Water resources for urban areas come from either surface water or ground water .
Many urban areas derive their water from adjacent rivers or lakes.
Some of the surface water is diverted to local water treatment plants for purification before being used. In these cases problems arise only if the demand for water by the population being served by the treatment facility exceeds either the amount of surface water available, or exceeds the treatment capability of the water facility. It is incumbent upon urban planners to keep in mind the limits of local water resources. If these limits are exceeded, urban areas must find additional water from other resources. Urban areas that derive their water from ground water sources must always be mindful of the possibility of aquifer drawdown , which results from excessive ground water extraction.