Forests and water—Ensuring forest benefits outweigh water costs
Introduction
Throughout the world there is increasing interest in land and water developments and policies in which forestry is a central focus. But are these policies always based on our best scientific understanding of the forest and water interactions? Do the benefits always outweigh the costs? Watershed development projects, often with forestry as an important theme, are widely promoted as the means for alleviating poverty, raising food production and for improving the environment. Bio-energy plantations are being investigated with renewed vigour in many countries in both the developing and developed world following recent rapid increases in the costs of fossil fuels. Bio-energy plantations are promoted as an alternative and possibly cheaper and more environmentally friendly source of fuel. But to what extent do these energy production benefits outweigh other possible water resource, societal, food production and conservation costs? Payments for environmental services schemes are frequently based around maintaining or increasing forest cover. They are often promoted as ‘win–win’ mechanisms where not only can environmental services be maintained at less cost than man-made structural interventions but equity and poverty alleviation benefits can also be achieved through payments to the upland poor. The success of these policies is heavily dependent upon how forests interact with the water environment. Particularly critical is our understanding of how forests may influence and modify extreme events such as floods, droughts and landslips—events which can sometimes have devastating societal consequences.
The public perception that forests are, in all circumstances, necessarily and always good for the water environment, that they increase rainfall and runoff, regulate flows, reduce erosion, reduce floods, ‘sterilize’ water supplies and improve water quality, has long been questioned by the scientific community. The evolving modern science perception suggests a more complex and generally less advantageous view of forests in relation to the water environment. However, within this complexity, within the range of forest and soil types throughout the world, within the different competing processes which can sometimes move outcomes in either positive or negative directions, and from the wealth of studies that have been carried out and summarised from around the world (Bosch and Hewlett, 1982, Hamilton and King, 1983, Hamilton, 1987, Bruijnzeel, 1990, Bruijnzeel, 2004, Calder, 1992, Calder, 1998, Calder, 2000, Calder, 2005, Best et al., 2003, Zhang et al., 2001, Jackson et al., 2005, Farley et al., 2005, Nisbet, 2005) there are perhaps some broad rules (reflected in the editorial review) that we can suggest to policymakers:
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Water use: Water use of forests is generally greater than that from other shorter and un-irrigated crops leading to reduced annual flows from catchments. Fast growing plantation species have often been found to cause major reductions in catchment flows—older plantations and slower growing indigenous forests are likely to exhibit lesser reductions in flow.
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Floods: At the small catchment scale and for small-scale flood events forests will have significant mitigating effects but for the largest, most damaging flood events and at large catchment scales benefits are likely to be minimal. Forest management activities such as roads and drainage ditches, through increasing the effective density of the stream network, can contribute to local flooding.
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Dry season flows: It is possible that the higher infiltration rates under forest soils will exceed the extra evaporation expected from forests leading to greater groundwater recharge and positively increased dry season flows. However, the vast majority of results reported from field studies around the world (see, e.g. the most recent review by Farley et al., 2005), particularly those dealing with plantation forests as compared with other well-managed land uses, indicate neutral to negative impacts, i.e. reduced low flows.
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Erosion: Erosion rates from natural forests are likely to be amongst the lowest of any land use but this is not necessarily the case for plantation forest. In poorly managed plantations, roads, drainage ditches, logging, and monoculture plantations with little understorey subject to splash-induced erosion, may cause erosion rates higher than those from well-managed agricultural land and pasture.
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Water quality: Water quality would normally be expected to be good from forests. An exception may occur in high pollution climates where deposition rates of atmospheric pollutants may lead to catchment acidification and high nitrate concentrations in soil and groundwater.
The questions arise as to whether forest and water policies in different countries in the world are always based on our best modern science perception and whether our best science is being effectively communicated to policymakers and, if not, how can we go about bridging the gap between science and policy. This paper discusses these issues together with the need to develop impact assessment methodologies to evaluate the multitude of positive benefits and consider these alongside any water costs.
Section snippets
China: the Natural Forest Protection Programme and the Sloping Lands Conversion Programme
The major floods in China in 1998 precipitated the early announcement of the Natural Forest Protection Programme (NFPP), which stipulated the protection of natural forests throughout the country and imposed a complete ban on logging in the upper reaches of the Yellow and Yangtze Rivers. The erosion-prevention programme named the Sloping Land Conversion Programme (SLCP) was announced at the same time. The stated purpose of the SLCP, as announced by Prime Minister Zhu Rongji, was to reduce
EXCLAIM dissemination tool
The GIS-based EXCLAIM tool was developed specifically as a means for disseminating knowledge of land and water interactions to non-specialists and policymakers (Calder et al., submitted). It has been applied to demonstrate the impacts of catchment interventions, including changes in forest cover, in a range of countries including India, South Africa and Costa Rica (Calder, 2005). Where the appropriate socio-economic data is available the tool can demonstrate how spatial changes in land use
The need for new initiatives
The role of forests in relation to the sustainable management of land and water resources remains a contentious issue in many parts of the world. This is despite a significant advance in scientific understanding of forest and water interactions based on almost a century of research in forest hydrology. Problems have often arisen from a failure both to communicate results effectively to policymakers and planners, and to challenge entrenched views, and new approaches and dissemination tools are
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