High-resolution UAV-SfM approach to quantify short-term hydro-dynamics in Mediterranean river systems.

Mountain headwaters play a critical role in both water and sediment production, transport, and storage. In Mediterranean regions, their geomorphological and ecological significance is amplified by pronounced climatic seasonality, hydrological variability, and anthropogenic pressures. Central Spanish Pyrenees exemplify these dynamics with steep gradients, land use and land cover (LULC) changes, and climatic oscillations in a natural-regime framework. Understanding short-term geomorphic adjustments in these systems requires high-resolution, event-based monitoring that provides a link between hydrological variability and morphological response.

This study examines hydro-morphological dynamics across four headwater reaches in Upper Aragón Basin (NE Spain), characterized by contrasting altitudinal, sedimentological, and topographical characteristics. Daily discharge records (2012 – 2024) were analysed to assess recent hydrological trends and stablish one- and five-year return period magnitudes. In order to stablish a sediment-mobilizing threshold for each reach, a critical discharge (Qc) was calculated based on D50 sediment fraction. To quantify hydraulic forcing in-between periods, the cumulative excess energy (ΔE) was calculated, integrating discharge exceeding Qc over given survey intervals.

High-resolution topographic surveys were conducted using UAV-SfM photogrammetry following competent flood events. Seven seasonal topographic surveys (2023 – 2025) were carried out with a DJI Mavic 3M drone equipped with multispectral sensors and RTK positioning. To ensure topographic correction and evaluate precision and accuracy, ground control points (GCP) and checkpoints (ChP) were surveyed with an EMLID GNSS-RTK rover, achieving centimetric values (average RMSE: 0.08 m; σ: 0.07 m).. Dense point clouds (130 000 points/m² average) were processed within a 0.05 m grid to generate Digital Elevation Models (DEM). DEMs were filtered through the application of multispectral indices (GLI, NDVI, NDWI, and WI) to mask vegetation and debris. Successive DEMs were differenced to quantify aggradation – degradation patterns through models of difference (DoD). A variable minimum level of detection (minLoD) was applied across wet and dry areas along with a custom neighbour-based algorithm to identify and remove isolated outliers.

By integrating hydrological thresholds with fine-scale morphological monitoring, this study provides a detailed quantification of sediment mobility and channel adjustment in Mediterranean mountain rivers. The results highlight the influence of discharge variability on short-term morphological response, offering a process-based framework to support sediment management and channel evolution under changing hydro‑climatic conditions.

ACKNOWLEDGMENTS: This work is funded by the European Research Council (ERC) through the Horizon Europe 2021 Starting Grant program under REA grant agreement number 101039181 - SEDAHEAD.