Surface water is water collecting on the ground, in a stream, river, lake, wetland, sea or ocean.
Surface water is naturally replenished by precipitation and naturally lost through discharge to evaporation and sub-surface seepage into the ground.
Surface water supplies, primarily river runoff, are about 300 cubic miles from the total water supply of the Earth, which is 320 million cubic miles.
Surface runoff plays an important role in the water recycling process. Not only does it replenish lakes, streams, and groundwater; it also creates the landscape by eroding topography and transporting the material elsewhere.
A stream typically transports three types of material: dissolved load, suspended load, and bed load. Chemical weathering of rocks produces ions in solution. High concentrations of Ca2+ and Mg+ are also known as hard water.
Suspended sediment makes water look cloudy or opaque. The greater the suspended load, the muddier the water.
Bed load (silt- to boulder-sized)settles on the bottom of the channel. Bed load sediment moves by bouncing or rolling along the bottom.
Surface water quality is classified into groups 1−5:
- Class 1 is extra clean fresh surface water resource used for conservation, not necessarily required to pass through water treatment process, and requiring only an ordinary process for pathogenic destruction and ecosystem conservation where basic organisms can breed naturally.
- Class 2 is very clean fresh surface water resource used for consumption, which requires ordinary water treatment process before use, for aquatic organism of conservation, fisheries, and recreation.
- Class 3 is medium clean fresh surface water resource used for consumption, but requires passing through an ordinary treatment process before use, for agriculture.
- Class 4 is fairly clean fresh surface water resource used for consumption, but requires a special water treatment process before use, for industry.
- Class 5 is the source which is not classified in class 1−4 and used only for navigation.