Multi-decadal sediment dynamics of a tropical watershed – The relative roles of surface erosion and shallow landsliding

Sedimentation represents a key risk to the world's artificial reservoirs upon which humans rely to satisfy vital water needs. Improving our understanding of watershed sediment dynamics is crucial in managing water reservoirs, particularly in small, high-standing tropical islands characterized by high sediment yields. This study addresses such a need by comparing potential contributions from surface erosion on active cropland and rain-driven shallow landsliding to sediment yields in an artificial reservoir in Puerto Rico for a 60-year period. The study relied on various data sources including land cover and landslide maps, high-resolution digital elevation databases, long-term rainfall data and bathymetric surveys to document the sediment budget of a 43.9 km² watershed. Results indicate that sediment delivery associated with landsliding may explain 47–79% of the 1,195 Mg km⁻² yr⁻¹ long-term, watershed-scale average sediment yield with surface erosion from active cropland capable of sustaining about 20%. Rainfall not directly associated to tropical cyclones appears to be as important as that directly related to named storms in driving landslide mobilization. As cropland cover declined and as the frequency of days with sufficient rainfall to trigger landslides increased through time, the relative importance of landsliding consequently increased and presently appears to be about 85% of sediment yield rates that remain well above background levels. The current high incidence of landsliding in this mostly forested watershed appears to be linked to its abundant rainfall (~1,980 mm yr⁻¹; ~0.92 landslide triggering rains per year), high topographic relief (~90% of watershed > 30°) and high road densities (19.4 km km⁻²). Most of the roads are farm access roads and largely represent a legacy of now abandoned agricultural activities, yet they still induce much slope instability that generates significant quantities of sediment.