Chappell, N.A., Nik, A.R., Yusop, Z., Tych, W and Kasran, B. 2000. Spatially-significant effects of selective, tropical forestry on water, nutrient and sediment flows: a modelling-supported review. In Forests-Water-People in the humid tropics, UNESCO-IHP / IUFRO2000 workshop 30 July - 8 August 2000, Kuala Lumpur, Malaysia. (for publication by UNESCO / Cambridge University Press 2002)



There is an increasing desire to improve the environmental sustainability of timber harvesting within tropical Natural Forests. These efforts are, however, hampered by the dearth of robust interpretations of the impacts of selective (non-clearfell) forms of tropical forestry on water, nutrient and sediment flows at meaningful, landscape-scales. This review, therefore, seeks to (a) assimilate the results of reliable studies of selective forestry impacts on catchment-scale, ecohydrological flows, and (b) examine the value of modelling in the assimilation of these reliable case studies.

The findings indicate that there are very few tropical studies with high quality river data (i.e., water, nutrient and sediment flows) on which to base the analysis of selective forestry impacts. Statistical modelling and numerical modelling (using Data- Based-Mechanistic or DBM approaches) of those data that do exist, indicate that: (i) catchment water-yield and nutrient losses are increased but by less than a factor of two, (ii) the rate of migration of rainfall through a catchment to a river and hence the 'river responsiveness' may be increased, but only slightly, (iii) nutrient flows increase by a factor of 0.5 to 5 in the harvesting year (iv) river sediment flows are significantly increased by a factor of 2 to 50, though more precise explanation of this range remains unclear, (v) the effect of natural cycles (e.g., El Niño Southern Oscillation) and trends in the rainfall may exert as strong an influence on the river records as the forestry activities, (vi) differences in the physical impact of different forms selective forestry remains poorly quantified at the landscape-scale, and (vii) the rate of recovery of the catchment system from the forestry impacts, in terms of water-yield, nutrient loss and sediment delivery, also remains poorly quantified at the landscape-scale. Further field studies to support, scientifically, explanations of the differential impacts of different selective forestry operations, particularly on sediment delivery, are urgently needed, as are the extension of water, nutrient and sediment studies over 10-30 years, so that recovery of the ecohydrological system can be quantified. DBM and other modelling approaches provide a method to (i) undertake quality assurance of the existing and new river data, (ii) extract forestry impacts from natural cycles and trends, and tentatively, (iii) to forecast likely ecohydrological changes.

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