Although the areal extent of tropical rainforests has changed markedly through natural fluctuations in climate at a geological time scale, the rate of tropical forest harvesting and clearance during the second half of the twentieth century, has been unprecedented. Fuelled by the soaring demands for tropical hardwoods by ‘northern’ economies, timber harvesting relies heavily on the use of mechanised felling and extraction. This, in turn, has greatly disturbed the remaining vegetation, the soils and therefore the hydrological functioning of the forest. Further, the economic necessity for an adequate return on the capital invested in equipment, vehicles, roads and wood-processing mills makes it desirable to harvest all marketable logs during a single felling cycle, often at the cost of future growth. At the same time, traditional shifting cultivation practices of local communities have become unsustainable in many places due to the increased pressure on the land exerted by a growing population, resulting in gradual degradation or even total disappearance of closed forest. In addition to such ‘unplanned’ forest degradation and conversion there is an increasing trend towards planned, government-led conversions of tropical forest to apparently more profitable cattle ranching or commercial plantations.
The extensive disappearance of tropical forests during the last five decades has raised global alarm over the threats to climatic stability and the hydrological functioning of river basins posed by continued forest conversion, next to the well-being of forest dwellers and the conservation of biodiversity.
This section contains four chapters providing a critical assessment of existing guidelines (also called best management practices) that have been designed to minimise adverse effects on soils and streams during timber harvesting and land clearing operations in areas with tropical forest, as well as under rainfed cropping in deforested tropical steeplands. In addition, the main factors hampering the widespread application of these best management practices are examined in some detail and areas indicated requiring additional research.
Cassells and Bruijnzeel provide an overview of guidelines aiming to minimise adverse impacts on the residual vegetation, soils and streams during tropical timber harvesting operations. Research and land management experience over many decades have demonstrated that poorly planned or managed logging operations generally have deleterious environmental impacts. Most commonly, logging and the associated road construction lead to problems with erosion and sedimentation, and thus to a reduction in the quality of streamwater and aquatic habitat. At the same time, there is considerable evidence indicating that, provided forest managers and planners respect broad land capability limits, appropriately managed logging operations can be compatible with the maintenance of good quality water supplies. In this regard, experience has shown that particular attention needs to be given to the careful location of roads, extraction trails and stream crossings; minimising ground disturbance and maintaining effective ground cover; as well as maintaining undisturbed buffer strips (as stressed by Connolly and Pearson; Hamilton) around key streams and waterways.
This part contains eight chapters dealing with new methods to detect, evaluate or predict the hydrological effects of land-use change. They range from remotely sensed observations of terrain and vegetation characteristics, through statistical analysis of trends in streamflow data, to various model approaches simulating hydrological and related processes such as erosion and deposition at the hillslope to catchment scale. Also examined is the usefulness of isotope tracers as a tool to enhance hydrological process understanding under humid tropical conditions. The final chapter explores the possibility of using aquatic organisms as an indicator of water quality.
Held and Rodriguez present an overview of new remote sensing technologies for the derivation of key forest and terrain attributes. These can be divided into three groups: (i) those relevant to assessment (vegetation cover, forest type and structure, age of regenerating forest, and fire history); (ii) indicators of stress; and (iii) those more directly relevant to hydrology (terrain attributes, soil characteristics, photosynthesis and transpiration). Airborne systems (balloons, planes) allow for higher spatial resolution image collection than satellite systems and contain sensors (e.g. laser detection and ranging, LDR) capable of collecting data under cloud cover, which would make them particularly useful in (tropical) areas experiencing frequent cloud. Satellite systems provide a more stable platform for operational data collection and have the advantage of covering larger areas more rapidly at a lower cost per unit area.
The extensive conversion of tropical forests to other land uses during, especially, the last four decades has raised global alarm on the threats posed by continued forest conversion to climatic stability and the hydrological functioning of river basins, next to the well-being of forest dwellers and the conservation of biodiversity. This part consists of nine chapters setting the scene for this book. It starts off with an account of the rates and underlying causes of deforestation in the three main tropical rainforest regions during the last two decades. This is followed by a critique of neo-classical market-based economics which are held responsible for stimulating environmental degradation. The next three chapters (Chapter 3-5) describe the adverse socio-economic consequences of large-scale planned forest conversions for forest dwellers and other poor strata in society in Latin America and South East Asia. After concluding that governments and donor organisations, whilst well aware of tropical environmental degradation, generally have no new ideas on how to mitigate the effects of adverse practices (Chapter 6), the final three chapters highlight ways of using economic theory to improve the negotiating position of upland farming communities and of actively involving these communities in the identification and solving of local environmental problems.
Setting the pan-tropical scene, Drigo discusses how the latest FAO Tropical Forest Assessment and various related efforts (i.e. TREES II high resolution survey) to quantify the extent and rates of tropical deforestation reveal a rather complex picture of a reduction in higher biomass densities during the last two decades, despite different definitions of various vegetation classes used in the respective surveys.
This part contains eight chapters, the first three of which deal with the soil and water impacts of various kinds of forest disturbances, in order of increasing intensity. These are then followed by two chapters discussing the impacts of forest clearing and burning for the establishment of other land uses (pasture, cropping, plantations) at the small catchment, and river basin scale, respectively. The final three chapters focus on the changes in hydrology and soil nutrient reserves associated with forest regeneration, tree planting and the mixed crop-tree systems collectively known as agroforestry.
It is commonly overlooked that humid tropical forests are subject to a range of natural disturbances, including treefalls, landslides, hurricanes, floods, droughts, fires, volcanic eruptions, earthquakes and, in coastal areas, tsunamis. With the exception of treefalls and landslides, all of these have a significant impact on hydrological functioning and nutrient cycling at the catchment scale, with the geomorphology of an area often affected too. Scatena, Planos-Gutierrez and Schellekens provide an overview of the hydrological impacts of the principal natural disturbances occurring in humid tropical forest, emphasising that most forests experience disturbance-generating rainfalls at least once every decade. Rainfalls of c.200 mm day-;1 may cause treefall gaps, landslides (especially in steep, tectonically-active areas) and localised flooding. Events in the order of 400–500 mm of rain day-;1 (commonly associated with hurricanes) can cause widespread landscape modification, mainly through landsliding.
This book reviews the current state of knowledge of forest hydrology and related land-water management issues in the humid tropics. As happened earlier in the related field of soil erosion and conservation, the days are long gone when land—water issues could be approached in a purely technical manner (cf. Hudson, 1971; Critchley, this volume), so much so that in a recent overview of responses to land degradation (Bridges et al., 2001), the majority of chapters dealt with socio-economic, institutional and policy-related aspects rather than the physical aspects of soil erosion. In view of the importance of policy and governance aspects in environmental management, in particular the involvement of local communities and other resource managers, the present book also aims to bring together scientific, policy and management perspectives. Such perspectives address tropical forest—land—water management issues and concurrently also seek optimum solutions for the benefit of all interest groups involved. Of late, the term ‘Blue Revolution’ has been coined to describe the shift from the traditional technical approach to one that gives due consideration to socio-economic factors as well (Calder, 1999).
The contents of this book are based on contributions made to a joint UNESCO International Hydrological Programme (IHP) – International Union of Forestry Research Organisations (IUFRO) Symposium and Workshop Forest—Water—People in the Humid Tropics: Past, Present, and Future Hydrological Research for Integrated Land and Water Management, hosted by Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia, 30 July – 4 August 2000.
Callaghan and Bonell introduce the main features of the tropical atmospheric circulation as a step towards linking different synoptic-scale, rain-producing phenomena, their associated rainfall characteristics and the subsequent impacts on runoff hydrology.
Within the monsoon regions, three systems of convergence are identified, the northern monsoon shearline, the southern monsoon shearline and the maximum cloud zone associated with the monsoon westerlies in the vicinity of the equator. The most active monsoon shearline (otherwise known as the monsoon trough) is identified with the summer hemisphere. It is along this system that tropical cyclones often develop in response to convergent, opposing equatorial westerlies and trade wind easterlies, coupled with sea surface temperatures in excess of 26 οC. Low latitude tropical cyclones can, however, occur more rarely within 5ο of the equator, despite the common belief that the Earth's deflection (Coriolis) force is too weak in this zone for these storms to form. An alternative explanation is the short duration of the year when the monsoon trough is resident near the equator so that the chances for tropical cyclones to form are much reduced. It is noted that the zone of deepest convection and persistent cloud is associated with the maximum cloud zone of the equatorial westerlies due to the convergence of inter-hemispheric airstreams. Activity waxes and wanes in this sector in response to the varying strengths of the inter-hemispheric trade wind systems and the eastward propagation of a Kelvin wave known as the Madden—Julian Oscillation.
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