Landslide hydrology: from hydrology to pore pressure

Rainfall-triggered landslides are among the most widespread hazards in the world. The hydrology in and around a landslide area is key to pore pressure buildup in the soil skeleton which reduces shear strength due to the buoyancy force exerted by water in a saturated soil and to soil suction in an unsaturated soil. Extraordinary precipitation events trigger most of the landslides, but, at the same time, the vast majority of slopes do not fail. The intriguing question is: 'When and where exactly can a slope become triggered to slide and flow downwards?' The objective of this article is to present and discuss landslide hydrology at three scales-pore, hillslope, and catchment-which, taken together, give an overview of this interdisciplinary science. We argue that 'filling, storing, and draining' of water are all equally relevant to understand and to quantify landslide behavior. Furthermore, by addressing landslide hydrology from both earth sciences and soil mechanics perspectives, we aim to manifest the hydrological processes in hillslopes and their influence on behavior and triggering of landslides and vice versa. The challenge of landslide hydrological research is matching, at hillslope scale, causal hydrological processes with detailed physics of triggering mechanisms. At the same time, mass movements initiate changes in the hydraulic character of the soil which are important for better understanding short-and long-term hydrogeomorphic responses of soil and hillslope. Interdisciplinarity is key in advancing our knowledge on water flows in (un)stable slopes.