|Forests, Climate, and Hydrology: Regional Impacts (UNU, 1988, 217 pages)|
|4. Effects of forests on precipitation in India|
Opinions differ on the effects of forests on rainfall; some maintain that they cause no appreciable increase in precipitation (Walker 1916, among others) while some believe that the disappearance of forests leads to desertification. It has been conjectured that a lighted cigarette dropped in a Siberian or Canadian forest could bring about climate change, leading even to the onset of glaciation (Borisov 1973)! Wadia (1955) pointed out that the forests in Chinese Turkistan are not attracting rainfall but are drying out due to the general dryness of the climate. However, Kaulin (1962) asserted that the precipitation was higher in the forest than in the neighbouring steppe.
Nicholson (1929) contended that because in certain localities and under certain circumstances forests do not induce rain it does not mean that forests cannot induce rain. Harrington (see Luna 1981), on the other hand, stated that the evidence of higher rainfall within the forest does not mean that forests increase the rainfall, while Marsh (see Luna 1981), reviewing the literature, found evidence of decreasing precipitation subsequent to forest removal.
Different values have been attributed to the forest in augmenting rainfall. Thus Kittredge (1948) asserted that forest increased rainfall by 3%, 1% due to the trees of 30 m or higher obstructing air movement and 2% due to the effect of the friction of the canopy. Schubert's (1937) figure for Germany was 6%. Others estimate increases of 10 to 12% in the plains and up to 25% in the hills (Pinchot, see Luna 1981; Nicholson 1929; Hursh and Connaughton 1933; Fedorov and Burov 1967).
Recently the consensus of the hydrological community was that land use changes had no effect on rainfall (Pereira 1973). However, still more recent computer studies have shown significant changes in regional climate, including rainfall, due to surface changes. Dickinson (1980) explains that the differences are due to scale: hydrologists consider small areas and use limited observational data, whereas if the area studied was larger (of the order of several hundred square kilometres), the effects of vegetational change on atmospheric processes would be detected. Also, in the tropics crop yields and water resources over a large area are affected by what appears to be a negligible change of rainfall amounting to a few percentages. Moreover, convective rainfall (thunderstorms) is so variable that it is difficult to detect even moderate changes.
Lockwood (1980) thought that the disappearance of tropical forests would at most modify local climates and any effect on global climate would probably be swamped by natural changes and through an increase in the carbon dioxide content of the atmosphere. The most marked changes would be reflected in the hydrological cycle with increased runoff and an increased tendency to droughts in view of reduced soil water storage. However, he included changes in the vegetation cover among factors responsible for climatic variations such as fluctuations in solar radiation, for volcanic activity, tectonic movements, and changes in sea-level.
Dickinson (1980) considered that tropical deforestation would seriously modify local microclimates and, if sufficiently extensive, could change the climate of large regions in the vicinity of the deforested areas. If huge areas of rain forest disappear, even the global heat balance could be affected significantly. The effects of increases in carbon dioxide on radiation balance would outweigh the effects of albedo increase, at least for a few centuries until much of the carbon dioxide released due to clearance had been absorbed by the oceans. Climatic changes at the global level brought about by forest loss may be of the same magnitude as the natural climatic variability or modification brought about by burning fuels. However, Dickinson (1980) warns that deforestation combined with carbon dioxide released by the combustion of fossil fuels would add so much carbon dioxide to the atmosphere that it would worsen the situation at a global scale and regional changes following deforestation would be of greater magnitude.
Certainly not all arid zones are due to the absence of forest. Generally deserts owe their origin to their geographic location at subtropical latitudes on the western side of continents and are zones of large-scale descending air masses. Semi-arid zones, in certain cases, are located in special topographic situations, such as the areas in the lee of the Western Ghats (Bellary or Coimbatore) or in the shadow of Sri Lanka (Pamban, Tuticorin). Tropical coastal deserts as in Peru are deprived of rainfall by anomalously large annual ranges of sea surface temperature derived from cold advections. The coastal desert of southern Angola owes its existence to the coastal upwelling of cold water (Guilcher 1982), though the heavy rains occasionally recorded at Baia dos Tigres and Moçâmedes in March may be linked to the meandering of the warm current temporarily reaching the coast and suppressing the upwelling.
Delannoy (1982) has demonstrated the influence of the sea surface temperature on coastal rainfall. The rain-producing efficiency of disturbances coming from elsewhere and the pressure field are controlled by the surface temperature of the sea-water close to the continent. The difference in temperature between sea and land is important in breeze formation. Up to a certain distance from the coast changes in direction or velocity of the breezes and their dissipation are greatest where the land surface warms up quickly, as near towns as opposed to forests (Escourrou 1982).
Anomalies in the tropical sea-surface temperature of only a few degrees may induce large changes in circulation, cloudiness, and rainfall (Julian and Chervin 1978; Rowntree 1978). An increase in ocean temperature by several degrees could raise evaporation rates by as much as 20 to 50%. If the change in evaporation rate involves an area of several thousand square kilometres, it can modify climate over and downwind of the perturbed area.