Abstract. The various ways in which the forest cover nay influence the atmospheric and soil processes controlling the hydrological cycle are examined. Case studies of extensive deforestation affecting the rainfall pattern are reviewed.
This paper analyses the putative relationship between forests and rainfall and especially between decline in forest cover and disruption of rainfall regimes. Not every shower is due to forest influence, otherwise it would never drizzle in the desert. On the other hand, Cherrapunji, located in N.E. India, is totally devoid of forests yet receives over 1 m of rainfall per annum.* The quantity of rain that falls in the humid tropics depends on several factors, of which forest cover appears to be one. Other rain-causing phenomena include geographic location, topographic features, sea-surface temperature and land temperature, together with other forms of vegetation cover.
Modification of Microclimates
What is the role of forests and of deforestation in climatic change? Among the most far-reaching and immediate effects of deforestation is the modification of microclimates. The daily variation in ground temperature is much higher in denuded areas compared to land under forest; the soil is also less protected from torrential rains. Studies from Singapore reveal higher temperature at all depths from 3 to 50 cm under bare soil, slightly less under grassland, and the lowest and the least variable under mixed Dipterocarp forest (Hill, 1966).
In the Palni Hills and the Nilgiris in India, the night temperature during winter is as low as -10deg.C in open grassy areas, and frost destroys whatever seeds of forest trees that may have germinated in the open. At the same time, the temperature under the forest canopy remains above 0deg.C (Legris and Blasco, 1969). However, frost is not the only adverse factor at work. There are rainless periods during which the relative humidity in grasslands drops very low, favoring the spread of fires. One to three months may pass without the region receiving a drop of rain. The seedlings of forest species are eliminated by fire. The forest itself enjoys a comparatively higher value of relative humidity, about 80%. Thus, eco-climatic conditions are different under a forest and within a grassland, though both may occur side by side.
Deforestation and Decline in Rainfall
1. Evidence from India
In order to determine the effects of deforestation, Meher-Homji (1980a, 1980b) selected two groups of meteorological stations with long records of rainfall and rainy days in the Western Ghats. One group of stations had undergone considerable loss of forests in their vicinity (in recent years); the other group was free of such devastation. A statistical comparison produced at least some evidence of declining tendency of rainfall and rainy days becoming more apparent after large-scale deforestation.
For the Nilgiri district, Padmavalli (1976) noted a change in the pattern of the normal rainfall years with the reduction in the wooded land. The area under forest diminished from about 65% in 1944 to 50% in 1961 and 43% still later on. Corresponding to these changes, the normal rainfall years waned from 47% in the earlier period to 36% in the later. Udhagamandalam (Ootaeamund), the hill station of this district, also presents a waning trend of rainfall in association with declining forest cover. Average rainfall and number of rainy days of successive 20-year periods, as well as 20-year moving averages, show a net decline for recent years, the record lowest rainfall being 800 mm in 1982 (Meher-Homji, 1984). The average 20-year rainfall has declined from 1415 mm in 1902-1921 to 1200 mm for the 20-year period 1965-1984. The corresponding decline in rainy days is from 106 to 89, a decline of 16%. Legris and Blaseo (1969) observed diminishing rainfall for Udhagamandalam from the turn of the century. The frequency of dry years with less than 1300 mm of rainfall increased from eight years between 1902 and 1922 to 12 years between 1954 and 1964, with six consecutive years receiving less than this mean value. von Lengerke (1977) stated that the sub-normal rainfall of the years 1967-1970 at Udhagamandalam led farmers and planters to suggest a change of climate, but he himself found no clear indication of a large-scale shift in rainfall. At the same he noted a decrease from 1965 onwards.
The number of rainy days reported for this station for the five-year period 1886-1890, excluding the months June, July and August, when the rains are of monsoonic origin and not local, was 417 (Meher-Homji, 1984). The figures for the successive five-year periods presented in table I bring out the diminishing tendency of rainy days, though the decline is not gradual. The percentage decline in rainy days from the period 1886-1890 to the period 1978-1982 is of the order of 35% although one should make allowance for instrumental and observational difficulties. figure 1 depicts the vegetation of Nilgiri district. It shows (a) the area remaining under forest at present and (b) the area that was under forest till recently (according to the Survey of India topographic map) but that has been subsequently converted into degraded vegetation and grassland or transformed into plantations of tea, coffee and forest species, food-crop cultivation or lakes (Bellan, 1985; Gaussen et al., 1961).
It is doubtful whether the plantations of exotic trees play the same role as the indigenous trees in the water cycle, because these exotics have xeromorphic features to reduce the rate of evapotranspiration. Pines have needle-like leaves and in some species of wattles (Australian Acacia) the leaf stalk (phyllode) takes up the function of leaves. In Eucalyptus hybrids the transpiration rate, which is high under flooded conditions, is reduced considerably under restricted soil moisture supply by stomatal closure (Rawat et al., 1985).
Among other studies on the effect of deforestation on waning tendency of rainfall, mention may be made of Warren (1974) for the Ranchi plateau, Sarmah (1976) for Dibrugarh, Biswas (1980) for Andaman-Car Nicobar Islands, Mishra and Dash (1984) for Sambalpur, Mukherjee et al. (1976) for Santa Cruz (Bombay), Raju (1981) for Uttara Kannada district of Karnataka, and Singh et al. (pers. comm.) for Kumaun.
Soman et al. (1988) have observed decreasing rainfall over the highlands of Kerala following deforestation in recent decades. Although the authors are unable to account for the physical mechanisms of deforestation affecting the climate of the region, it is significant that the zones registering the maximum decline in rainfall are those having undergone appreciable loss of forest cover.
Nicholson (cf. Ranganathan, 1949) observed that the Chota Nagpur region, which had a good area under forest towards the turn of the century, used to receive fairly frequent afternoon showers known as instability rain during summer, favoring tea plantations. Consequent upon the destruction of private forests, in spite of no apparent reduction in the monsoon rainfall, the instability rain has decreased so much that tea gardens have disappeared.
Warren (1974) attributes the decline in rainfall of the pre-monsoon months (May and June) over the Ranchi plateau to the degradation of forests over an extensive area. The thunderstorm activity of the pre-monsoon season provides showers at the crucial moment when the water supply is rapidly dwindling towards the end of the long dry season. During the prevalence of long droughts during the (summer) monsoon season in weak monsoon years, conditions resemble those of the premonsoon months with high temperatures and no rains. Under such conditions the wooded areas are likely to benefit from the convective rains which may not be high in amount but are sufficient to maintain the crops and water supply for the local economy; the regions devoid of forests may not derive any benefit from the convective showers.
Given the large-scale variation in rainfall over space and time, not always related to forest cover, meteorologists in general do not accept the link between the changing pattern of precipitation and the man-made shifts in land-use by consultation of even long-term statistics of both rainfall and percentage of forests, cultivated land and wasteland.
Gadgil and Prasad (1986) have pointed out the utility of realistic models capable of simulating the various effects of deforestation such as changes in albedo and soil moisture. Computer experiments show that model-simulated climates are influenced by land-surface boundary conditions. The areas bordering the deserts were particularly prone to rapid desertification following deforestation in the wake of biogeophysical feedbacks.
2. Evidence from other Asian Countries
In the southeastern part of Yunnan Province of China, Zhang (1986) adduces meteorological data before and after severe deforestation (more than 50%) around the station Jhing Jhono (the 'before period' being 1954-1962, and the 'after period' being 1962-1980). He compares these data with those of the neighbouring stations Lancan and Simao, which are located at a distance of 100 km from each other, these being areas where forest cover has remained unchanged.
Zhang (1986) finds that in the wake of forest clearing the annual rainfall has been reduced by 4% with a range of 2-8%. But the daily maximum rainfall in the early rainy season has increased, leading to more frequent floods. There is a decrease in mean annual relative humidity of the order of 2% but there has been a consequent decrease of 28 in the number of foggy days. The author makes no mention of the difficulties in the way of making accurate rainfall measurements in and out of the forest which could render a 4% decline of no great significance. In order that a canopy may not intercept the rainfall, the rain gauge is placed in a forest opening where the wind velocity is considerably affected by the trees. The rainfall measured in a forest clearing may be as much as 10% higher than in an open area in its vicinity because the rain gauge in the forest clearing is shielded from wind.
The turbulence caused by the opening of the canopy might decrease the rainfall or, on the contrary, a downward component of the over-riding wind may lead to an increase.
Fox (1979) noted that in Sandakan (Malaysia), average rainfall in a forest rain gauge was less than or equal to that installed in an open area 200 m away on 94 out of 132 days of observations, but rainfall in the forest tended to be higher with greater falls of rain.
Legris and Blasco (1969) have cited the example of the Kudiraiyar basin of the Palni hills. A rain gauge at the border of the Kukkal forest at 2000 m recorded rainfall about 50% higher than that of the two rain gauges in the grasslands at the same altitude and the same northern slope during the period 1956-1966. However, in the hilly terrain, the topography itself could account for the variation in the rainfall over short distances. Besides, the length of the dry period differs between the two locations as shown in the climatic map of Blasco (1971).
In northwestern Peninsular Malaysia, Chan (1986) has noted evidence that in recent years the area has become drier with a greater frequency of droughts. The reason advanced for this change is the large-scale forest clearance during the past decade. Bearing in mind the important contribution of convectional storms to the rainfall of the region (Lockwood, 1967), the drastic decline of forest cover could have significantly affected the rate of evapotranspiration and brought about desiccation.
Reports of deforestation-related declines in rainfall totals and disruption of rainfall regimes have also been reported for parts of the Philippines (Alfonso, pers. comm., 1988). During the 1960s there was considerable deforestation on Mount Apo, for instance, due to the establishment of coffee plantations; and the apparent result is increasing drought.
Elsayam (1987), considering the period 1953-1983, shows that a change in forest density in equatorial Africa may affect the air-mass characteristics of central Sudan.
In southwestern Ivory Coast, cultivation has been replacing forest during the last three decades, with attendant modification in evapotranspiration rates (Myers, 1988). In some sectors the rainfall has been reduced so much that wells have dried up and cocoa plantations have been abandoned (Spears, pers. comm.).
The impacts of forest clearing in various parts of Africa have been documented by Mann (1987). In Ghana the area under closed forest has been reduced by 64%. In the Southern Province of Zambia tree cover has greatly decreased and grasslands have been converted into croplands, as a result of which the River Magoye is now completely dry. In Gambia the forest area declined from 28% in 1946 to less than 3% in 1968; and in northern Senegal forest cover in various areas declined by between 20 and 80% between 1970 and 1974 (Anon., 1974); in both instances the result appears to be more erratic rainfall and increasing dust levels.
4. Tropical America
Annual rainfall has been declining in lowland Guanacaste Province in northwestern Costa Rica over the last 40 years. Fleming (1986) hypothesized that secular changes in rainfall are functionally related to the clearing of lowland forests.
Whereas the stations in the mid-Isthmus of Panama, like the Laboratory Clearing on Barro Colorado Island and Monte Lirio 8 km further north, present a decreasing tendency of rainfall ranging from 6 mm to 10 mm per annum, the coastal stations of Panama show no waning trend (Windsor and Rand, 1985). Weakened convection could account for the decrease of turbulence and thunderstorm activity over the mid-Isthmus. On the other hand, the coastal stations depending on the moisture brought in by the prevailing winds from the oceans and on the orographic precipitation would not be subject to a declining trend. The weakening of convection due to the cooling of land surface in the wake of the creation of Lakes Gatun and Madden has been ruled out because the effect of lakes should appear in the climatic record as a step-wise change. However, the change is progressive in nature, and is parallel to deforestation.
Probable Mechanism Involved in Forest-Rainfall Linkages
The ascent of a moisture-bearing air mass to a height where it allows condensation of water vapour is associated with several meteorological phenomena after which various types of rainfall are named. Thus we have 'convection rains' or 'thunderstorms' when, due to the excessive heating of some areas (mostly in the premonsoon months April-May), currents of air rise to great heights carrying cloud moisture. Rainfall of cyclonic or depression origin is caused by the differential heating of sea and land masses resulting in converging winds. Orographic rainfall occurs when an air mass strikes a vertical obstacle which it tries to 'climb over.'
Whereas forests do not seem to influence cyclonic or orographic types of rainfall, they seem to influence convectional rainfall as shown by the study by Nicholson (cf. Ranganathan, 1949) for the Chota Nagpur plateau of Bihar. It may be added that the convectional rainfall could be generated by the forests at some distance and not locally. It may not be the absolute decline in monsoon rainfall that affects agriculture, flora and fauna, and water supply, but rather the lack of rains at critical stages. However marginal may be the increase in rainfall due to forest cover, it makes a difference in sustaining agricultural crops and maintaining ecosystems.
Shukla and Mintz (1981) report that modifications in vegetation cover due to the deforestation of large-magnitude areas surely influence precipitation. The hypothesis is based on correlation between precipitation and evapotranspiration. The determinant factor is not only the vegetation but also the relationship between the moisture content of soil, vegetation and the solar energy necessary for transforming water into atmospheric water vapour. This is, however, an unverified computer simulation.
Charncy et al. (1977) emphasize the role of reduced albcdo (i.e., the proportion of the radiation reflected back to the amount striking a surface) in inducing higher rainfall in forested areas. The albedo of deserts or barren soil is considerably greater (30-35%) than that of vegetated or forested surfaces (15-25%). So the actual net energy imparted to the atmosphere over a desert is less than that over green belts. The albedo depends on vegetation, which absorbs more heat than does bare soil. Because of less evapotranspiration over bare soil, the absorbed energy heats up the air causing dry thermals.
In the case of orographic rainfall, forests act as an obstructing medium and increase the effective height of the land surface in providing an obstruction to air movement. Forests also reduce wind speed through their aerodynamically rough, undulating canopy; with the decrease in wind velocity, the air masses are forced to stack and rise. Garrat (1978) states that the eddy diffusivities are considerably greater just above tall vegetation than at corresponding heights above smooth surfaces. Sud and Smith (1985) have stressed the importance of the mechanical friction effect of forests in lifting the moist-air and enhancing the rainfall. Pereira (1986) maintains that the desiccation following large-scale deforestation is due to the important influence of forests on the reception of rainfall, and not on its generation. Evapotranspiration is no doubt important from the forest, but drought is caused less by lack of moisture in the air than by a lack of a cooling mechanism to condense the water into precipitation.
An essential condition for rainfall to take place is that the warm moisture-bearing air should be able to rise. But deserts are the zones of large-scale descending air motion. The large amount of dust over the desert increases the subsidence rate by as much as 50%. Deforestation through erosion increases the dust content of the atmosphere. By cutting off a good portion of shortwave solar radiation by scattering and reflection, the dust particles prevent it from reaching the ground. Another major effect of dust loading is a greater cooling and radiation divergence in the troposphere (Chakravarti, 1978).
Pollen grains, debris and other parts of plants serve as condensation nuclei. Their role as seeds of crystallization is more effective than the inorganic debris like dust because the ice is formed on the inorganic debris at a much lower temperature (Glantz, 1987).
Finally there is the role of cloud forests, mossy forests and stunted woodlands occurring in the tropical montane belts and along coastal fog zones. This role should be emphasized in harnessing the moisture from the clouds through the mechanism of cloud or fog stripping (Stadtmuller, 1986). Even a single tree or a group of trees can trap a substantial quantity of rainwater through the process called horizontal precipitation (Zadroga, 1981). The amount so trapped can vary from 7 to 18% of the rainy-season precipitation and up to 100% of dry-season rains (Hermann, 1970; Vogelmann, 1973; Juvik and Ekern, 1978; Vis, 1986). The destruction of such cloud forests (as in the Western Ghats of India) can diminish stream flows and ground-water recharge (Bruijnzeel, 1986).
Dickinson (1980) states that none of the numerical studies reviewed by him has treated the effect of deforestation on increasing temporal and spatial fluctuations between wet and dry conditions. However, he feels that such a change in surface conditions could in turn increase the intensity and decrease the duration of tropical rainfall, enhancing run-off even if the mean rainfall were unchanged.
If the intensity increases without a change in the annual quantum of rainfall, the result is a smaller number of rainy days with long spells of dryness and erratic distribution. Meher-Homji (1980a, 1980b) has already shown that large-scale deforestation reflects more through a reduction in the number of rainy days than through the volume of rainfall.
Soil erosion in its turn provokes two major problems. If rainfall continues to be 'normal' but irregular, with occasional torrential falls, the consequences are the silting-up of river beds and floods; if drought years prevail in succession, not only do the streams and rivulets depending on the gradual release of water from the forest soil dry up, resulting in desertification of at least the marginally sub-humid zones, but increased dust particles in the atmosphere lead to desiccation and drought at least on the margins of the zones that are not so humid. Even the humid zones are in danger of getting progressively drier if droughts continue to recur over a series of years.
* However, a recent declining trend of rainfall has been reported even for Cherrapunji. In the wake of intensified shifting cultivation and consequent shortening of the fallow period from 25-20 years to 3-4 years, this hill station faces severe water shortage in winter.
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(Received 28 October, 1988; in revised form 3 April, 1990).