Modelling surface and sub-surface connectivity with a mechanistic soil-landscape evolution model

Sediment redistribution dynamics interest geoscientists greatly as their assessment can help address issues such as soil loss in agricultural areas or the movement of pollutants and sediments toward streams. In mountain landscapes, the variety of gravitational and water-based erosion processes makes redistribution assessments particularly challenging.

The concept of sediment connectivity was defined generally as the degree of linkage between sediment sources and downstream sink areas, and was further distinguished into structural (SC) and functional (FC) connectivity.

SC is the potential connectivity due to the existing topography, most widely assessed using the Index of Connectivity, developed by Borselli et al. (2008) and later improved by Cavalli et al. (2013).

It is an empirical calculation, which is not intended to quantify sediment rates, to capture sub-surface dynamics and changes in time or even whether sediment transfer actually occurs.

The FC has been introduced to represent the actual transfer of water and sediments within the landscape, on the basis of the geomorphic processes and their magnitude.

FC is not as widely adopted and few authors have adopted existing Landscape Evolution Models or LEMs to quantify sediment dynamic across the landscape. So far, FC has only been simulated at the surface.

The aim of this work is to a) assess how time, depth in the soil, grain size, and landscape development affect surface and sub-surface sediment dynamics; b) determine the impact of bioturbation on sediment connectivity; and c) compare sediment redistribution to the Index of Connectivity (Borselli et al. 2008, Cavalli et al. 2013) dynamically after each timestep.

This work was carried out using an adapted version of the mechanistic soil-landscape evolution model LORICA and applied to a mountainous catchment in Tirol, Austria.

The results confirmed that simulating bioturbation affects the vertical redistribution of sediments, transporting surface sediments deeper in the soil profile and bringing deeper sediments to the surface. It also showed that different grain sizes are transported at different rates across the landscape, with finer grainsizes reaching both deeper layers and the outlet of the catchment more rapidly compared to larger ones.

Lastly, transport of material in deeper layers was delayed compared to those on the surface.

The mean Index of connectivity across the catchment was mostly unaffected by bioturbation.