Impact of deforestation on catchment hydrology and nitrogen losses

Forest status in natural catchment is substantially important for hydrology and water quality, but it has been increasingly altered by human activities and climatic factors. Due to recent rapid changes in forest cover, there is an urgent need for hydrological water quality models which can adapt to these changing environmental conditions. The objective of this study was to analyse the impact of rapid continuous forest decline on nitrogen losses in a temperate mountain range catchment using a dynamic setting of the HYPE (HYdrological Predictions for the Environment) model. The modified model was applied to the Große Ohe catchment, Germany, which has experienced severe forest dieback (caused by bark beetle infestations) and its recovery over the last three decades. The model was validated by using also additional 25 years data from an internal gauge station (Forellenbach) and two soil measurement sites. Three scenarios, namely, no forest change, deforestation with subsequent regeneration, and deforestation without regeneration, were compared to identify key factors influencing catchment discharge and nitrogen export due to deforestation and regeneration. Results showed that the model performed well at the Große Ohe catchment scale, with Nash-Sutcliffe Efficiency values of 0.77 and 0.57 for discharge and IN concentration, respectively, and percentage BIAS values of -11.6% and 0.5%, respectively, during the validation period. Similar good performances were also observed at other scales. The simulation results proved that the improved model was able to (1) well capture the timing of peak flows and the seasonal dynamics of inorganic nitrogen (IN) concentration, and more importantly, (2) reflect the first increasing and then decreasing trend of discharge and IN concentration, in accordance with the deforestation and forest regeneration, respectively. By comparing scenarios, after experienced forest dieback without regeneration, the discharge and IN concentration exports were 24.9% and 160%, respectively, greater than those of scenario without forest change. However, the discharge and IN concentration exports were only 3.63% and 39.6% greater, respectively, with the help of continuous regeneration, indicating that forest regeneration is important for restoring hydrological and water quality status in the catchment. Compared to non-change scenario, the deforestation scenario exhibited decreased annual plant uptake of 34.7%, and strong increase in annual denitrification and N mineralization suggesting that the increased nitrogen export was likely induced by the reduction in vegetation uptake and the increased availability of soil nitrogen from tree residues. Overall, the adapted mechanistic modelling under the changing catchment forest conditions can strongly support forest management in terms of water quality.