|Forests, Climate, and Hydrology: Regional Impacts (UNU, 1988, 217 pages)|
|9. Résumé and conclusions|
Resumes of the review papers and discussions on terrestrial hydrological processes (chaps. 3, 4, 5, and 6) were made by Walther Manshard and Keith McNaughton and on the papers on modelling and inputs to the atmosphere (chaps. 7 and 8) by Anne Henderson-Sellers. These resumes were discussed by the workshop and some of the points raised are given in the following sections.
Deforestation is commonly less extensive and complete than is generally imagined. Also, hydrologists and climatologists are rarely conversant with the social and economic reasons that govern this activity and with the complete range of resultant vegetation types, their uses, or the degrees of impoverishment that result. These two problems could be remedied by investigations to give the historic perspective of land use change in areas selected for their hydrological and climatological significance.
The contrast between the very large immediate changes in water yield in the first year following deforestation and the subsequent, often marginal, effects when a new vegetative cover has become established were reviewed by Oyebande (chap. 3). These longer term effects on water yield are rarely as great as 20% of the original value. However, continuing interference with the regrowth of vegetation could produce very large effects, and recovery might become impossible.
In relation to afforestation or reforestation, apart from work in Russia, most observations have been on forest less than 20 years old. They therefore neither corroborate nor contradict the observations in chapter 5 that evaporation from the forest reaches a maximum at about 20 years. One result from older Australian eucalypt forest suggests that the Russian finding may apply to other areas. Another suggestion is that forest water use is correlated with the density of the tree stand. It is important to note that, increasingly, studies are examining the underlying processes involved rather than being simply observations of phenomena (chap. 3).
Although changes in the time course of runoff after rainfall may have little relevance to the effects of vegetation on climate, they can be an important result of vegetation change. Vegetation change may lead to modifications in water quality, soil erosion, and flooding all of great significance to the local inhabitants.
On a small scale, some of the effects of forest on precipitation may be explicable in terms of the redistribution of rain or snow; this would cause little effect at the 20 km x 20 km scale. There is a possibility that recycling of precipitation could enable changes in vegetative cover to cause changes in precipitation significant on a 500 km x 500 km scale. However, there are likely to be difficulties in proving and quantifying this.
It is arguable how studies on the climatological effects of afforestation or deforestation might best be furthered by institutional and organizational means. There is a great deal to be said for informal but productive meetings of scientists exchanging information at small seminars held in third world countries. Another approach might be to establish a documentation centre. This would need a well-informed staff and adequate financial backing to provide an efficient, up-to-date service. Ideally, the centre would distribute validated data, lists of all specialists and their work, and authoritative reviews. It would also be able to give advice on priorities, techniques, and software. There is much to be said for such a centre to be integrated with a training or research institute, or even with an international network of such institutes.
Data needed for GCMs are available from several independent sources; however, there is sufficient disagreement between some data sets to materially affect the simulations of the models. These discrepancies are not confined to a single parameter: they apply, for instance, to estimates of areas of vegetation types, areas of deforestation, and magnitudes of carbon sinks. Clearly there is a need to resolve these differences.
Some of the incompatibility between data sets is related to the different scales of the sets. That is, the number of classes of soil, vegetation, or land use often increases with scale. Perceived spatial heterogeneity is thus intimately linked with scale. If data are simply entered as the value at specified grid points, they may well lead to errors in the simulation. An improvement in this case would be to also include information on the spatial pattern of the parameter. There are indications that an appropriate pattern can be related to drainage basins. Within a basin there is a degree of orderliness among land surface parameters, while a considerable amount of the heterogeneity is associated with distinctions between basins. An initiative by the WMO is moving in the direction of making greater use of this relationship. River basins, as well as being generally well defined topographically and governing the hydrology of water flow (see chap. 7), are also being increasingly used as the basis for planning and development. It seems entirely appropriate that ground surface data used in GCMs should attempt to combine these grid and river basin approaches. There are, of course, other problems of scale (chap. 7) and problems due to heterogeneity in time, both systematic and random, that need more attention if the outputs from GCMs are to be improved.
Enough experience has been gained from simulation experiments (chap. 8) to indicate that land surface parameters can certainly affect the regional climate. Thus vegetation change by man, provided it causes changes in albedo and evaporation to approach the values inserted in the models, could cause regional changes in climate. Admittedly, one of the weakest parts of the models is the actual simulation of precipitation, in comparison to other atmospheric processes and states. However, the models consistently predict that if albedo increases, evaporation descreases and precipitation markedly decreases.
If the way forward is to refine the models and conduct many more experiments with them, then simulating earth surface changes closer to values possible with society's current economy and technology is needed. This in turn requires more accurate and detailed data collection for the whole of the earth's surface. Morever, the reliability of the models must be firmly established if planning and investment decisions are to be based on them. This gives support to the three extant proposals to collect mesoscale data that are relevant to regional atmospheric circulation and the terrestrial inputs involved. It seems desirable that initially a test site should be established in the Brazilian Amazon. At least one of the proposals envisages an extension to other regions at a later stage.
One proposal is outlined in chapter 7. It involves a test of the parametrization in models of forest evaporation, soil moisture, and runoff. The scale of a pilot study would be 50 km x 50 km and would most appropriately be based on the existing Brazilian/lnstitute of Hydrology, UK observation station at the Duce Reserve, Manaus.
Another proposal (ALVIM, "Isotope Aided Studies of the Effects of Changing Land Use on the Ecology and Climate of the Brazilian Amazon") is for the quantitative study of the water, nitrogen, carbon dioxide, and other nutrient cycles. It also aims to identify the origin of water-vapour producing precipitation on the Amazonian region and to establish better water vapour circulation models.
The third study, the "Diurnal Amazon Regional Climate Experiment," proposed under the names of Molion and Dickinson, intends to relate radiosonde measurements of climatic parameters to measurements of rainfall and forest microclimate at surface sites. These combined measurements would be related to global weather patterns and together with satellite radiation measurements be used to test and improve GCM and mesoscale models.
Hopefully, following further development and critical tests of GCMs, within a decade the scientific community will be in a position to predict with some confidence what would be the climatic effect of any proposed large-scale change in land use. This will enable both national and international planners and investors to take into account the effect on the regional or even global climate of their activities.