Field-based identification of near-surface lateral flow in a forested hillslope catchment

Climate change is expected to increase the frequency and volume of short-duration, high-intensity rainfall events in many regions. These changes can strongly affect runoff generation processes in hilly catchments, where rapid subsurface flow responses may play a key role during extreme precipitation events. However, direct field-based evidence of near-surface lateral flow (interflow) at the hillslope scale remains limited.

This study presents field-based evidence of near-surface lateral flow in a small forested hillslope catchment in southern Hungary. The investigation combines multi-depth soil moisture monitoring with a plot-scale controlled infiltration experiment to analyse subsurface water movement under intense water input conditions. Volumetric soil water content was measured at depths between 10 and 100 cm using frequency domain reflectometry (FDR) sensors, providing high temporal resolution data during infiltration events. In parallel, an artificial rainfall experiment was carried out on a bordered hillslope plot to enable event-based water balance estimation.

Soil profile observations revealed a vertically heterogeneous soil with a shallow humic layer, an underlying permeable horizon, and a clay-enriched subsoil showing signs of temporary saturation. This vertical structure creates a hydraulic contrast that restricts vertical percolation during intense infiltration. As expected, soil moisture measurements showed rapid wetting in the upper soil layers, while deeper layers responded more slowly. Water balance calculations indicated that a considerable part of the water applied to the surface could not be detected in the change of the vertical soil water storage, suggesting a subsurface lateral flow distribution within the near-surface soil layers.

The timing and depth distribution of soil moisture responses, together with the water balance results, provide consistent evidence for the activation of near-surface lateral flow along soil horizon boundaries with contrasting hydraulic properties. The findings highlight the importance of subsurface flow processes in forested hillslope hydrology and underline the need to consider near-surface lateral flow when assessing runoff generation under increasingly extreme rainfall conditions.