A numerical method to account for the vegetation layer in the numerical modelling of runoff process
Authors: Tagarelli Vito , Cecconi Manuela , Grimaldi Salvatore , Cotecchia Federica 
Published: March 2024

The effects of the weather conditions on soil erosion and the stability of slopes have become the subject of intense research activity recently, since both advancements in the monitoring of the soil state in the slope and developments in the numerical modelling of the slope processes connected to the climatic conditions have been achieved. All the thermo-hydro-mechanical processes driven by the climatic variables acting at the ground surface of the slope, i.e., rainfall, solar radiation, relative humidity, wind speed and air temperature, acting on both the vegetation covering the slope and the top soils where the vegetation is rooted, are concurring to determine the interaction which involves the top soil skeleton and the pore fluids (Gens, 2010); this interaction, together with the transient seepage that is activated in the slope has been defined as slope-vegetation-atmosphere (SLVA) interaction (Elia et al., 2017).
Elia and co-Authors reviewed the modelling strategies that can be employed to assess the effects of SLVA interaction on the stability of slopes, highlighting that the complexity of the modelling relates to the coupling of all phenomena taking place in the slope, and also to the disciplinary broadness of the subject, particularly due to the vegetation layer, whose impact is still far from being fully understood and modelled.
This contribution addresses numerically the impact of the vegetation layer on the hydraulic balance at the slope surface by simulating artificial controlled rainfall tests; as an example of application, the data reported in Apollonio et al. (2021) have been taken into consideration. These Authors found that the runoff was significantly affected by the vegetation layer, causing a reduction of the runoff flux of 1/3 on average with respect to the bare condition.
The idea behind this work was to back-analyse numerical testing results by simulating the presence and the features of the vegetated layer adopting a soil-like region within the model aimed at representing the hydraulic behaviour of the vegetation layer. As such, the back analyses have been carried out to retrieve the most proper value of the coefficient of saturated conductivity to be applied to the vegetated region in the numerical mesh.
Hydraulic uncoupled (Elia et al., 2017) back analyses were carried out by using Plaxis LE Groundwater, whose last release is able to calculate the surface runoff parallel to the top model boundary, i.e., the climate boundary, allowing to retrieve the most proper coefficient of saturated conductivity to simulate the presence and the effects of the vegetated layer. It is believed that the proposed numerical method may inform the numerical modelling of the SLVA interaction, by explicitly accounting for the vegetated layer by using and characterize a proper region in the mesh above the slope. In this perspective, this approach may be further generalized and applied to other case studies.

Apollonio C. et al., (2021), https://doi.org/10.3390/su13116058
Elia G. et al., (2017), https://doi.org/10.1144/qjegh2016-079
Gens A., (2010), https://doi.org/10.1680/geot.9.P.109