Assessing sediment and nutrient transport in a South Brazilian catchment: hysteresis, fingerprinting, and management implications.

Effective management and protection of natural resources - especially soil and water - require a thorough understanding of the occurring hydro-sedimentological processes in a catchment. In South Brazil, the 18.5 km Guarda Mor catchment, characterized by strong topographic gradients, is vulnerable to soil and water degradation, despite much of its land use under no-till agriculture. Situated at the transition between volcanic to sedimentary lithologies and featuring an escarpment with a nearly 300-m altitude difference, this catchment is part of a fluvial system which was heavily and unprecedentedly impacted by extreme rainfall and flooding in April and May 2024. To quantify and analyze the catchment s hydro-sedimentological behavior, we combined traditional hydrological monitoring of rainfall, streamflow, sediment, and solute (potassium and phosphorus) concentrations with tools for hysteresis analysis and sediment source tracing. This approach allowed us to examine sediment-related water quality over four years, during and in-between rainfall events. We also assessed the hysteresis between streamflow discharge and sediment and solutes concentrations, calculated sediment and nutrient yields, and used the sediment fingerprinting technique to estimate the main land-use-based sediment sources. Extreme events were responsible for most of the sediment yield, with one event in November 2023 yielding 18% of the monitored total ( tons) between 2020 and 2024. Crop fields were estimated as the main sediment source (44%), followed by stream channels (31%), and unpaved roads (25%). Total potassium concentrations were linked to runoff volume, while total phosphorus concentrations were related to sediment yield. During a drought period, between August 2022 and April 2023, increases in phosphorus concentrations impacted water quality. These results highlight the need for sustainable management strategies and long-term monitoring to mitigate extreme flood and drought impacts in headwater catchments.