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Although the consequences of deforestation on rainfall are contentious and difficult to assess in quantitative terms at present, there appears to be rather more of a consensus of opinion on two adverse effects of forest clearance, namely an increase in storm intensity and the associated soil erosion. Dickinson (1980) states that in none of the numerical studies reviewed by him has the effect of deforestation on increasing temporal and spatial fluctuations between wet and dry conditions been examined. He feels that deforestation could increase the intensity and decrease the duration of tropical rainfall, enhancing runoff even if the mean rainfall were unchanged. If the intensity increases without a material change in the annual quantity, the result is accentuation of dryness and erratic spatial distribution. Circumstantial evidence of lasting and profound changes being initiated by forest clearing is provided by long-term observations on rainfall in private rubber plantations in Malaysia (Unesco 1978). After large-scale forest clearing, the number of rainfall incidents decreased, the rainfall intensity substantially increased, while the total rainfall appeared to remain unaffected. Meher-Homji (1980a, b) has shown, further, that large-scale deforestation has its effect more in the reduction of the number of rainy days than in the volume of rainfall. The result is soil erosion and, in turn, major disasters. If rainfall continues to be normal in amount but irregular, with occasional torrential falls, the consequences are floods and the silting of river beds. If successive drought years prevail, not only do streams and rivulets dependent on the gradual release of water from the soil dry up but dust becomes a problem. This may result in desertification of marginally subhumid zones. Even humid zones are in danger of becoming progressively drier if drought years continue to recur.
Many of the suggested effects of forests on rainfall in the literature remain to be tested by actual data from India. For instance, if tropical forest contributes only 3% to the global water cycle, what would the decrease be if the forests were cleared?
I wish to thank the organizers of the Workshop for having given me the opportunity to participate. My sincere thanks are due to Frank Thompson and Evan Reynolds for going through the manuscript and making useful suggestions.
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No doubt due to the variety of ways in which forest vegetation influences the rain climate, the alleged effects of forests on rainfall are not clearly revealed by changes in gross parameters. Indeed, annual values often obscure those features of the precipitation that produce ecological and agricultural responses. Meher-Homji has elucidated a series of parameters relating to the rainfall and to rainy days that might be supposed to be sensitive to specific mechanisms through which forest affects local climate. He regards trends of the majority of these with deforestation within an arbitrary 16 km radius of the meteorological station as signifying a proven effect, particularly where a control station in the region, with minimal deforestation, shows more equivocal trends in the parameters.
From these areas of the Indian subcontinent examples of the persistence of man's influence in deforesting the landscape demonstrate the subtleties of interplay between local exacerbation of drought, resultant fires, frost prevalence, and the production of a grassland subclimax.
It is to be hoped that the provision of a large number of references to the original literature, including many in French, will encourage development of these studies in a critical manner.