Stemflows and Preferential Flows: A Historical Review and Challenges for the Future

ABSTRACT This paper provides a historically‐grounded review and research agenda on a generally neglected issue in hillslope hydrology and hydrological modelling: the role of stemflow in initiating preferential flow in soils. While stemflow typically represents a small fraction of incident rainfall, it can concentrate water fluxes by up to ~20‐fold at tree bases, creating localised infiltration intensities that exceed those from throughfall. Some new historical context for throughfall and stemflow studies from the 19th Century is presented, including a summary of stemflow research in a wide range of vegetation types and environments. The evidence for preferential flows resulting from stemflows as a ‘double‐funnelling’ effect is reviewed, emphasising tracer‐based studies used to follow flow pathways. Although stemflow‐driven preferential flows have been shown to occur commonly and, in some cases, to rapidly transport water to zones of saturation and consequent downslope flows, such processes have not been included in hydrological models to our knowledge. Thus, their significance at hillslopes and catchment scales remains an open question. The paper concludes with a needs analysis that identifies key observational and modelling challenges required to quantify stemflow‐preferential flow impacts at larger scales.