How do modeling choices and erosion zone locations impact the representation of connectivity and the dynamics of suspended sediments in a multi-source soil erosion model?

Magdalena Uber,Guillaume Nord,Cédric Legout,Luis Cea

doi:10.5194/esurf-9-123-2021

Abstract

Abstract. Soil erosion and suspended sediment transport understanding is an important issue in terms of soil and water resources management in the critical zone. In mesoscale watersheds (>10 km2) the spatial distribution of potential sediment sources within the catchment associated with rainfall dynamics is considered to be the main factor in the observed suspended sediment flux variability within and between runoff events. Given the high spatial heterogeneity that can exist for such scales of interest, distributed physically based models of soil erosion and sediment transport are powerful tools to distinguish the specific effect of structural and functional connectivity on suspended sediment flux dynamics. As the spatial discretization of a model and its parameterization can crucially influence how the structural connectivity of the catchment is represented in the model, this study analyzed the impact of modeling choices in terms of the contributing drainage area (CDA) threshold to define the river network and of Manning's roughness parameter (n) on the sediment flux variability at the outlet of two geomorphologically distinct watersheds. While the modeled liquid and solid discharges were found to be sensitive to these choices, the patterns of the modeled source contributions remained relatively similar when the CDA threshold was restricted to the range of 15 to 50 ha, with n restricted to the range 0.4–0.8 on the hillslopes and to 0.025–0.075 in the river. The comparison of the two catchments showed that the actual location of sediment sources was more important than the choices made during discretization and parameterization of the model. Among the various structural connectivity indicators used to describe the geological sources, the mean distance to the stream was the most relevant proxy for the temporal characteristics of the modeled sedigraphs.

Highlights

Soil erosion and suspended sediment transport are natural processes that can be exacerbated by human activities and are a major concern for soils and water resources management
In order to take into account the fact that the width of the river varies from upstream to downstream, we introduced a distinction between the perennial river network defined using a contributing drainage area (CDA) of 500 ha and the intermittent river network obtained using a CDA of 15 ha
Our results show that the location of the sources in the catchment highly influenced the temporal dynamics of suspended solid discharges at the outlet

Summary

Introduction

Soil erosion and suspended sediment transport are natural processes that can be exacerbated by human activities and are a major concern for soils and water resources management They cause on- and off-site effects such as the loss of fertile topsoil, muddy flooding, freshwater pollution due to the preferential transport of adsorbed nutrients and contaminants, increased costs for drinking water treatment, reservoir siltation, and aggression of fish respiratory systems (Owens et al, 2005; Brils, 2008; Boardman et al, 2019). The distribution of sources within the catchment and different travel times of sediment from sources to the outlet as well as rainfall dynamics are assumed to be the dominant reasons for the observed suspended sediment flux variability (Legout et al, 2013)

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Conclusion