JOIN USThere have been many articles in international journals during the past decade to show that reforestation has a negative impact on water yield. Based on observations in plots upto two square kilometres in catchment areas in several river basins across the world, it has been brought out that removing forests would increase downward water availability. However, a second group of researchers argue the opposite.
The forest-water relationship is poorly understood, although not under-researched. David Ellison, Martin Futter and Kevin Bishop, who belong to the second group referred to above, reviewed the forest cover-water yield debate in a paper in “Global Change Biology” (2012). They concluded that trees can reduce run-off at small-scale but are more clearly linked to increased precipitation at large scales.
While the first group argues that trees and forests are consumers of water, like agriculture, industry, energy, and households, the second group puts forth that evapo-transpiration from vast stretches of forests, especially on the sea shore, consume water on one scale but facilitate cross-continental transport of atmospheric moisture, which promote rains at local, regional and global scales.
Forests provide a number of ecosystem services, the most important of them being the ability of vegetation to return water vapour to atmosphere and thus intensify the water cycle. Total precipitation is said to be the result of evapo-transpiration from trees as well as evaporation from water bodies.
Some findings reveal that evapo-transpiration contributes 59-64% of precipitation. The disturbing news is that urbanisation and expansion of agriculture is taking its toll on forests. Between 2000 and 2005, the world lost 3% of forest cover. Further, wetlands declined by 50% in the 20th century.
Deforestation also prevents moisture from sea being trapped along the coast. Further, the land heats up, resulting in rising vertical circulation columns that carry storms over surrounding mountains, thereby reducing orographic rains.
Biotic Pumps
Researchers of the second group have said that large forest expanses responsible for precipitation at the global level should be seen as ‘Biotic Pumps’. Precipitation over extensive natural forests is independent of distance from sea. Low level air moves from areas of weak evaporation to intense evaporation.
The argument that deforestation would lead to decline in evaporation was supported in another important paper by Makrieva and Gorshkov of Petersburg Nuclear Physics Institute, published in 2007 in “Hydrology and Earth System Sciences”. Forest precipitation increases upto a level when run-off losses from optimally moistened soil are compensated at any distance from the ocean. However, if there are no forests along the coastline, evaporation fluxes damp out exponentially. The ‘biotic pump’ works only when there are forests along the ocean.
Evaporation fluxes in two adjacent areas are different; the ascending fluxes of moist air are different as well. Precipitation is enhanced in intense evaporation area. This explains the existence of deserts like Sahara bordering an ocean.
In less arid zones like the Savannas, Steppes and irrigated lands, though there is some ground moisture, the absence of continuous tree cover and high leaf area prevents evaporation to the level of atmospheric moisture from oceans to compensate run-off. The ‘Biotic Pump’ does not work and the area does not contribute to evapo-transpiration and precipitation.
The northern river basins of Russia, Canada and Alaska are covered by natural forests. If the areas are deforested upto 600 km from the coastline, the remaining inland forests cannot pump atmospheric moisture from the ocean. This would mean that there would be no surplus run-off and ground water recharge. Soil water goes to the ocean as run-off or is blown away via the atmosphere through transpiration from existing vegetation. The soil will dry up and there will be no flow of water in the rivers. Forests will dry up and vanish.
Forest-cutting originated in Western Europe and was picked up all over the world. Remarkably, no area is separated from ocean or inner coast by more than 600 km, the distance of exponential weakening of ocean to land moisture fluxes in non-forested areas. Although everything is fine on paper, Western Europe has been witnessing an increase in catastrophic droughts, fires and floods, facilitated by removal of natural forests from mountains.
Deforestation in any part of the world will lead to smaller amounts of recycled precipitation and reduced total precipitation at continental scale. This leads to higher likelihood of drought and expansion of drought-prone area. Reforestation can increase precipitation and intensify the water cycle.
It is necessary to stop the destruction of natural forests, especially on the borders of oceans or inner sea, to ensure that precipitation does not decline.
(The writer is a retired principal chief conservator of forests, Karnataka)