|Forests, Climate, and Hydrology: Regional Impacts (UNU, 1988, 217 pages)|
|9. Résumé and conclusions|
With the objectives of stimulating projects in the most significant areas of the subjects considered by the workshop and publicizing the current state of knowledge regarding the effects of vegetation change and regional climate, the experts agreed on the following conclusions.
The Description of Vegetation Change. Large-scale changes in vegetation induced by man, of which deforestation is the most extreme example, often result in site impoverishment (in terms of productivity, fertility, flora, and fauna) and an alteration of regional heat and water balances. Deforestation also results in an increased input of CO2 to the atmosphere, though not as large as that resulting from the burning of fossil fuels.
To understand the role of vegetation change in these effects data are needed relevant not only to the size of the area involved, about which there is considerable disagreement, but also to the significant physical, chemical, and biological parameters (perhaps remotely sensed and monitored): for example, albedo, aerodynamic roughness, and biomass. It may be possible to assess the relevant parameters from an appropriate ecological description. There is a real need to specify and quantify more precisely vegetation change in terms of the regrowth of vegetation and/or subsequent land use. For selected areas, the history of human interference and system response need to be thoroughly investigated.
Hydrological Processes in Temperate and Tropical Regions. In the tropics more rapid hydrological cycling occurs than in temperate regions, convective rainfall is more important, as are the meteorological responses to surface fluxes of heat and moisture. The net effect of these processes during land changes and climatic anomalies may well be more critical in tropical regions as atmospheric demand is high and persistent. In addition, the most extensive land use changes are currently occuring in these areas.
The Importance of Forest Succession and Age on Hydrology. With forest clearance there is an immediate, short term increased discharge (of up to 450 mm per year). Subsequent tree growth produces well-known and important changes in surface hydrology. In relation to the detailed Russian findings on the effects of forest age and succession on site hydrology, it would be useful to obtain comparable data for other regions and to establish the mechanisms involved. In the tropics, rapid growth rates mean that any analogous effects would operate over greatly reduced time-scales: for natural tropical forest there may well be no effect of stand age due to the small-scale heterogeneity of species and age in these forests.
Forests and Changes of Precipitation. Many apparent local variations in precipitation due to forests over a few tens of kilometres at most are essentially caused by redistribution of precipitation and as such are edge effects. There is some evidence for about a 5% increase in precipitation due to aerodynamic roughness of forests. Although the mechanisms of this need to be clarified, it is probably due again to precipitation redistribution rather than to an actual increase. In the tropics, modification by forests of the surface hydrology is likely to have some effect on precipitation but probably to a small degree. All other possible changes of precipitation caused by forest are scale dependent.
Changes of vegetation cover on a massive scale (e.g. a GCM grid scale) may result in precipitation change on this scale in areas where recycling via evaporation is important and the Charney mechanism may operate. The magnitude of this change of precipitation would depend on the change in vegetation and soil characteristics such as albedo, surface roughness, interception capacity, infiltration rate, and water storage capacity. Calculations predict that the conversion of a large area of tropical forest to pasture might result in a decrease in precipitation of about 200 mm per year. Feedback processes involving surface albedo, vegetation development, and soil and atmospheric moisture might amplify these changes.
The Future Role of Catchment Studies. Catchment studies will continue to be needed as an integrated description of the performance of surface hydrological systems. However, the uniqueness of each catchment limits the predictive value of these investigations for other catchments. They therefore need to be supplemented by shortterm process studies that in time can be validated by the catchment studies. Catchment studies in the tropics should be multiplied and also extended into larger flat basins for comparison with flatland micrometeorology.
The Use of Isotopes to Study the Recycling of Precipitation. The use of stable isotopes as an independent approach has promise for assessing the recycling of water, especially in the tropics where precipitation is largely of convective origin. The measurement precision must be adequate and fractionation processes completely understood. Detailed process and modelling studies of recycling should be used to complement the isotope method.
Extrapolating Hydrological Results in Space and Time. The heterogeneity of important hydrological parameters and processes make extrapolation difficult. For instance, local soil moisture measurements cannot be applied to the whole catchment due to the small-scale variability of soil and our poor understanding of runoff processes. Simplistic models for runoff and soil water availability have to be adopted. Although widely used, these are poor predictors of macro-catchment behaviour.
Ideally, appropriately scaled models should be used for extrapolating to a larger scale. For example, when proceeding from microscale to mesoscale, the model should include boundary layer considerations defined from tower and plot studies. Such models of the atmospheric boundary layer processes are needed to integrate plot studies over larger catchments and to interface surface processes to GCM models. Larger catchments need to be studied due to the increased numbers of interactions and feedbacks as the scale increases.
As most models work with short time steps, integration over time is not generally a problem. However, the model must be demonstrably valid for the time scale of the simulation. A further complication is that vegetation succession could alter surface properties over a long period.
The Importance and Design of Mesoscale Experiments. Mesoscale experiments are essential, on the one hand to explore the hypotheses of macro-hydrology at the scale of GCMs and, on the other, to test mesoscale data synthesized from microscale and watershed data. Mesoscale experiments must include studies of horizontal integration in the lower atmosphere.
The pilot experiment proposed under the World Climate Research Programme is an example of a mesoscale experiment on an area of 50 km by 50 km. Land surface fluxes will be estimated by a number of independent methods. Radiation, precipitation, evaporation, runoff, and soil moisture will be measured. During the period of transition from soil saturation to large soil moisture deficits, about a hundred hours of measurement of the boundary layer will be made from an aircraft.
The Importance of Increased Carbon Dioxide in the Atmosphere on the Global Hydrologic Cycle. Anthropogenic increase of atmospheric carbon dioxide and the predicted global warming may have significant effects on forests and thus on the hydrologic cycle. Recent GCM simulations of regional climatic response to atmospheric carbon dioxide increases, though preliminary, indicate that marked changes, both negative and positive, might be expected for precipitation and evapotranspiration for certain regions of the earth. Improved simulations should indicate the regions where this effect of carbon dioxide change is most likely to produce detectable hydrologic changes. It is possible that in some areas the effects of carbon dioxide increase will exceed the direct hydrological impact of land use changes.
Under laboratory conditions raised carbon dioxide levels increase carbon assimilation by plants and decrease transpiration, that is, increase the water use efficiency of at least some species. However, the increased temperatures could be accompanied by increasing respiration and stress. The potential of these interactions on the hydrology of areas under natural and crop species is difficult to assess.
Although land use changes may significantly alter global biomass and must be considered in any interpretation of the partitioning of carbon from fossil fuel between the major carbon sinks, the past, present, and future losses of biospheric carbon are unlikely to have a major effect on atmospheric carbon dioxide fifty to a hundred years hence.
Global Climatic Effects of Vegetation Change. Vegetation change may contribute to changes in the global mean temperature in a number of ways:
Other impacts on climate have not been assessed, but it should be noted that changes in the distribution of tropical heat sources have been shown by other modelling experiments (e.g. sea temperature anomaly) to affect the global circulation at all latitudes. However, from the current evidence it would seem that changes in vegetation are more likely to have important effects on climate at the regional scale.