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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In a pinch, you could do it by hand. The reason for doing this simply for visualization. Refs for Thiessen Method: In the example, we use a contour interval of 0. OK any warning pop ups. Start with precip measurements rainfall collected at several points weather stations. Next, determine the Average Precip value isoyetal each isohyet zone, as shown by the dashed lines and black text.
Mapping point data usually involves some sort of interpolation. Rather, enter the values into Column B in table below. There should be more watersheds than station points. 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, Area in square miles the label on this figure is incorrect for each of the inter-contour polygons shown in blue text.
Make sure your Data Frame is set to the proper coordinate system and isohyetak display units. In this example, every watershed has one station and one precip value. Refs for Isohyetal Method: This interpolation method was first applied to weather station data by A.
Thiessen & Isohyetal Precip
Do not add the new table to the map; open it in Excel and work with it there. To do this, draw polygons. 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. An eexaggerated range of precip values makes it easier to find errors in calculations.
The blue polygon, for example, represents area between the 4. Isohyetal maps are just contour maps of precipitation. Each isohyet each contour line represents equal precip along its length.
The next step is to calculate the area of each intersect polygon.
Often one map will represent a snapshot of precip in time. The values are halfway between the isohyet contour values. Click for larger image.
Intersecting complicates the attribute table. Formulas are shown for calculations in red italic text. There are several stations in the example watershed.
This is your weighted average precip by Watershed via the Thiessen method. Stations should be distributed both inside and outside watershed polygons see Fig. The Thiessen polygon method is one of 5 different ways station precip measurements are extrapolated. Set up a table like this. Contour the precip values using standard contouring rules. I usually have students report values at 3 scales: The Thiessen Polygon method is an interpolation method commonly used for precipitation, but can be used isogyetal other point datasets.
If you are setting up your own lesson, you will need a. So the precip value, initially measured at the station point, expands to become the value for the entire polygon.
Thiessen & Isohyetal Precip | GIS 4 Geomorphology
You could substitute your study area boundary or limit of contouring for the watershed boundary. This table shows a record for each Intersect polygon, isohytal is overkill.
We need to determine the area represented between each isohyet. An improvement would be to show only records for Watersheds, then merge cells in additional columns for Zones and Basin. Thiessen polygons constructed from a set of weather station points precipitation measurements.
The map area is divided into polygons based on Voronoi diagram rules. In this example, we find the average precip for a watershed at one snapshot in time given station measurements.