every watershed has one station and one precip value. Stations should be distributed both inside and outside watershed polygons see Fig. Each isohyet each contour line represents equal precip along its length.

In a pinch, you could do it by hand. 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.

Discuss results with respect to mwthod and measurement density. OK any warning pop ups.

There should be more watersheds than station points. The blue polygon, for example, represents area between the 4.

Contour the precip values using standard contouring rules. Isohyetal maps are just contour maps of precipitation. Each point gets it own polygon and the point value is distributed throughout the entire polygon. Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. Often one map will represent a snapshot of precip in time.

Refs for Isohyetal Method: Mapping point data usually involves some sort of interpolation. Intersecting complicates the attribute table.

Start with precip measurements rainfall collected at several points weather stations. Refs for Thiessen Method: There are several stations in the example watershed.

### Thiessen & Isohyetal Precip | GIS 4 Geomorphology

Formulas are shown for calculations in red italic text. So the precip value, initially measured at the station point, expands to become the value for the entire polygon. This interpolation method was first applied to weather station isojyetal by A. The map area is divided into polygons based on Voronoi diagram rules. This is easily done in Excel.

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. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets.

You could substitute your study area boundary or limit of contouring for the watershed boundary. Thiessen polygons constructed from a set of weather station points precipitation measurements. Provide only one precip measurement per station to keep things simple. I usually have students report values at 3 scales: 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.

The next step is metohd calculate the area of each intersect polygon. To do this, draw polygons.