Natural watershed systems maintain a balance amongst the various processes of 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 of water from the atmosphere 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 water ecosystem functioning and the amount available for human uses.
The two water cycle processes responsible for moving the greatest quantities of water are precipitation and evaporation, globally transporting approximately 505,000 km³ of water each year.
It is necessary to understand this "water balance" in order to maintain the resource and its environmental and human connections in the watershed. The understanding of the water cycle on a watershed basis is important for developing watershed management policies and procedures.
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