Soil erodibility and processes of water erosion on hillslope

The importance of the inherent resistance of soil to erosional processes, o r soil erodibility, is generally recognized in hillslope and fluvial geomor phology, but the full implications of the dynamic soil properties that affe ct erodibility are seldom considered. In Canada, a wide spectrum of soils a nd erosional processes has stimulated much research related to soil erodibi lity. This paper aims to place this work in an international framework of r esearch on water erosion processes, and to identify critical emerging resea rch questions. It focuses particularly on experimental research on rill and interrill erosion using simulated rainfall and recently developed techniqu es that provide data at appropriate temporal and spatial scales, essential for event-based soil erosion prediction. Results show that many components of erosional response, such as partitioning between rill and interrill or s urface and subsurface processes, threshold hydraulic conditions for rill in cision, rill network configuration and hillslope sediment delivery, are str ongly affected by spatially variable and temporally dynamic soil properties . This agrees with other recent studies, but contrasts markedly with long-h eld concepts of soil credibility as an essentially constant property for an y soil type. Properties that determine erodibility, such as soil aggregatio n and shear strength, are strongly affected by climatic factors such as rai nfall distribution and frost action, and show systematic seasonal variation . They can also change significantly over much shorter time scales with sub tle variations in soil water conditions, organic composition, microbiologic al activity, age-hardening and the structural effect of applied stresses. P roperty changes between and during rainstorms can dramatically affect the i ncidence and intensity of rill and interrill erosion and, therefore, both s hort and long-term hillslope erosional response. Similar property changes, linked to climatic conditions, may also significantly influence the stabili ty and resilience of plant species and vegetation systems. Full understandi ng of such changes is essential if current event-based soil erosion models such as WEPP and EUROSEM are to attain their full potential predictive prec ision. The complexity of the interacting processes involved may, however, u ltimately make stochastic modelling more effective than physically based mo delling in predicting hillslope response to erodibility dynamics. (C) 2000 Elsevier Science B.V. All rights reserved.