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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The values are halfway between the isohyet contour values.

Thiessen & Isohyetal Precip | GIS 4 Geomorphology

This is easily done in Excel. Watersheds A,B,C,D in blue. The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. The next step is to calculate the area of each intersect polygon.

Set up a table like this. Provide only one precip measurement per station to keep things simple.

Thiessen & Isohyetal Precip

Discuss results with respect to scale and measurement density. Each point gets it own polygon and the point value is distributed throughout the entire polygon.


Mapping point data usually involves some sort of interpolation. Often one map will represent a snapshot of precip in time. In the example, we use a contour interval of 0. 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 Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated.

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

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

OK any warning pop ups. Refs for Thiessen Method: 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. Thiessen polygons are constructed from the station points not the precip values. An eexaggerated range of precip values makes it easier to find errors in calculations.

Formulas are shown for calculations in red italic text. We need to determine the area represented between each isohyet. Make sure your Data Frame is set to the proper coordinate system and map display units.


Do not add the new table to the map; open it in Excel and work with it there. This table shows a record for each Intersect polygon, which is overkill.

Isohyetal maps are just contour maps of precipitation. Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text. Stations should be distributed both inside and outside watershed polygons see Fig. Thiessen polygons constructed from a set of weather station points precipitation measurements.

Rather, enter the values into Column B in table below. This is your weighted average precip by Watershed via the Thiessen method. An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Methld. If you are setting up your own lesson, you will need a.

Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text. Contour the precip values using standard contouring rules.