Most landslides and debris flows worldwide occur during or following periods of rainfall, and many of these have been associated with major disasters causing extensive property damage and loss of life [1
]. Given concerns about the effects of climate change on precipitation regime, in the future, some mountainous areas may likely experience more landslides with a faster response to rainfall; however, most such projections are weakly based and remain untested [10
Subsurface hydrology is usually the main triggering mechanism of these landslides and associated debris flows. While the effects of hillslope hydrology on runoff generation have been thoroughly studied, much less attention has been paid to these effects on landslide and debris flow initiation. Recent syntheses demonstrate that it is no longer appropriate to view the subsurface as a static media which facilitates the transit of subsurface water, rather a variety of factors affecting the dynamics of subsurface hydrology need to be considered [12
]. This dynamic nature of subsurface hydrology depends on the complex interactions among precipitation inputs, physical properties and heterogeneity of soils and bedrock, local geomorphology, and vegetation and associated biomass. These factors influence the timing of landslides with respect to precipitation inputs and antecedent soil moisture [14
], the mass and mode of failure [18
], and the extent of runout or transformation of landslides into debris flows [19
Both the infiltration of rainwater and snowmelt and bedrock exfiltration provide the local trigger of these landslides, while drainage and evapotranspiration tend to stabilize hillslopes by rerouting and removing subsurface water. Subsurface hydrology is strongly affected by preferential flow within the soil, substrate topography, and exfiltration from fractures in bedrock [2
]; the overall regolith moisture regime and recharge rates are influenced by evapotranspiration, soil development processes, soil water-groundwater interactions, and landform aspect and shape [14
]. The dynamic behavior amongst these interacting hydro-eco-geomorphic components evolves across spatial and temporal domains creating the conditions for landslide initiation. As such, the resulting hydrologic dynamics that induce changes in soil moisture and pore water pressure remain an important focal area of investigation and are addressed in this special issue along with hydro-meteorological thresholds for landslide assessment and early warning and modelling [15
Incorporation of complex hydrological processes into landslide simulations is still lacking and has significantly lagged the development of reliable predictive hydrodynamic models for landslide occurrence. This conundrum is largely due to the complexities of both the regolith properties and the different modes of failure. In addition to the complications of simulating subsurface flow, monitoring groundwater levels and soil moisture in unstable terrain is challenging due to the large areas involved and diverse topographies.
Furthermore, challenges remain associated with landslide runout, i.e., the spatial propagation of landslide sediments. In mountainous terrain, a threshold appears to exist between landslide dam formation in receiving channels and debris flow occurrence associated with topographic conditions, water content, and the lithology from which sediment is derived [19
]. Hydraulic modeling studies have reasonably predicted the spatial propagation of pumice debris flows [26
]. Findings from flume experiments were used to develop a hydro-mechanical model for debris flow initiation, including rainfall thresholds [27
]. Other modeling investigations included in this special issue examined the role of exfiltration from bedrock fractures on slope failure [18
] and the influence of variable bedrock topography coupled with rainfall intensity on slope stability [22
The thirteen papers presented herein address numerous landslide hydrology issues in five different continents. These studies cover a variety of soils and lithologies in climates ranging from tropical to Mediterranean to temperate. Failure modes include the full range from progressive soil creep to shallow, rapid landslides and debris flows. As such, these papers provide a significant contribution to the developing literature on landslide hydrology.