Are one-dimensional infiltration models suitable for simulating soil moisture in landslide-prone hillslopes?

The dynamics of water in hillslopes influence the processes that govern slope stability and the triggering of landslides, particularly under intense rainfall events. Understanding hydrological processes across multiple scales, from the watershed to the microscopic level of the soil, is essential for identifying the causes and triggers of slope instability. Hydraulic anisotropy, the presence of discontinuities, textural variability, and slope angle control the direction and intensity of water flows over time, generating both vertical and lateral flow components.

On steep slopes, water flow in the soil can be described by two distinct infiltration fronts: a transient front, which propagates perpendicular to the ground surface (associated with vertical flow), and a stationary front, which develops parallel to the slope and is governed by lateral flow. The predominance of transient front or stationary front depends on variables such as the initial depth of the water table, soil hydraulic conductivity, constant infiltration rate, and slope angle.

In this context, the present study evaluates the validity of the hypothesis of predominant one-dimensional flow in simulating infiltration on landslide-prone hillslopes, focusing on periods of intense rainfall, during which the short duration of events tends to limit the contribution of lateral flows. Simulations of volumetric soil moisture were performed using observed rainfall data and hydraulic parameters derived exclusively from pedotransfer functions. However such type of simulation has not satisfactorily reproduced the observed hydrological behavior (mean Spearman correlation coefficient ρ = -0.27). On the other side, when the hydraulic parameters have been adjusted based on soil moisture field in situ measurements, the calibrated simulations showed fairly acceptable and good agreement between both, simulated and observed soil moisture, depicting positive and statistically significant correlations at all monitored depths (mean Spearman correlation coefficient ρ = 0.80).

The results indicated a predominance of downward vertical flow during intense rainfall events, depicting that, despite the fact that hillslope hydrology is inherently multidimensional, one-dimensional model approach still can adequately represent soil moisture dynamics under transient conditions associated with rapid infiltration events. Furthermore, the results highlight the need for site-specific calibration of soil hydraulic parameters. 

Overall, the findings highlight the importance of site-specific calibration of soil hydraulic parameters and reinforce the value of continuous soil moisture monitoring as an effective tool for identifying hillslope areas susceptible to shallow landslides.