Localized area of impact

Not only must we know the increase or decrease of the water balance components due to a change of vegetation, we must also know how extensive these effects may be. If the water balance is only materially altered within the confines of the area of changed vegetation, then the problem is greatly reduced. If, however, there are significant global effects, then the international community needs to be concerned in local land use policies. To underline the significance of the scale of the effects, we speak of "regional impacts" of the hydrological and climatological changes caused by vegetation change. The well-known effects on the microclimate and purely local water balances are adequately treated by others. At the other end of the scale, global climatic effects are unlikely to be significant if we are unable to demonstrate an effect at the regional scale.

It is inevitable that our considerations are concentrated mainly on extensive tropical deforestation and afforestation since there is a concentration of developing countries in these regions and a great potential for land use change. The exploitation of the forests of these regions to finance development and release land for agriculture and plantation crops is an attractive proposition in many ways. The fact that, relative to other regions, larger areas of the land surface may be altered hydrologically focuses attention on the tropics as being currently the most likely area to initiate regional or even global climate changes by land use change. The food demands of the increasing populations in tropical areas result in permanent land use changes following deforestation leading to changes in the radiation balance and evaporation. Additionally, if we wish to test the possibility that land use change affects the climate of distant areas, the nature of the general atmospheric circulation of the earth makes the tropical zone one of the more likely areas for such changes to lead to significant effects elsewhere. A more local effect of land use on climate in need of further study is that due to the internal convective circulation cells of tropical regions producing rainfall only some tens of kilometres from its evaporative source.