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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Click for larger image. Formulas are shown for calculations in red italic text.

There are several stations in the example watershed. The values are halfway between the isohyet contour values. 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.

### Thiessen’s Polygon & Isohyetal Methods by jacob Baby on Prezi

An eexaggerated range of precip values makes it easier to find errors in calculations. This is your weighted average precip by Watershed via the Thiessen method. Do not add the new table to the map; open it in Excel and work with it there. Isohyetal maps are just contour maps of precipitation. Mapping point data usually involves some sort of interpolation. Area in square miles the label on this figure is incorrect for each isohyeta, the isohyetxl polygons shown in blue text.

### Thiessen & Isohyetal Precip | GIS 4 Geomorphology

Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text.

The map area is divided into polygons based on Voronoi diagram rules. Refs for Thiessen Method: So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

If you are setting up your own lesson, you will need a. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. In the example, emthod use a isohyefal interval of 0. The blue mthod, for example, represents area between the 4. Make sure your Data Frame is set to the proper coordinate system and map display units.

Often one map will represent a snapshot of precip in time. To do this, draw polygons.

Refs for Isohyetal Method: Each isohyet each contour line represents equal precip along its length. Start with precip measurements rainfall collected at several points weather stations.

This is easily done in Excel. In this example, we find the average precip for a watershed at one snapshot in metuod given station measurements. Contour the precip values using standard contouring rules. Each point gets it own polygon and the point value is distributed throughout the entire polygon.

The reason for doing this simply for visualization. In this example, every watershed has one station and one precip value.

The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through 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, An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin.

Intersecting complicates the attribute table. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated. This table shows a record for each Intersect polygon, which is overkill.

## Thiessen & Isohyetal Precip

Provide only one precip measurement per station to keep things simple. Thiessen polygons are constructed from the station points not the precip values. Discuss results with respect to scale and measurement density.

OK any warning pop ups. Rather, enter the values into Column B in table below. Set up a table like this. There should be more watersheds than station points. This interpolation method was first applied to weather islhyetal data by A. Metnod usually have students report values at 3 scales: