Watershed Data

presented in Figure 5.4 for the mean daily data, and Figure 5.5 for the 15-minute data.

5.1.5.1.4 Watershed Data

An area of land that drains to a single outlet or waterbody is called a watershed. Watershed

boundaries follow the ridgelines and topographic divides that separate lands draining to different surface

waters. Before a watershed plan can be created, whether for erosion control, water quality, or some other

purpose, it is essential to know what exists in the watershed. The following sections briefly describe the

fundamental information needed to characterize watershed hydrology and define boundary conditions for

any subsequent hydraulic analyses.

5.1.5.1.5 Watershed Boundaries and Areas

A fundamental step in any watershed analysis is to delineate the boundary of the watershed above

some point of interest and determine the contributing land area within that boundary. Watershed

boundaries may be delineated by several means at various levels of accuracy. Watersheds may also be

defined at many different scales and sizes from the scale of the Mississippi River watershed to the scale of

the many thousands of streams and rivers that make up smaller watersheds within the larger Mississippi

Basin. Perhaps the most common approach is to use USGS topographic maps at either 1:24,000 or

1:100,000 scales to identify the contributing area above the watershed outlet by tracing ridgelines

determined from elevational contours. The technique for determining watershed boundary on a topographic

map is to start at the base level and, working uphill, mark the ridgeline. The decision as to whether a

particular piece of ground is "in" or "out" of the watershed may be determined by examining whether the

area of interest flows to the stream above the base level or watershed outlet. Three simple rules help in this

determination:

1. Water tends to flow perpendicularly across contour lines;

2. Ridges are indicated by contour "V"s pointing downhill; and

3. Drainages are indicated by contour "V"s pointing upstream.

Once the watershed boundary has been delineated, the area within may be estimated using a

planimeter, tracing paper and a grid overlay, or by digitization using a digitizing tablet and an appropriate

software package. More sophisticated approaches include the use of Geographic Information Systems

(GIS) and digital elevation models to compute watershed areas. While it is recognized that most of the

watershed is, in all likelihood, on a slope, the area that is reported is the horizontal projection of the

watershed boundary. In regions where extensive drainage or supply networks are linked across former

watershed divides, care must be taken to adequately represent

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