Hydrological cycle
Committed by UNESCO for WATER ROOMS link project: thewaterooms.org/ Water is a finite resource, in the sense that the amount of water present on our planet has always been constant throughout its history. Nevertheless it has always been moving around: changing state and distribution. This continuous closed-loop process of water moving on, above, and below the surface of the Earth is called the hydrological cycle. It is the journey of a water drop that moves and transforms by the physical processes of precipitation, runoff and infiltration, subsurface flow, evaporation and condensation. This cycle can be dramatically altered under the pressure of given natural or anthropogenic factors, in turn affecting the availability of freshwater (both quantitatively and qualitatively). The arrival of humans and the increasing impact of their actions on the environment have altered this cycle in direct and indirect ways. Direct impacts are caused by groundwater abstraction – causing groundwater levels to drop, saltwater intrusion along coasts to develop – , land use changes through deforestation and urbanization – leading to increased runoff, decreased evapotranspiration, reduced groundwater recharge –and man-made pollution of rivers and water bodies – leading to eutrophication and low water quality – . The most important indirect impact occurs through climate change by which an increase of the average earth surface temperature leads to a speeding up of the hydrological cycle. This leads to a wide range of effects such as higher evaporation rates combined with an increased capability of the atmosphere to hold water moisture, thereby leading to higher intensity rainfall, and an increased frequency and intensity of storms or hurricanes.
