Relationships between vegetation characters and saturated hydraulic conductivity at catchment scale

Shallow landslides susceptibility assessment by physically based methods relies on the parametrization of both hydraulic and geotechnical features of soils. Among these, saturated hydraulic conductivity (Ks) influences infiltration rates, runoff, groundwater recharge and drainage processes, which makes it of particular concern in the prediction of natural hazards, including catastrophic floods and shallow landslides (Hao et al., 2019). Moreover, it is well known that vegetation plays a role towards slope stability and over the last decades many efforts have been done in order to quantify the effects of the vegetation towards protection from shallow landsliding (Giadrossich et al., 2017). Basically, two main vegetation effects may be considered: hydrological (e.g., modification of the pore water pressure through tree rainfall interception) and mechanical ones (increase of the soil strength due to the root effects and increase of the subsurface shear stress due to the vegetation load). Soil Ks is expected to be an important factor that influences plant growth, plant-available water, root system and root biomass and, therefore, this work focuses on the quantitative assessment of the relationship between vegetation characters and hydraulic conductivity. Study areas affected by shallow landslides were chosen in the Garfagnana and Alpi Apuane regions (Northern Apennines, Italy), as well as in the Mt. Amiata volcano (Southern Tuscany, Italy), where field measurements (about 200) of below-ground vegetation (Root Area Ratio - RAR), above-ground vegetation (Leaf Area Index - LAI and vegetation load) and Ks were carried out inside, in the neighbour and far from shallow landslide locations. Acquisition of data within landslides areas was possible taking advantage of a multi-temporal landslide inventory already available for the study areas. Below-ground vegetation data were collected in trench profiles, while above-ground vegetation data were acquired by means of both digital relascope and digital cover photography. Measurements of Ks were carried out by means of both constant and falling head approaches. Results show that Ks related to RAR and soil depth, with decrease of Ks as the depth increases and root area decreases. Moreover, Ks varies with different vegetation types, indicating that shifts in aboveground vegetation features may impact the water dynamics of soil. The weight of above-ground vegetation plays a “mild” negative role on slope stability. Instead, root reinforcement to soil in terms of root-related cohesion plays a relevant role for the depths involved in shallow landslides.