Unlike any other wetland system on earth, the Everglades are sustained primarily by the atmosphere. Evapotranspiration – the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere – associated with thunderstorms, is the key mechanism by which water leaves the region. During a year unaffected by drought, the rate may reach a year. When droughts take place, the rate may peak at over , and exceed the amount of rainfall. As water leaves an area through evaporation from groundwater or from plant matter, activated primarily by solar energy, it is then moved by wind patterns to other areas that border or flow into the Everglades watershed system. Evapotranspiration is responsible for approximately 70–90 percent of water entering undeveloped wetland regions in the Everglades.