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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If you are setting up your own lesson, you will need a. To do this, draw polygons. Often one map will represent a snapshot of precip in time.

An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin. Each isohyet each contour line represents equal precip along its length. Set up a table like this. An eexaggerated range of precip values makes it easier to find errors in calculations. The blue polygon, for example, represents area between the 4.

In the example, we use a contour interval of 0. Watersheds A,B,C,D in blue.

The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through There should be more watersheds than station points. Discuss results with respect to scale and measurement density. Rather, enter the values into Column B in table below.

The reason for doing this simply for visualization. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated.

Contour the precip values using standard contouring rules. In a pinch, you could do it by hand. Each point gets it own polygon and the point value is distributed throughout the entire polygon. We need to determine the area represented between each isohyet. Isohyetal maps are just contour maps of precipitation. OK any warning pop ups. Refs for Isohyetal Method: So the precip value, initially measured at the station point, expands to become the value for the entire polygon.

Formulas are shown for calculations in red italic text. The next step is to calculate the area of each intersect polygon. This is your weighted average precip by Watershed via the Thiessen method. There are several stations in the example watershed. This interpolation method was first applied to weather station data by A.

## Thiessen & Isohyetal Precip

Make sure your Data Frame is set to the proper mdthod system and map display units. Thiessen polygons are constructed from the station points not the precip values.

Intersecting complicates the attribute table. I usually have students report values at 3 scales: The map area is divided into polygons based on Voronoi diagram rules. Start with precip measurements rainfall collected at several points weather stations.

Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text. This is easily done in Excel. Provide only one precip measurement per station to keep things simple. In this example, we find the average precip for a watershed at one snapshot in time given station measurements. Next, determine the Average Precip value for each isohyet zone, as shown by the dashed lines and black text.

You could substitute your study area boundary or limit of contouring for the watershed boundary. 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. Thiessen polygons constructed from a set of weather station points precipitation measurements.

Mapping point data usually involves some sort of interpolation. 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 & Isohyetal Precip | GIS 4 Geomorphology

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