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. We need to determine the area represented between each isohyet. This is your weighted average precip by Ishoyetal via the Thiessen method.

This is easily done in Excel.

Watersheds A,B,C,D in blue. You could substitute your study area boundary or limit of contouring for the watershed boundary. Stations should be distributed both inside and outside watershed polygons see Fig. There are several stations in the example watershed. This table shows a record for each Intersect polygon, which is overkill.

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

OK any warning pop ups. Isohyetal maps are just contour maps of precipitation. Intersecting complicates the attribute table. An improvement would be to show only records for Watersheds, isohyetl merge cells in additional columns for Zones and Basin. Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text. The next step is to calculate the area of each intersect polygon. The Thiessen polygon method isohyegal one of 5 different ways station precip measurements are extrapolated.

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The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through Often one map will represent a snapshot of precip in time.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

There should be more watersheds than station points. If you are setting up your own lesson, you will need a. Thiessen polygons constructed from a set of weather station points precipitation measurements. Formulas are shown for calculations in red italic text.

Thiessen & Isohyetal Precip

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, So the meethod value, initially measured at the station point, expands to become the value for the entire polygon. Refs for Thiessen Method: I usually have students report values at 3 scales: Mehhod up a table like this.

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


This interpolation method was first applied to weather station data by A. The blue polygon, for example, represents area between the 4. Contour the precip values using standard contouring rules. Make sure your Data Frame is set to the proper coordinate system and isohhetal display units.

Mapping point data usually involves some sort of interpolation. The map area is divided into polygons based on Voronoi diagram rules. Rather, enter the values into Column B in table below.

Discuss results with respect to scale and measurement density. Click for larger image. In this example, every watershed has one station and one precip value.