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Abstract: South Africa is poor in natural forests and therefore heavily dependent on forest plantations for timber. These were first established in 1875, and currently cover in excess of 1,257,000ha, of which 39.1% comprise Eucalyptus spp. They are concentrated in limited areas of high rainfall which are critically important source areas for rivers. Originally covered by seasonally dormant grasslands and Fynbos (Macchia shrub), their replacement by deep rooted evergreen trees has increased evapotranspiration, and reduced catchment water yields. Concern over impacts of forest plantations on catchment water yields in South Africa were already being expressed by 1915. Research on this subject started in 1937 and continues today. Methodologies have changed over the years, providing increasingly realistic estimates of the hydrological impacts of forest plantations, and changing perceptions of their water-use. Regulations were introduced by Government in 1972 to curb expansion of forest plantations in catchments where water supply is fully committed. A new Water Act in 1998 laid the foundation for a more equitable water allocation system based on the principle of integrated water resource management. Among other adopted principles, it recognizes that water resources can only be effectively managed within larger catchments, that local interested and affected parties should participate in the management of the water resource, and that water must be allocated equitably and used beneficially in the public interest. This has shifted the research effort towards measurement and modeling of ET at large catchment scale, as well as linking water-use to the economic benefits obtained from forests and crops. A clearer and more balanced picture of the efficiency of water-use by forest plantations and other major crop species is now emerging, but further studies are required. The forestry industry showed considerable foresight in initiating research into the water-use efficiency (WUE) of Eucalyptus clones in the 1990s. These investigations need to continue, not only to search for genotypes that limit the current hydrological impacts of forest plantations, but also to explore options for sustainable management of forest plantations into a future with potentially significant climate change. Environmental stresses in plantation trees may increase in response to such factors as a reduction in rainfall, changes in rainfall intensity, changes in rainfall distribution and increased vapor pressure deficits. WUE is likely to decrease in response to greater physiological stress, and clones may be available to minimize production and economic losses under such a scenario. [Copyright &y& Elsevier]

Abstract: Tree canopies partition rainfall into temporary canopy storage, throughfall and stemflow. Knowledge of this partitioning process is needed to predict the hydrological effects of the large areas of tree plantations that are being established in the tropics. In this study, we compared throughfall, stemflow and interception in four Neotropical and one exotic tree species growing in selection trials in the Republic of Panama. We sought to answer four questions: (1) Are there interspecific differences in total throughfall and stemflow, and throughfall and stemflow for a range of rainfall depths?, (2) How do crown traits influence interspecific differences in throughfall?, (3) Does the spatial heterogeneity of throughfall differ among species? and (4) How do species affect litter biomass and other variables that influence rainfall erosivity? Rainfall depth mediated interspecific differences in throughfall and stemflow, the relative importance of crown traits in the interception process, and spatial heterogeneity of throughfall. Total throughfall was between 10.9 and 16.2% less in Acacia mangium than Gliricidia sepium, Guazuma ulmifolia, Ochroma pyramidale or Pachira quinata. Increasing rainfall also changed relative quantities of throughfall and stemflow among species. For example, throughfall was similar in Gliricidia and Acacia for small rain events, but increased more rapidly in Gliricidia with increasing rainfall depth. Interspecific differences in throughfall were driven, in part, by canopy traits. Leaf area index (LAI), crown depth and crown openness all affected throughfall from smaller storms, but live crown length was the only significant predictor of throughfall in storms that were deeper than 20mm. The spatial heterogeneity of throughfall beneath individual tree canopies increased with rainfall depth, but was always lower in Gliricidia than in Acacia, Ochroma, or Pachira. High litter biomass and cover in Acacia and Ochroma relative to other species would be likely to buffer the erosive effects of raindrop impacts. These complex interactions between rainfall and species traits may affect local hydrology, and may need to be explicitly considered in reforestation projects in the seasonal tropics. [Copyright &y& Elsevier]

Abstract: South Africa is poorly endowed with natural forests, and is heavily dependent on plantations of exotic forestry species to meet its timber needs. The first forest plantations of exotic trees were established in South Africa in 1875. Since then, there has been a steady growth in the total area of forest plantation, culminating in the approximately 1.5 million hectares that are today spread over the higher-rainfall eastern and southern regions of the country. Concerns over the effects of these forest plantations on streamflows and catchment water yields arose as far back as 1915, and were thoroughly debated during the Empire Forestry Conference that took place in South Africa in 1935. A decision taken at this conference led to the establishment of a network of long-term paired catchment experiments in various catchments located in the major forestry areas of South Africa. Since the mid 1980s, these experiments have been supplemented by a wide variety of process studies, where transpiration or evapotranspiration in particular have been directly measured above forest, grassland and other vegetation types, using heat pulse and micrometeorological techniques. Information from all these studies has been used to calibrate catchment hydrological models that have provided estimates of the hydrological impacts of forest plantations in all quaternary catchments in which some degree of afforestation has taken place. Outputs from such studies have been simplified and tabulated to permit planning authorities to predict the likely hydrological impacts of afforestation in any given catchment, and to limit the spread of further afforestation in catchments where available water resources are fully or over-committed. This paper reviews the present South African forest water use situation, and in particular the attitude towards new afforestation, outlining the requirements of the National Water Act that was passed in 1998. Recent efforts to implement an integrated water resource management system that meets the requirements of this Act are described, as are the new institutional arrangements in the process of being implemented. The concept of streamflow reduction activities is presented, and the process by which the national Department of Water Affairs and Forestry currently regulates plantation forestry in the context of all forms of land and water use, is described. Additional aspects of the new system, such as the protection of an ecological reserve (to maintain aquatic systems and ensure supply for basic human needs), water pricing and trading, Compulsory Licensing and the need for equity in the allocation of water resources are highlighted, as well as some of the difficulties hindering implementation efforts. [Copyright &y& Elsevier]

Abstract: To assess the potential impacts of land use change on river water users at both the local and regional scale requires models used in unregulated areas to be linked to river planning models for regulated river systems. This paper illustrates how a land use change model can be linked to the Integrated Quantity and Quality Model (IQQM) for the Murrumbidgee River system in southeastern Australia. Linking the two models allows the impact of potential plantation expansion to be assessed at various points throughout the river system and allows changes in streamflow in upland areas to be converted into impacts on allocations and diversion for downstream water users. To derive an envelope of responses, two plausible plantation expansion scenarios are considered. These are the planting of 30,000ha in areas that will have the largest reduction in mean annual water yield, and the planting of 30,000ha in areas that will have the smallest reduction in mean annual water yield. This paper shows that, at the regional scale, the impacts of these plantation scenarios are small on a mean annual basis, with reductions on allocations, diversions and end-of-system flows being 0.7%, 0.4% and 2.6%, respectively for the maximum impact scenario. However, when there is a large increase in the area of plantations in one sub-catchment, the local scale mean annual streamflow reductions can be significantly higher (up to 23% reduction for the modelled scenario), with larger percentage reductions seen in low flows than in the higher flows. Analysis of the annual impacts in the regulated system highlights the importance of looking at sequences of years and impacts in critical years with maximum annual reductions in allocations, diversions and end-of-system flows of 3.5%, 10% and 18% respectively for the maximum-impact water yield scenario. Linking the two models gives an insight into how management rules that control the regulated system can be triggered. This triggering of rules can result in small changes in upland area having large impacts downstream if key threshold values are affected. These impacts have the potential to be felt not only in the Murrumbidgee River system, but also outside in the Murray River system. [Copyright &y& Elsevier]

Abstract: Some land use changes are driven by the need to improve agricultural production and livelihoods. Rainwater harvesting and management is one such change. It aims to retain additional runoff on agricultural lands for productive uses. This may reduce river flows for downstream users and lead to negative hydrological, socio-economic and environmental impacts in a river basin. On the other hand, rainwater storage systems may lead to positive impacts by reducing water abstractions for irrigation during dry periods. This paper presents a conceptual framework for assessing the impacts of land use changes in the upper Ewaso Ng’iro river basin in Kenya. It is based on a people–water–ecosystem nexus and presents the key issues, their interactions and how they can be addressed. The paper presents hydrological assessment of up-scaling rainwater harvesting (HASR) conceptual framework, which assesses the impacts of land use changes on hydrological regime in a river basin. The results will enhance formulation of sustainable land and water resources management policies and strategies for water-scarce river basins. [Copyright &y& Elsevier]