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Köppen Scheme of Classification Modified by Trewartha (1961)
A. Tropical forest climates; all months above 18°C.
C. Humid sub-tropical.
a. Warmest month above 22°C.
b. Warmest month below 22°C.
c. Less than 4 months above 10°C.
d. Less than 4 months below 10°C.
f. Constantly moist, rainfall every month of year.
s. Dry season in summer.
w. Dry season in winter.
m. Monsoonal rain, short dry season, yearly rainfall sufficient to support tropical rain forest.
E0 evaporation from an
open water surface calculated by Penman's equation.
I interception loss
IITA International Institute of Tropical Agriculture
K coefficient of soil permeability
MAB Man and the Biosphere
S stemflow
T throughfall
WMO World Meterological Organization
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Assessments of the rate and degree of global and regional deforestation are generally accepted uncritically; any deforestation tends to be equated with absolute forest destruction. Regrowth may be so rapid, however, especially in the tropics, that at any time the area cleared due to forest exploitation may be hydrologically and climatologically insignificant. On the other hand, we must identify the vegetation types and the methods and objectives of forest clearance that preclude regrowth or, at any rate, delay it for a significant period. Forests that owe their existence to the microclimate they produce (such as cloud forests) or to the soils that in turn need their protection are examples of forests that are unlikely to regenerate in the short term after clearance, so that for many practical purposes it becomes irreversible. Where forest clearance is followed by periodic fires, often associated with grazing, a grass fire climax vegetation frequently replaces the forest. Thus the term deforestation should be carefully defined. Different interpretations of the term forest clearance wil apply depending on whether it relates to such diverse interests as forest resources surveys, the conservation of wildlife, soil conservation, landscape aesthetics, or surface parametrization for general circula tion models. Consequently, though very dissimilar, primary forest, secondary forest, plantation crops, and even orchards may be identical from the point of view of some hydrological or climatological studies.
For the purpose of relating vegetation change to regional climate, ecological classes and measurements of a vegetative cover related to its microclimatology, site hydrology, and plant-water relations are extremely useful Ecological methods and descriptions are too frequently considered to be ends in themselves rather than being possible tools to relate to ecosystem processes. Descriptions of vegetation incorporating canopy aerodynamic resistance and the water use strategy of the constituent plants (possibly based on rooting system and stomata! behaviour in relation to water stress) would be valuable, as would descriptions of the carbon reserves of different vegetation covers. While more detailed classification of vegetation types below formations and the life forms and plant architectures of different ecosystems undoubtedly distinguish vegetations with different hydrological and climatological properties, too little attention has been given to them. It is to be hoped that projects such as MAB and the Brazilian RADAM might be reinterpreted along these lines.
One ecological measurement that has been correlated with site hydrology is biomass (more specifically, that above ground). Thus the dry weight of above ground, living plant material may be related, for instance, with transpiration (although the major proportion of forest biomass will commonly be in the stems). However, biomass is a very heterogeneous quantity and is qualitatively different even between temperate and tropical forests. For instance, epiphytes such as the Bromeliads of tropical forests contribute little to the biomass, yet their water trapping adaptations are probably of considerable hydrological significance (incidentally this would be appreciated by most ecological classifications of vegetation types). Similarly biomass measurements fail to give recognition to the deciduous and evergreen components of the vegetation. The leaf area index (LAI) is suggested as a better ecological parameter of vegetation than biomass for correlating with such processes as transpiration, interception, and micrometeorological parameters such as rs and ra.
As yet too little attention has been paid to the infiltration of precipitation into the soil. Often earth surface models simply assume that no water drains from the site until the soil is wetter than field capacity. Thus the available soil water and, in turn, evaporation tend to be overestimated and runoff underestimated. The considerable effort by agronomists to measure infiltration and model it have been neglected by hydrologists relating earth surface and atmospheric processes. We feel that a study of the process in forest soils should reveal the cause of some of the anomalies in reports of water yields. It would also be helpful if data on hydrological processes were reported as absolute values rather than, or as well as, relative values. For instance, interception simply reported as a percentage of gross precipitation has little meaning unless one knows the local conditions. Similarly collation of data from different regions is helped if the measurements are independent of other variables.
It should be emphasized that a change in land use imposes a variety of effects on the site hydrology, some tending to increase and others tending to decrease the water yield. Forests behave like any other land use in this respect. Thus the prediction of the effects of tropical deforestation or afforestation is a complicated task. Successful prediction depends on detailed studies being carried out on every process contributing to the water balance of tropical catchments. Meanwhile the available information indicates that there is no evidence as yet for catastrophic climatic effects on a global or local scale resulting from afforestation. The local hydrological effect of tropical forest clearance, however, will usually be an increase in water yield that will relate to the proportion of the area that has not been covered by a perennial (tree) cover by regeneration or planting. That having been said, such clearance must take into account the effects on surface runoff, soil erosion, and flooding, which can frequently be more serious than a minor climatic change.