The general understanding about forests reducing flood peak discharge is not supported with recent observations in the Alptal in Switzerland. Theoretical considerations show that forest influence depends highly on the local situation and, as known since a long time, on the rainfall pattern.
To understand how forest influence on runoff generation varies with different conditions, an in depth analysis was carried out in the Sperbelgraben. In a nested approach soil profiles were analysed, plots of 1 m² were irrigated artificially, surface runoff plots with an area of 50 to 110 m² were installed and runoff in two neighbouring zero order basins was observed.
The study showed that forests have only little influence on flood runoff generation in the Sperbelgraben. Either the soils are so wet due to an impermeable subsurface that only little storage capacity is available, or the subsurface is so permeable, that water easily seeps deep into the ground when the storage capacity of the soil is still far from being saturated. The highest impact of forests on flood runoff generation is expected on soils showing a layer with reduced permeability in a depth between 20 and 60 cm.