sheds. Agricultural data is also compatible with watershed boundaries because the Census of Agriculture (of the U.S. Department of Agriculture), reports production data and livestock populations according to counties. For smaller watersheds, census data collected on divisions smaller than counties are also available, with data aggregated according to census tracts and census blocks that have resolution to a scale equal to about four city blocks. Watershed approaches are therefore geographical perspectives that can integrate of physical, chemical, some biological, economic, and social data.
The use of watershed boundaries is less effective for wildlife and vegetation applications. Many types of ecosystems do not have boundaries determined by drainage divides, but this problem should not be detrimental to watershed approaches. For small ecosystems, vegetation patches, and limited habitats for some wildlife species, even small watersheds will completely contain the area of concern. In the case of bald eagle nesting places, for example, some watershed will contain the critical habitat, while others do not, and so the management issue will be present for some watersheds and not others. Some ecosystems are either larger than easily defined watersheds, or they span several, such as the greater Yellowstone ecosystem. In these cases, special considerations will be required to account for the overlap.
Watersheds are open systems, with mass and energy being exchanged with places outside the defined area. Groundwater, for example, migrates along groundwater gradients that do not always respect the surface configuration, so that an aquifer may transcend watershed boundaries. The hydroelectric power generated in one watershed may be conducted to distant markets, so that the consumers of that power become legitimate stakeholders in the management of the watershed of origin, even if they never see it. Water may also be diverted from the watershed of origin and transported by artificial means into neighboring watersheds. These interbasin transfers occur commonly in the United States, and they extend the interests of managers beyond the nearest drainage divide. In effect, most watersheds circumscribe areas of primary interest containing most of the environmental resources and human users, but in many cases a secondary, larger region also merits consideration, a sort of expanded shadow of the original watershed. In some cases, the watershed is not the appropriate model. Airsheds, for example, with their attending issues of air quality and visibility, define regions that do not depend on watersheds. For these issues, the watershed may not be the appropriate framework for management, and other administrative regions must be constructed.
These exceptions not withstanding, the watershed is clearly useful a scientific and administrative tool for organizing the natural and human landscape. The watershed approach provides an organizational framework that integrates natural, social, and economic components shared in a geographic area.