Joris Eekhout

To get a full understanding of the impacts of global change on the catchment-scale sediment balance, models are needed that combine hillslope soil erosion processes with channel morphodynamics. Here we present a modification to the SPHY-MMF model that includes a novel channel morphodynamics module, which determines erosion and deposition in rills and channels. We applied the model to a Mediterranean study area in southeast Spain, in which we show that channel erosion contributes substantially (35%–40%) to the total sediment yield, highlighting the importance of accounting for channel erosion in catchment-scale sediment budget estimations. The climate change scenarios show that the different erosional processes (i.e. sheet, rill, channel) are projected to decrease or increase, depending on the projected change in annual and extreme precipitation. From this we conclude that interactions between different erosional and depositional processes should be considered when studying the impact of global change on the catchment-scale sediment balance.

Soil erosion is a natural process that can be accelerated by natural and anthropogenic disturbances and lead to land degradation and geomorphological changes. Analyzing soil erosion and catchment sediment dynamics is a complex process. In such cases, simplified methods can be applied to analyze soil erosion and sediment connectivity variations and to understand sediment flux in a river basin to inform watershed management. In this study, we tested the combined method of the Revised Universal Soil Loss Equation (RUSLE), the Index of Connectivity (IC), and the Sediment Delivery Ratio (SDR) to estimate sediment yield (SY) and investigate the spatiotemporal variation of soil erosion rates and sediment connectivity in the Mediterranean Rogativa catchment (∼53 km2), Southeast Spain. In this ‘RUSLE-IC-SDR’ approach, the sediment delivery ratio was estimated from the spatially distributed index of connectivity, calculated using SedInConnect and accounting for the trapping efficiency of 58 check dams in the channels, while assuming 100 % sediment delivery in other parts of the channels. The sediment delivery ratio was calibrated, and sediment yield was verified for the year 2001 using observed sediment yield (in 2003) behind the non-silted check dams. Predicted soil erosion, connectivity (IC, SDR, and SY), and soil erosion-connectivity maps were quantified and compared over time and space, revealing the impacts of rainfall, land use, and check dams. These maps show higher values for areas closer to the channels than on the hillslopes, and higher values on croplands than other land use types, as well as a decrease over time due to land use change and the construction of check dams. The relatively simple ‘RUSLE-IC-SDR’ approach was found to be effective in identifying the sources and hotspots on the hillslopes of a complex Mediterranean catchment. Future studies should consider the channel erosion processes as the RUSLE-IC-SDR does not take these into account.

Understanding erosion and sedimentation processes along the drainage network, from hillslopes to rivers and reservoirs, is essential for water resources management and river restoration. This work proposes a novel dynamic evaluation of landscape factor from modeled runoff and erosion rates from physically-based distributed hydrological modelling, to estimate event-scale sediment connectivity. Four precipitation events of moderate intensity were selected and used for model calibration. The results were used to analyze the temporal variability of connectivity and comparison with indices based on catchment relief or land-uses. Although the headwater areas of the hillslopes presented similar values for all simulated events, a progressive increase in sediment connectivity, proportional to the runoff magnitude of the event, was observed. The variability of the event-scale connectivity index was mainly controlled by parameters related to flow (riverbed roughness, rill erodibility and particle diameter) and less by land use and vegetation cover (cover fraction or interrill erodibility). Although features affecting functional connectivity caused variations between events, the obtained results agreed with indices based on relief as landscape factor. This highlights the important role of structural connectivity represented by the catchment topography. However, the proposed methodology is subject to several sources of uncertainty related to event-scale model calibration, the erosion and transport processes considered and the spatial distribution of runoff. Furthermore, the geomorphological threshold for hillslope and rivers can also affect sediment connectivity, especially along the fluvial system. The results of this work highlight important future challenges in a more dynamic understanding of sediment connectivity river basins.