Quantifying suspended-sediment cascade and dynamics in a Mediterranean mountainous river

Sediment discharges quantify the pace of landscape evolution and provide a baseline for assessing the impacts of land use changes on soil erosion and sediment discharge. Estimates of the long-term sediment discharge from catchments are important because they provide a measure of basin-averaged erosion rates considering the understanding of erosion and sediment transport is crucial for the sustainable management of water and soil resources. Geomorphologists have currently a significant understanding of sediment delivery in catchments, but there is still a lack of comprehension of how these processes are coupled or decoupled in different types of catchment cascades. Besides, catchment monitoring is complicated by the fact that the relevant processes operate at different temporal and spatial scales. This requires that a nested approach is adopted and that catchment data are collected on the micro to macro scale. This approach has been applied in three gauging stations (3.4, 52.6, and 145 km²) of the Sant Miquel River, a Mediterranean mountainous catchment (151 km²) highly shaped for the human activity (i.e. intensive agriculture, terracing, check dams, channelization) and land abandonment (i.e. revegetation processes). It has an intermittent hydrological regime due to the predominance of karstic processes, despite groundwater from these karstified formations contribute with large water volumes in the middle and downstream part of the catchment. This communication aims to assess the runoff and suspended-sediment transport as well their dynamics (hysteresis) at different time-scales during a 7-year study period for these three nested catchments where water and sediment fluxes were continuously measured and recorded to evaluate the sediment cascade at annual, seasonal and event scales. A discharge (Q; m³ s⁻¹) and suspended sediment concentration (SSC; mg l⁻¹) database with 56 (3.4 km²), 36 (52.6 km²), and 99 (145 km²) events was used to perform a hysteretic analysis using the h index developed by Zuecco et al. (2016). Results showed an annual average suspended-sediment yield of 7.5, 40.8, and 26.2 t km^-2 yr⁻¹ from headwaters to downstream, evidencing transmission loses between the middle and downstream part of the catchment. Clockwise Q-SSC hysteresis loops (with the suspended sediment peak leading the discharge peak) were recorded most frequently (~50 %) at the two headwater sites, being only 31 % in the downstream site where the figure-of-eight pattern (clockwise and anti-clockwise) predominated with 62 % of the events involving that sediment concentration continued to rise despite the falling hydrograph; pattern promoted by the sediment contribution from different tributaries and sources. The need to obtain long-term monitoring programmes provides the key value of this communication to reach a comprehensive understanding about the sediment cascades on catchments at different scales. For this reason, it is necessary to continue exploring the sediment origin dynamics in the future, like an opportunity to reach the scalar integration between processes and techniques to better understand and validate erosion models in Mediterranean catchments.