# Sentinel-2 Reveals Record-Breaking Po River Shrinking Due to Severe Drought in 2022
## 1 Introduction  
Rivers are dynamic systems where abiotic and biotic elements interact across spatial and temporal scales [1]. They are fundamental in the global hydrological cycle and are crucial in providing essential goods and services to human society and ecosystems [2]. However, many rivers worldwide are heavily impacted by anthropogenic pressures, population growth, and climate change, leading to significant alterations in channel patterns and hydrology [34]. These alterations often result in a reduction in freshwater quality and quantity, further driven by extreme events. In Europe, projections suggest increased seasonality of river discharge and more intense summer droughs and heat waves, affecting the kinetics of solute, pollutant, and oxygen dissolution and precipitation, which in turn may degrade water quality and disrupt plant and fish habitats [5]. Variations in discharge also impact fluvial dynamics and sediment transport, impairing vegetation growth andinfluencing meander development and channel parameters (slope, width, depth, etc. ), thereby modifying the morphology of river systems [6]. Therefore, quantifying the spatio-temporal variation in surface water bodies and monitoring hydomorphological processes are essential to understand river changes induced by climate change and human activity, and to manage the current and future water resources and land use, supporting effective biodiversity conservation and climate adaptation strategies [7; 8]. Traditionally, river monitoring techniques have involved in situ observations and measurements, which are labor-intensive, costly, and often lack a uniform spatial distribution of measurement stations (e.g., gauges). Effective integrated river management requires methodologies that exploit near-real-time data over wide geographic areas for both long and short-term intervention planning, especially with the increasing frequency and intensity of extreme events (like floods and droughts) caused by climate change [9]. Consequently, there has been a shift to explore new and cost-effective approaches for targeted monitoring over longer periods, with more detailed explicit spatial information over large geographic areas. Satellite remote sensing has emerged as a promising solution for detecting surface waterbodies and delineating their changes [2; 10], which is the most widely used technique to date for the morphological assessment of rivers [1]. Recent advances in approaches to remote sensing data analysis address the need for a global understanding of the physical river hydomorphological processes [11]. Satellite imagery has been used to monitor drought impacts on surface water resources in specific regions, enabling improved water resource management, drought detection, and preparedness efforts in resource-limited environments [12]. The implementation of the EU Water Framework Directive, which mandates the classification of water bodies' hydrological and morphological conditions, has stimulated the development of new cross-disciplinary methods [13; 14]. For example, satellite remote sensing data are integrated with numerical models to fill gaps when data series are incomplete, generating indices and maps that facilitate the characterization of rivers in response to natural or anthropogenic pressures and climate change [14]. Recent years have seen an increase in inland water Earth observation products, which provide valuable information at the global level, including remote and hazardous areas [15]. Indeed, numerous national and international initiatives, such as the International Precipitation Working Group (IPWG), NASA Energy and Water Cycle Study (NEWS), European Union Water and Global Change (WATCH), the Global Energy and Water Exchanges (GEWEX 2018) initiative, the EU Copernicus Programme, the Copernicus Italian IRIDE program (part of the Copernicus Mirror Programme), have been developed over the years to improve the hydomorphological monitoring [16; 17].