A watershed includes all water, water-dependent land features such as wetlands and valley woodlands, humans and other living things. A watershed is separated from adjacent watersheds by a land ridge or divide. On a smaller scale, a subwatershed is the land area that is drained by an individual tributary to the main watercourse.

    Humans affect the way a watershed naturally works. We have changed the way the land drains, and removed the moderating influence of forests and wetlands. Mining, aggregate removal, and construction of major works, such as highways and settlements, have immediate and visible effects on the natural system. Other impacts may be more subtle, building up over many years and may have a harmful cumulative effect upon the environment.

How does the hydrologic cycle affect a watershed?

 

Natural watershed systems maintain a balance between precipitation, runoff to lakes, rivers and wetlands, infiltration to the groundwater system, evaporation from open water surfaces and evapotranspiration from vegetation. This balance completes the natural cycle back into atmospheric moisture and precipitation.

As a general rule, in natural watersheds approximately 60% of the precipitation evaporates or transpirates back into the atmosphere, 30% infiltrates the soil and 10% runs off into surface water bodies.

The water balance for the watershed or subwatershed determines the amount of water available for aquatic ecosystem functioning and the amount available for human uses.

It is necessary to understand this "balance" or "water budget" in order to sustain the resource and its environmental and human connections in the watershed. The understanding of the hydrologic cycle on a watershed basis is essential for development and implementation of appropriate watershed management policies and procedures.