Water Supply
Planting trees has long been an expression of intent to do something of substance in the climate change debate; scientists have found a new rationale for this
atmosphere to which trees and wetlands contribute importantly and in quantities that can be measured, and ii) a starting point for condensation of vapour into cloud droplets and rain drops. Trees are a source of volatile compounds that can become cloud condensation nuclei and trees are also a source of bacteria that form ice nuclei.“ In clean, dust-free air, cloud droplets may cool down to-40 ° C, high up in the atmosphere, before freezing occurs if there are no ice nuclei present that can catalyse freezing”, says Dr. Cindy Morris, one of the coauthors.“ But trees and forests release‘ ice nuclei’ into the atmosphere including certain fungal spores, pollen and bacteria that can initiate rainfall at much warmer temperatures, sometimes as warm as-4 ° C. This means that rain initiation can take place more readily in lowaltitude clouds.”
As forests modify and contribute to the atmospheric flows of moist air they influence downwind rainfall. While coasts derive most of their rainfall from oceanic evaporation, downwind continental surfaces are increasingly dependent on upwind terrestrial sources of atmospheric moisture. On average 40 % of rainfall over land is recycled from evapotranspiration over land surfaces.
Long-distance dependencies
Important examples of long-distance dependencies have been documented between the Congo basin and East Africa providing rain to the Ethiopian Highlands and the Sahel; the Amazon supporting rain in NW Argentina; and mainland Southeast Asia feeding atmospheric moisture to China. In all these cases, major changes in tree cover can break the chain and reduce precipitation in downwind basins.
“ Where most water studies have focused on the‘ blue water’ in rivers and the‘ green water’ used by plants, water in the atmosphere is now recognized as‘ rainbow water’”, says Dr Meine van Noordwijk, co-author and Chief Scientist with the World Agroforestry Centre( ICRAF).“ The policy arena may have to adjust to the idea that rainfall is not simply the result of large scale air mass movements, but depends importantly on how upwind neighbours care for their forests.“ Reliable rainfall in the continental interiors of Africa and South America, as well as in other downwind locations, may depend on maintaining relatively intact and continuous tree cover from upwind coasts. The geopolitics of these relations can become a source of conflict, but can also lead to new types of cooperation.”
With these and more fascinating“ cool insights” and evidence from research, the authors point out that there is a strong basis for a hydro-climate policy that involves forests and trees. This policy would be much wider than what has so far been shaped by scientific understanding of the greenhouse-gas dominated climate and been incorporated in international agreements.
The review concludes with a cry to action on forests, water and climate:“ Climate policy must take these waterprocessing, cooling and rainfall-generating effects of trees and forests more explicitly into account.”
Significant revision of national, regional and continental climate change mitigation and adaptation strategies are urgent as next steps.
Water resources
Water resources are sources of water that are useful or potentially useful to humans.
It is important because it is needed for life to exist. Many uses of water include agricultural, industrial, household, recreational and environmental activities.
Virtually all of these human uses require fresh water. Only 2.5 % of water on the Earth is fresh water, and over two thirds of this is frozen in glaciers and polar ice caps.
Water demand already exceeds supply in many parts of the world, and many more areas are expected to experience this imbalance in the near future.
It is estimated that 70 % of world-wide water use is for irrigation in agriculture.
Climate change will have significant impacts on water resources around the world because of the close connections between the climate and hydrologic cycle.
Due to the expanding human population competition for water is growing such that many of the worlds major aquifers are becoming depleted.
Many pollutants threaten water supplies, but the most widespread, especially in underdeveloped countries, is the discharge of raw sewage into natural waters.
Africa Water, Sanitation & Hygiene • May- June 2017 35