Sensitivity of the landslide model LAPSUS_LS to vegetation and soil parameters

The influence of vegetation on slope stability is well understood at the slope level but scaling up to the catchment
level is still a challenge, partially because of a lack of suitable data to validate models. We tested the physical
landslide model, LAPSUS_LS, which models slope stability at the catchment scale. LAPSUS_LS combines a hydrological
model with a Limit Equilibrium Method model, and calculates the factor of safety of individual cells
based on their hydrological and geomorphological characteristics. We tested two types of vegetation on slope
stability: (i) coffee monoculture (Coffea arabica) and (ii) a mixed plantation of coffee and deep rooting Erythrina
(Erythrina poeppigiana) trees. Using soil and root data from Costa Rica, we performed simulations to test the
response of LAPSUS_LS to root reinforcement, soil bulk density, transmissivity, internal friction angle and depth
of shear plane. Furthermore, we modified the model to include biomass surcharge effect in the calculations.
Results show that LAPSUS_LS was most sensitive to changes in additional cohesion from roots. When the depth of
the shear plane was fixed at 1.0 m, slopes were not unstable. However, when the shear plane was fixed to 1.5 m,
the mixed plantation of coffee and trees stabilized slopes, but the coffee monoculture was highly unstable,
because root reinforcement was low at a depth of 1.5 m. Soil transmissivity had a limited impact on the results
compared to bulk density and internal friction angle. Biomass surcharge did not have any significant effect on
the simulations. In conclusion, LAPSUS_LS responded well to the soil and vegetation input data, and is a suitable
candidate for modeling the stability of vegetated slopes at the catchment level.