Available freshwater resources are either groundwater or surface water (rivers and lakes). Water that flows on the surface of the land is surface runoff. The relationship among surface runoff, precipitation, evaporation, and percolation is summarized in the following equation:
Surface runoff = precipitation - (evaporation + percolation)
When surface runoff resulting from rainfall or snowmelt is confined to a relatively narrow, well-defined channel, it is called a river or stream.
Groundwater is that water that has percolated downward through the soil and is present within porous spaces in soil and bedrock. It has been estimated that the global groundwater resource is equivalent to about 34 times the volume of all surface waters (i.e., rivers and lakes) of the world. This resource is present nearly everywhere and has the additional advantages of typically needing no storage or treatment. Utilization does require the construction of a well, sometimes presenting a problem in the most needy locations.
Water utilization efficiency is measured by the ratio of water withdrawal and its subsequent consumption. Water withdrawal is water pumped from rivers, reservoirs, or groundwater wells, and is then transported for use. Water consumption is water that is withdrawn and actually used for some specific purpose. It is then returned to the environment through evaporation, transpiration, discharge to a river or lake, or in some other way.
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