Urban Watershed Hydrology

Understanding the basics of stormwater and watershed hydrology

A watershed is an area of land that drains all of the streams and rainfall to a common outlet, such as the outflow of a reservoir or mouth of a bay.

Watersheds are organized hierarchically, with smaller subwatersheds nested within larger drainage basins. For example, the Anacostia River watershed functions as a subwatershed of the Potomac River basin, which itself is part of the Chesapeake Bay watershed.

In an urban context, the urban drainage network consists of overland flow, ditches and channels, storm drain pipes, and stormwater outfalls. All of these elements can cause precipitation to concentrate, flow to speed up, and pollutants to wind up in the watershed.

Defining Land Cover

Urban and suburban areas generally have four different types of land cover: impervious cover, turf, forest, and construction. Land cover is specific to what you might see flying over the site, as opposed to land use, which breaks these into categories such as parking lot, road, or industrial. Each land cover has a different hydrologic response, meaning they each react differently to precipitation. This can affect both the quality and quantity of stormwater runoff coming from various sites.