Water repellency under coniferous and deciduous forest — Experimental assessment and impact on overland flow

• Water repellency can lead to considerable overland flow under forest.
• Coniferous forest shows higher WDPT than deciduous forest.
• The humus horizons show higher WDPT than the Ah.
• In the Of-horizon, the highest WDPT was measured.
• Water repellency is strongly related to soil moisture and antecedent rainfall.

Current climate change makes it necessary to gain a deeper understanding of the runoff generation processes in Central European forests. A changing climate might affect soil water repellency (SWR) which can be seen as an import trigger for overland flow generation in forested areas. In this study the differences between coniferous and deciduous forest concerning SWR and overland flow generation were investigated in a small catchment in the Hunsrück low mountain range, Rhineland-Palatinate, Germany.To investigate the occurrence and persistence of SWR and its influence on overland flow generation, a combination of two experimental methods was applied: water drop penetration time (WDPT) test, and rainfall experiments. The field WDPT test results ranged from wettable (WDPT < 5 s) up to more than 900 second persistence of water repellency in both forest types. The median WDPT was 30 s for the coniferous forest and 1 s for the deciduous forest sites. On the deciduous forest soils, only the Of-horizon showed considerable water repellency. Rainfall experiments with 40 mm h− 1 rainfall intensity yielded runoff coefficients between 0% and 63%. The lowest measured infiltration rate of the rainfall experiments was 11.6 mm h− 1. The highest runoff coefficients were measured on water repellent (WDPT > 300 s) coniferous forest sites. The overland flow starts significantly earlier with water repellent soil conditions. The median runoff rate for the wettable forest soils is 2.7%, whereas the water repellent sites show a median runoff coefficient of 11.4%.The results suggest that the occurrence of SWR can lead to considerable overland flow generation under forest.