Effects of Forest Harvesting Operations on Hydrology: Experiences From the Palatinate Forest Biosphere Reserve

The use of timber harvesting and skidding machines significantly alters a catchment's hydrological processes due to soil compaction effects. Although it is obvious that the use of heavy forest machines increases surface runoff and water yield, quantifying these effects remains challenging. This research aimed at exploring how physically based hydrological models are suited for investigating the effects of soil compaction on hydrologic responses at the catchment scale. We employed a process-based SWAT+ hydrological model to assess the impact of temporary (e.g., skid trails) and more permanent timber harvest infrastructure on the hydrologic response of the Palatinate Forest Biosphere Reserve in Germany. We specifically analysed the role of soil compaction in hydrological processes by simulating water fluxes under three scenarios: uncompacted soil (baseline), compacted soil during harvest and skidding operations (Scenario-1) and due to permanently constructed infrastructures (Scenario-2). Results demonstrated substantial alterations in water fluxes under Scenarios-1 and -2 compared to the baseline, with annual average surface runoff increasing by 74% and 125%, respectively, and lateral flow decreasing by 14% and 26%. These changes varied notably between steep and low slope areas. Steep slope regions exhibited significantly larger increases in surface runoff, while low slope areas experienced greater reductions in lateral flow and percolation. This differentiation underlines the combined influence of management scenarios and slope, highlighting the critical need for forest harvesting strategies that account for spatial variability and the types of machinery employed.