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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Often one map will represent a snapshot of precip in time. Discuss results with respect to scale and measurement density.
An eexaggerated range of precip values makes it easier to find errors in calculations. This table shows a record for each Intersect polygon, which is overkill.
I usually have students report values at 3 scales: Each isohyet each contour line represents equal precip along its length. There should be more watersheds than station points.
Watersheds A,B,C,D in blue. Next, determine the Average Precip value for each isohyet zone, as shown by meethod dashed lines and black text. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.
We need to determine the area represented between each isohyet. Mapping point data usually involves some sort of interpolation. Refs for Thiessen Method: Set up a table like this.
OK any warning pop ups.
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 values are halfway between the isohyet contour values.
The intersection of Watershed polys and Thiessen polys creates many new polygons ID numbers 1 through Each point gets it own polygon and the point value is distributed throughout the entire polygon.
You could substitute your study area boundary or limit of contouring for the watershed boundary. Contour the precip values using standard contouring rules.
In a pinch, you could do it by hand. If you are setting up your own lesson, you will need a.
Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text. Thiessen polygons constructed from a set of weather station points precipitation measurements. This interpolation method was first applied to weather station data by A. Start isohyteal precip measurements rainfall collected at several points weather stations. Formulas are shown for calculations in red italic text.
This is easily done in Excel. The reason for doing this simply for visualization. In this example, every watershed has one station and one precip value.
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. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated. In this example, we find the average precip for isihyetal watershed at one snapshot in time given station measurements.
There are several stations in the example watershed. Isohyetal maps are just contour maps of precipitation.
The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used on other point datasets. Intersecting complicates the attribute table. Stations should be distributed both inside and outside watershed polygons see Fig.