FOREST EVAPORATION MODELS - RELATIONSHIPS BETWEEN STAND GROWTH AND EVAPORATION

The relationships between forest stand structure, growth and evaporati on were analysed to determine whether forest evaporation can be estima ted from stand growth data. This approach permits rapid assessment of the potential impacts of afforestation on the water regime. The basis for this approach is (a) that growth rates are determined by water ava ilability and limited by the maximum water extraction potential, and ( b) that stand evaporation is proportional to biomass and biomass incre ment. The relationships between stand growth and evaporation were mode lled for a set of catchment experiments where estimates of both growth and evaporation were available. The predicted mean evaporation, over periods of several years, was generally within 10% of the measured mea n annual evaporation (rainfall minus streamflow) when the model from o ne catchment was applied to other catchments planted with the same spe cies. The residual evaporation, after fitting the models, was correlat ed with rainfall: above-average rainfall resulted in above-average eva poration. This relationship could be used to derive estimates for dry and wet years. Analyses using the models provide additional evidence t hat Eucalyptus grandis may be depleting groundwater reserves in catchm ents where its roots can reach the water table. The models are designe d to be integrated into a plantation management system which uses a ge ographic information system for spatial analysis and modelling. The us e of readily available growth parameters as predictor variables may re duce our dependence on intricate process-based models. This is seen as an efficient way of extrapolating existing catchment data - reflectin g the impacts of forestry on water supplies across a range of sites, c limatic zones and species. This approach has the potential for further development, especially in dealing with low flows and faster growing species. (C) 1997 Elsevier Science B.V.