The terrestrial water cycle describes how water within rivers, lakes, and wetlands gain and lose water. Water comes down from the atmosphere in the form of precipitation and, globally, around two-thirds of this goes back into the atmosphere through evaporation. The rest restocks groundwater and provides surface and subsurface runoff, which flows down rivers into lakes and reservoirs to provide freshwater. This freshwater is critical for human life and nature but only accounts for around 2.5% of the total water on Earth (Vörösmarty 2009).