Fundamentals of Engineering Design
the area contributing water to the basin outlet. Field reconnaissance is an essential component of most
watershed studies, particularly with regard to urban and agricultural drainage networks and the behavior
of hydraulic structures.
Existing digital coverages of watershed boundaries sometimes preclude the need for watershed
delineation. In most areas of the country, the boundaries of large watersheds (on the order of 1,000 square
miles) have already been digitized by federal and/or state agencies. These boundaries are widely available
at the scale of river basins and major subbasins but availability of small watershed boundaries varies from
state to state. The USGS assigns a system of numerical codes that provides information on location, scale,
and hierarchy/nesting for each basin. River basin scale watersheds with eight and eleven digit codes are
widely available whereas fourteen digit "hydrologic units" are much less available. Fourteen digit
watersheds, when available, are an excellent resource for watershed planning and management activities
and usually depict watershed areas on the order of 10-100 mi2. It has been estimated that over 93 percent
of streams nationwide have drainage areas of less than 23 square miles (Leopold et al., 1964). Planning
on the scale of these small watersheds, therefore, has the potential to positively effect a great majority of
surface waters.
5.1.5.1.6 Watershed Attributes Geographic Information Systems
Once a watershed has been identified, a number of parameters can be calculated that aid in
describing and quantifying the characteristics of the watershed. The determination of several watershed
parameters provides information that is useful in making decisions about how to manage the watershed in
addition to simply describing it. Organizations in the United States that offer GIS resources include:
Alaska Geospatial Data Clearinghouse
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Arkansas, Center For Advanced Spatial Technologies
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Arizona Geographic Information Council
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California
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- Teale Data Center
- California Geographic Information Council
Colorado Geographic Information Coordinating Committee
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Florida Data Directory
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Idaho Geographic Information Center
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Illinois Natural History Survey
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Iowa
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- Iowa Geographic Information Council
- Iowa Geospatial Data Clearinghouse
- Iowa Dept. of Natural Resources
Kansas
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- Kansas Data Access and Support Center (DASC)
- Kansas Geographic Information Systems Policy Board
Kentucky Geographic Information Advisory Council
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Louisiana Geographic Information System Council
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