Pit-mound microrelief on a forested slope drives infiltration and preferential flow after heavy rainfall-experiments with soil resistance monitoring and dye tracing

With the changing climate, short heavy rainfalls (SHRs) are becoming more frequent and intensive. The distri-bution of rainwater and subsurface runoff formation in forest soils can be strongly affected by soil disturbances and surface microrelief. Rainwater redistribution and the formation of preferential flow in response to natural and simulated SHRs were investigated in soil profiles on slopes forested by Norway spruce with a pit-mound microrelief formed by historical tree uprooting using two independent methods; soil electrical resistance monitoring and the application of dye tracer. In the deepest soil layers, resistance decreased in responses to natural SHRs on average by 46% in pits and by 11% on 'smooth slope' controls, reflecting the deeper redistri-bution of infiltrated water in pits. Dye tracing in the simulated rainfall revealed the formation of shallow biomat flow at the control plot. In contrast, the shallow lateral flow was focused by the pit and redirected into funnel flow underneath the pit bottom. The sum of medium and high dye concentration classes obtained with the support vector machine classification of profile photographs indicated a higher water entry into topsoil and subsoil layers in pit (44% and 31% of area, respectively) as compared to the control profile (16% and 8% of area, respectively). Leaving pit-mound microrelief as the natural legacy in forest soils can mitigate some of the negative hydrological effects of intensive forest management and may improve the water yields on forested slopes by redirecting shallow subsurface runoff and facilitating deep distribution of infiltrated water. Such beneficial effects of pit-mounds on the hydrology of forested slopes should be considered in future hydrological modelling and land management.

Automated summary

Due to climate change, short heavy rainfalls (SHRs) are increasing in frequency and intensity. Soil disturbances and surface microrelief can strongly influence rainwater distribution and subsurface runoff formation. This study investigated rainwater redistribution and the formation of preferential flow in response to natural and simulated SHRs in forested slopes with pit-mound microrelief. The results showed that the pit area had deeper redistribution of infiltrated water and the control plot had shallow biomat flow. Leaving pit-mound microrelief as a natural legacy in forest soils can mitigate the negative hydrological effects of intensive forest management and improve water yields on forested slopes.