Soil creep is common along the hillslopes of the tropical Andes of Colombia, where very heterogeneous soils develop on old debris flow deposits and are subjected to abundant rainfall with a bimodal annual regime. In particular, the western hillside of the city of Medellín, Colombia, is comprised of a series of debris and earth flow deposits in which landslides and soil creep are common. To explore linkages between soil creep and hydrology, we selected an experimental site in the western hillslope of the Medellín valley to assess the behavior of water within the soil mass, its relationship with rainfall, and its connection with soil displacement. In experimental plots, we systematically measured runoff, percolation, water table levels, and volumetric water content, for a period of almost 2 years; we also conducted several alti-planimetric positioning surveys to estimate relative displacements of the soil surface. Moisture content of the soil remained above field capacity for most of the year (~68% of the time) and active and quasi-permanent lateral subsurface flow occurred within the upper 80 cm of the profile. The shallow flow likely facilitates the downslope movement. Additionally, our results suggest that displacement magnitudes are largest during the wet season of September–October–November, when a highly humid soil experiences changes in water content, so it is during this time that the effects of expansion / contraction of the soil particles (associated to wetting / drying cycles) contribute the most to the movement. This observational study represents a contribution to the understanding of soil creep in tropical hillslopes, where it responds to the wetting / drying cycles, with the particularities of a rainy weather (>1500 mm/year), warm temperatures (~22 °C on average), and a bimodal precipitation seasonality.

Mass movements are quite common in the Northern Andes and constitute one of the major hazards in the region. In particular, along valley flanks where the city of Medellin (Colombia) is located, rainfall is the main trigger of these phenomena, but little is understood about how water in the soil and subsoil behaves. In this study, we show data from some basic soil hydrology measurements and conventional geophysical surveys within a ~4 ha experimental plot that is experiencing soil creep. The seasonally wet study site has an average slope gradient of 33%, and its surface geology consists of a series of older deposits of debris flows. Our measurements show a low surface runoff, which ranges from 4 to 11% of the rainfall; infiltration is 89–96% of the rainfall, and 15–33% corresponds to drainage water at shallow levels in the soil (20–50 cm); piezometric measurements reveal a mostly steady-state water table. About 14–54% of the rainfall becomes subsurface flow within the first ~1–2 m below the surface. Geoelectrical and seismic surveys suggest small temporal changes in the properties of materials shallower than 2 m, consistent with the steady-state water table and the permanent and high subsurface flow. These geophysical surveys also indicate the presence of a major discontinuity at ~4–6 m below the surface, which we interpret as the limit between different prior debris flows.