The hydrologic component of an ecosystem is important both as a single component for classification (Rosgen) and as a modifier to other components within the classification system. It is primarily considered when evaluating vegetation classes and wetlands. Specific hydrologic data can be accessed at http://water.usgs.gov/. More generalized information, including regional data and additional sources for understanding the role of hydrology in the environment, can be found at the http://terrassa.pnl.gov:2080/EESC/resourcelist/hydrology.html and at http://www.hwr.arizona.edu/hydro_link.html.
Classification systems that use hydrologic data to some extent include the HGM Classification of Wetlands, Cowardin, and the NVCS.