Potentials and limitations of natural flood retention by forested land in headwater catchments: evidence from experimental and model studies

Assessment of catchments water retention potential in the landscape is a common part of management plans as components of integrated flood risk management strategies. For forest land use, it has been stated that the promotion of sustainable forest management and the increase of forest land via afforestation will considerably improve flood retention. Notably, the latter should be efficient to re-establish the natural water retention potential in anthropogenically disturbed river basins and to decrease the human-made contribution to flood generation. However, due to obvious physical limitations (such as soil depth, porosity, and conductivity) and the frequent lack of available land for forest expansion, the role of afforestation in flood mitigation has been a topic of strong debate. This study assesses the 'forest effect' based on model calculations and suggests visualisations to communicate the results. For a catchment (6.8 ha) and two well-founded land-use scenarios, it could be shown that the peak reduction for flood events varies from 3% to 70% and is highly related to the event characteristics (especially pre-event soil moisture). The cross compliance to other protection aims (i.e. water quality and soil protection) is part of the considerations.