Hydrologic cycle (also called the water cycle) : the cycle of which water evaporates from oceans and other bodies of water, accumulates as water vapor in clouds, and returns to oceans and other bodies of water as rain and snow, or as runoff from this precipitation or as groundwater.
  
Runoff : water originating as rain or snow that runs off the land in streams, eventually reaching oceans, inland seas, or aquifers unless it evaporates first.
  
Aquifer : a layer or section of earth or rock that contains groundwater.
  
Groundwater : any water naturally stored underground in aquifers, or that flows through and saturates soil and rock, supplying springs and wells.
   
Water withdrawal : removal of water from any natural source or reservoir – such as a lake, stream or aquifer – for human use. If not consumed, the water may later be returned to the same or another natural reservoir.
   
Water consumption : use of water that allows its evaporation or transpiration (through plants), or leaves it unfit for any subsequent use.
  
Renewable water : water continuously renewed within reasonable time spans by the hydrologic cycle, such as that in streams, reservoirs or other sources that refill from precipitation or runoff. The renewability of a water source depends both on its natural rate of recharge and the rate at which the water is withdrawn for human ends. To the extent water is withdrawn faster than its source is recharged, it cannot be considered renewable.
  
Nonrenewable water : water in aquifers and other natural reservoirs that are not recharged, or are recharged so slowly that significant withdrawals will cause depletion.
  
Desalination : production of fresh water by removing salt from seawater, or brackish water through the application of energy, usually oil or other fossil fuels.
  
Water scarcity : as used in reference to countries by water engineers and in this report, condition in which the annual availability of renewable fresh water is 1,000 cubic meters or less per person in the population.
  
Water stress : condition in which the annual availability of renewable fresh water is less than 1,667 and greater than 1,000 cubic meters per person in the population.
  
Sources: Adapted from Robert K.Barnhart, The American Heritage Dictionary of Science,1986;Peter H.Gleick;World Resources Institute