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

An eexaggerated range of precip values makes it easier to find errors in calculations. Set up a table like this.

isohhyetal Thiessen polygons are constructed from the station points not the precip values. The blue polygon, for example, represents area between the 4. Rather, enter the values into Column B in table below. You could substitute your study area boundary or limit of contouring for the watershed boundary.

If you are setting up your own lesson, you will need a. This table shows a record for each Intersect polygon, which is mehhod.

Often one map will represent a snapshot of precip in time. Intersecting complicates the attribute table.

An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin. 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, To do this, draw polygons.

Each isohyet each contour line represents equal precip along its length. This interpolation method was first applied to weather station data by A. Mapping point data usually involves some sort of interpolation. Make sure your Data Frame is isonyetal to the proper coordinate system and map display units.

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

There should be more watersheds than station points. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated. Isohyetal maps are just contour maps of precipitation. Click for larger image.

Do not add the new table to the map; open it in Excel and work with it there. Watersheds A,B,C,D in blue.

## Thiessen & Isohyetal Precip

Refs for Isohyetal Method: Thiessen polygons constructed from a set of weather station points precipitation measurements.

Start with precip measurements rainfall collected at several points weather stations. Contour the precip values using standard contouring rules. In a pinch, you could do it by hand. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

Refs for Thiessen Method: The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. The values are halfway between the isohyet contour values.

Provide only one precip measurement per station to keep things simple.

The reason for doing this simply for visualization. Formulas are shown for calculations in red italic text. 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. Stations should be distributed both inside and outside watershed polygons see Fig.

Discuss results with respect to scale and measurement mehhod. This is your weighted average precip by Watershed via the Thiessen method.

The intersection of Watershed polys and Mehtod polys creates many new polygons ID numbers 1 through We need to determine the area represented between each isohyet. In this example, every watershed has one station and one precip value. OK any warning pop ups. I usually have students report values at 3 scales: In the example, we use a contour interval of 0.

The map area is divided into polygons based on Voronoi diagram rules. This is easily done in Excel. In this example, we find the average precip for a watershed at one snapshot in time given station measurements.

The isohuetal step is ksohyetal calculate the area of each intersect polygon. Each point gets it own polygon and the point value is distributed throughout the entire polygon. There are several stations in the example watershed.

Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text.