Identification of runoff processes – The impact of different forest types and soil properties on runoff formation and floods

In a changing climate extreme weather events including convective storm rainfalls are expected to generate an increasing number of floods threatening small catchments. To improve the understanding of soil–water interrelations in different forest types and associated flood generation, a series of sprinkling experiments was conducted in the low-mountain ranges of Southwest Germany. The experiments were at hill-slope scale and involved afforestations following agricultural land-use as well as established forest stands. Several different runoff processes were identified. The soils under recent afforestations still possessed the physical conditions and similar runoff formation processes of the former agricultural soils. The soils under established forests were relatively porous with high infiltration and water conductivity rates, and consequently reduced occurrence of surface runoff. However, compacted subsoil layers under both agricultural and forest use hindered or even prevented further vertical seepage. The subsurface flow above these compacted soil layers thus contributes to flood generation. Hence, soil properties were confirmed as one of the crucial factors for runoff generation. We strongly recommend to consider soil characteristics and, where applicable, the use of amelioration tools in further forest planning. This will help to improve water retention and storage capacities, and enhance the flood protection function of forests.