Forest and Water Programme

Flying Rivers – how forests affect water availability downwind and not just downstream



Forests are integral to the water cycle, acting as pumps that “recycle” water and transport water across landscapes and continents. Through the process of evapotranspiration, forests replenish the supply of water vapour in the atmosphere. As much as 70 percent of the atmospheric moisture generated over land areas comes from plants, which has important impacts on water availability across landscapes. Atmospheric moisture generated by forests not only affects water availability in the local catchment, it is also transported into other regions or even continents by prevailing winds.

This phenomenon of air currents bringing water vapour generated by forests into different regions is known as “flying rivers”. The Amazon region is an exemplary example for flying rivers. Indeed, the water vapour generated by the rainforest is transported as far south as Northern Argentina and the Amazon’s flying rivers are responsible for much of the rain that falls in the Centre-West, Southeast and South of Brazil. Researches have calculated the daily amount of water released by the rainforest into the atmosphere to be an estimated 20 billion tons of water daily – an amount that compares to the flow of the Amazon River. Flying rivers are also recorded in the Congo basin, where the climate is primarily influenced by moisture from the Atlantic Ocean and the forests of the Congo basin, which can evaporate up to 1 to 2 meters of water per year.

With forests contributing to water availability not just downstream, but also downwind through these “flying rivers”, the role of forests in supplying water takes up a new dimension – and with this, so should the management of forests to secure the provision of water. Droughts experienced in the South of Brazil in recent years have in fact been linked to the deforestation and conversion of the Amazon resulting in the reduction in rainfall brought by the “flying rivers”. It is thus important to inform on the impacts of ever-so-distant rainforests and the need for integrated resource management.