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Delineating Watersheds

How to Delineate a Watershed

This site is an excerpt from Appendix E of the Method for the Comparative Evaluation of Nontidal Wetlands in New Hampshire, 1991. Alan Ammann, PhD and Amanda Lindley Stone. This document and method is commonly called “The New Hampshire Method.” 

This site and Reading Topographic Maps are available together for download. This document requires Adobe Acrobat.

How to Read a Topographic Map and Delineate a Watershed (300 Kb)
  

The following procedure and example will help you locate and connect all of the high points around a watershed on a topographic map shown in Figure F-4 below (for a higher quality and larger image, click on the figure). Visualizing the landscape represented by the topographic map will make the process much easier than simply trying to follow a method by rote (first visit How to Read a Topographic Map).

  1. Draw a circle at the outlet or downstream point of Steps one and two in delineating a watershed on a topographic map are shown. the wetland in question (the wetland is the hatched area shown in Figure E-4 to the right) 
  2. Put small "X's" at the high points along both sides of the watercourse, working your way upstream towards the headwaters of the watershed. 
  3. Starting at the circle that was made in step one, draw a line connecting the "X's" along one side of the watercourse (Figure E-5, below left). This line should always cross the contours at right angles (i.e. it should be perpendicular to each contour line it crosses). 
  4. Continue the line until it passes around the head of the watershed and down the opposite side of the watercourse. Eventually it will connect with the circle from which you started. At this point you have delineated the watershed of the wetland being evaluated.

Steps 3 and 4 of the watershed delineation process are shown in this image. The delineation appears as a solid line around the watercourse. Click here or on the figure for a higher quality, larger image. Generally, surface water runoff from rain falling anywhere in this area flows into and out of the wetland being evaluated. This means that the wetland has the potential to modify and attenuate sediment and nutrient loads from this watershed as well as to store runoff which might otherwise result in downstream flooding.

Measuring Watershed Areas

There are two widely available methods for measuring the area of a watershed: a) Dot Grid Method, and b) Planimeter. These methods can also be used to measure the area of the wetland itself as required by The New Hampshire Methods.

a) The dot grid method is a simple technique which does not require any expensive equipment. In this method the user places a sheet of acetate or mylar, which has a series of dots about the size of the period at the end of this sentence printed on it, over the map area to be measured. The user counts the dots which fall within the area to be measured and multiplies by a factor to determine the area. A hand held, mechanical counting device is available to speed up this procedure.

b) The second of these methods involves using a planimeter, which is a small device having a hinged mechanical arm. One end of the arm is fixed to a weighted base while the other end has an attached magnifying lens with a cross hair or other pointer. The user spreads the map with the delineated area on a flat surface. After placing the base of the planimeter in a convenient location the user traces around the area to be measured with the pointer. A dial or other readout registers the area being measured.

Planimeters cost from several hundred dollars up to a thousand dollars or more depending on the degree sophistication. For the purposes of The New Hampshire Method, a basic model would be sufficient. Dot counting grids are more affordable, and are in the 10 to 20 dollar range. Both planimeters and dot grids are available from engineering and forestry supply companies. Users of either of these methods should refer to the instructions packaged with the equipment they purchase.