Controls on runoff processes in forested catchments: a global synthesis

Forested catchments represent major hydrological hotspots worldwide, supplying a large proportion of global freshwater resources while delivering the highest water quality among land-cover types and a wide range of water-related ecosystem services. Understanding the controls on runoff processes in forested catchments is therefore essential for land, water, and forest management. Despite nearly a century of experimental and modelling research on the hydrological functioning of forested catchments, existing knowledge is largely derived from individual sites or limited intercomparison studies, and a coherent global synthesis of runoff processes has been lacking.

Here, I compiled a global database comprising data of 691 forested catchments extracted from 267 peer-reviewed studies published between 1993 and 2024. I used this new and extensive dataset to identify and synthesize the dominant climatic, hydrological, pedological, vegetational, and geological/geomorphological controls on runoff generation, streamflow response, and streamflow prediction. I tested seven classic hypotheses in forest hydrology at the global scale, alongside an original one addressing the dominance of climate as an overarching control and its variability across humid and less humid regions.

The synthesis reveals that threshold behaviors are widespread across forested catchments globally, with soil moisture—often interacting with rainfall—emerging as the dominant driver of nonlinear runoff responses. Tracer-based studies confirm that pre-event water dominates streamflow generation, with groundwater constituting the largest fraction of this contribution, while soil water plays a secondary role. Subsurface flow, often involving preferential flow through macropores and soil pipes, is identified as the most frequent runoff mechanism. Contrary to conventional assumptions, overland flow is not rare in forested catchments: infiltration-excess overland flow, typically associated with arid and/or scarcely vegetated environments, occurs in many of the documented studies, particularly in catchments with low mean annual precipitation and with strong pedological control.

The analysis further shows that hillslope–stream hydrological connectivity is more strongly governed by topographic and vegetation patterns than by climate alone, highlighting the importance of landscape structure in forested environments. Streamflow response magnitude is primarily controlled by geomorphological characteristics and antecedent wetness conditions, in addition to meteorological forcing. Streamflow modelling performance is influenced by a broad combination of controls, with topography and geology exerting slightly stronger effects than soil, vegetation, or climate, reflecting both landscape dominance and model structural assumptions.

Overall, the results reveal the interaction of multiple factors on runoff processes in forested catchments across the planet, highlighting the larger role played by geological/geomorphological, pedological, and hydrological factors in certain processes compared to climate, while the relative importance of vegetation increases under humid conditions. This global synthesis provides new process-based insights, revises long-standing theories, and offers an empirical foundation for advancing our understanding of catchment functioning and improving hydrological modelling in forested catchments worldwide.