Contribution of satellite observations to continental hydrology

Depuis plusieurs décennies, un nombre croissant de satellites observe la variabilité spatio-temporelle de plusieurs compartiments des eaux continentales (eaux libres, manteau neigeux, humidité du sol...). Ces observations ont permis de mieux comprendre les échanges d'eau au sein des bassins fluviaux et le couplage avec l'atmosphère et les océans. Elles ont aussi contribué à améliorer les prévisions d'inondation, la prévention de risques naturels et le suivi de l'évolution géomorphologique des zones en eau. Les futures missions spatiales, en améliorant la précision et la récurrence des mesures, deviendront un outil incontournable pour gérer les ressources en eau.Over the last decades, an increasing number of satellite missions has helped to observe the spatial and temporal variabilities of continental waters (surface water, surface snow, soil moisture, etc.). These observations helped to better understand water fluxes within river basins and their coupling with the atmosphere and oceans. They also contributed to improve flood forecasting, prevent natural hazards and track geomorphological changes in water bodies. Future satellite missions, with improved measurement accuracy and temporal sampling, will become essential tools for water resources management

Variationnal data assimilation of AirSWOT and SWOT data into the 2D shallow water model Dassflow, method and test case on the Garonne river (France)

For river hydraulic studies, water level measurements are fundamental information, yet they are currently mostly provided by gauging stations mostly located on the main river channel. That is why they are sparsely distributed in space and can have gaps in their time series (because of floods damages on sensors or sensors failures). These issues can be compensated by remote sensing data, which have considerably contributed to improve the observation of physical processes in hydrology and hydraulics in general and, in particular, in flood hydrodynamic. Indeed, the new generation of satellites is equipped with sensors of metric resolution. Remotely-sensed images from satellites such as SWOT (Surface Water and Ocean Topography) would give spatially distributed information on water elevations with a high accuracy (able to observe river wider than100m with a vertical precision ~dm) and periodic in time (revisiting ~week at mid-latitude). Gathering pre-mission data over specific and varied science targets is the purpose of the AirSWOT airborne campaign in order to implement and test SWOT products retrieval algorithms. A reach of the Garonne River, downstream of Toulouse (FRANCE), is a proposed study area for AirSWOT flights. This choice is motivated by previous hydraulic and thermal studies (Larnier et al., 2010) already performed on this section of 100km reach of the river. Moreover, on this highly instrumented and studied portion of river many typical free surface flow modelling issue has been encountered, and this river reach represents the limit of SWOT observation capability. The 2DH (vertically integrated) free surface flow model Dassflow (Honnorat et al., 2005; Honnorat et al., 2007; Honnorat et al., 2009; Hostache et al., 2010; Lai and Monnier, 2009) especially designed for variational data assimilation, will be used on this portion of the Garonne River. Mathematical methodologies such as twin experiments (Roux and Dartus, 2005; Roux and Dartus, 2006) will be performed on several modelling hypothesis in order to identify main characteristic of the river. An identification strategy would allow to retrieve spatial roughness along the main channel, variation of the local topographic slope or else temporal evolution of the streamflow

Continental hydrosystem modelling: the concept of nested stream–aquifer interfaces

International audienceCoupled hydrological-hydrogeological models, emphasising the importance of the stream–aquifer interface, are more and more used in hydrological sciences for pluri-disciplinary studies aiming at investigating environmental is-sues. Based on an extensive literature review, stream–aquifer interfaces are described at five different scales: local [10 cm– ∼ 10 m], intermediate [∼ 10 m–∼ 1 km], watershed [10 km 2 – ∼ 1000 km 2 ], regional [10 000 km 2 –∼ 1 M km 2 ] and conti-nental scales [> 10 M km 2 ]. This led us to develop the con-cept of nested stream–aquifer interfaces, which extends the well-known vision of nested groundwater pathways towards the surface, where the mixing of low frequency processes and high frequency processes coupled with the complexity of geomorphological features and heterogeneities creates hy-drological spiralling. This conceptual framework allows the identification of a hierarchical order of the multi-scale con-trol factors of stream–aquifer hydrological exchanges, from the larger scale to the finer scale. The hyporheic corridor, which couples the river to its 3-D hyporheic zone, is then identified as the key component for scaling hydrological pro-cesses occurring at the interface. The identification of the hy-porheic corridor as the support of the hydrological processes scaling is an important step for the development of regional studies, which is one of the main concerns for water practi-tioners and resources managers. In a second part, the modelling of the stream–aquifer in-terface at various scales is investigated with the help of the conductance model. Although the usage of the temperature as a tracer of the flow is a robust method for the assess-ment of stream–aquifer exchanges at the local scale, there is a crucial need to develop innovative methodologies for as-sessing stream–aquifer exchanges at the regional scale. After formulating the conductance model at the regional and inter-mediate scales, we address this challenging issue with the de-velopment of an iterative modelling methodology, which en-sures the consistency of stream–aquifer exchanges between the intermediate and regional scales. Finally, practical recommendations are provided for the study of the interface using the innovative methodology MIM (Measurements–Interpolation–Modelling), which is graphi-cally developed, scaling in space the three pools of methods needed to fully understand stream–aquifer interfaces at vari-ous scales. In the MIM space, stream–aquifer interfaces that can be studied by a given approach are localised. The ef-ficiency of the method is demonstrated with two examples. The first one proposes an upscaling framework, structured around river reaches of ∼ 10–100 m, from the local to the wa-tershed scale. The second example highlights the usefulness of space borne data to improve the assessment of stream– aquifer exchanges at the regional and continental scales. We conclude that further developments in modelling and field measurements have to be undertaken at the regional scale to enable a proper modelling of stream–aquifer exchanges from the local to the continental scale

Evaluation of Regional-Scale River Depth Simulations Using Various Routing Schemes within a Hydrometeorological Modeling Framework for the Preparation of the SWOT Mission

The Surface Water and Ocean Topography (SWOT) mission will provide free water surface elevations, slopes, and river widths for rivers wider than 50 m. Models must be prepared to use this new finescale information by explicitly simulating the link between runoff and the river channel hydraulics. This study assesses one regional hydrometeorological model’s ability to simulate river depths. The Garonne catchment in southwestern France (56 000 km2) has been chosen for the availability of operational gauges in the river network and finescale hydraulic models over two reaches of the river. Several routing schemes, ranging from the simple Muskingum method to time-variable parameter kinematic and diffusive waves schemes, are tested. The results show that the variable flow velocity schemes are advantageous for discharge computations when compared to the original Muskingum routing method. Additionally, comparisons between river depth computations and in situ observations in the downstream Garonne River led to root-mean-square errors of 50–60 cm in the improved Muskingum method and 40–50 cm in the kinematic–diffusive wave method. The results also highlight SWOT’s potential to improve the characterization of hydrological processes for subbasins larger than 10 000 km2, the importance of an accurate digital elevation model, and the need for spatially varying hydraulic parameters

Evaluation of regional-scale water level simulations using various river routing schemes within a hydrometeorological modelling framework for the preparation of the SWOT mission

The ability of a regional hydrometeorological model to simulate water depth is assessed in order to prepare for the SWOT (Surface Water and Ocean Topography) mission that will observe free surface water elevations for rivers having a width larger than 50/100 m. The Garonne river (56 000 km2, in south-western France) has been selected owing to the availability of operational gauges, and the fact that different modeling platforms, the hydrometeorological model SAFRAN-ISBA-MODCOU and several fine scale hydraulic models, have been extensively evaluated over two reaches of the river. Several routing schemes, ranging from the simple Muskingum method to time-variable parameter kinematic and diffusive waves schemes with time varying parameters, are tested using predetermined hydraulic parameters. The results show that the variable flow velocity scheme is advantageous for discharge computations when compared to the original Muskingum routing method. Additionally, comparisons between water level computations and in situ observations led to root mean square errors of 50-60 cm for the improved Muskingum method and 40-50 cm for the kinematic-diffusive wave method, in the downstream Garonne river. The error is larger than the anticipated SWOT resolution, showing the potential of the mission to improve knowledge of the continental water cycle. Discharge computations are also shown to be comparable to those obtained with high-resolution hydraulic models over two reaches. However, due to the high variability of river parameters (e.g. slope and river width), a robust averaging method is needed to compare the hydraulic model outputs and the regional model. Sensitivity tests are finally performed in order to have a better understanding of the mechanisms which control the key hydrological processes. The results give valuable information about the linearity, Gaussianity and symetry of the model, in order to prepare the assimilation of river heights in the model

The challenge of unprecedented floods and droughts in risk management

Risk management has reduced vulnerability to floods and droughts globally1,2, yet their impacts are still increasing3. An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data4,5. On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change3

Panta Rhei benchmark dataset: socio-hydrological data of paired events of floods and droughts

As the adverse impacts of hydrological extremes increase in many regions of the world, a better understanding of the drivers of changes in risk and impacts is essential for effective flood and drought risk management and climate adaptation. However, there is currently a lack of comprehensive, empirical data about the processes, interactions and feedbacks in complex human-water systems leading to flood and drought impacts. Here we present a benchmark dataset containing socio-hydrological data of paired events, i.e., two floods or two droughts that occurred in the same area. The 45 paired events occurred in 42 different study areas and cover a wide range of socio-economic and hydro-climatic conditions. The dataset is unique in covering both floods and droughts, in the number of cases assessed, and in the quantity of socio-hydrological data. The benchmark dataset comprises: 1) detailed review style reports about the events and key processes between the two events of a pair; 2) the key data table containing variables that assess the indicators which characterise management shortcomings, hazard, exposure, vulnerability and impacts of all events; 3) a table of the indicators-of-change that indicate the differences between the first and second event of a pair. The advantages of the dataset are that it enables comparative analyses across all the paired events based on the indicators-of-change and allows for detailed context- and location-specific assessments based on the extensive data and reports of the individual study areas. The dataset can be used by the scientific community for exploratory data analyses e.g. focused on causal links between risk management, changes in hazard, exposure and vulnerability and flood or drought impacts. The data can also be used for the development, calibration and validation of socio-hydrological models. The dataset is available to the public through the GFZ Data Services (Kreibich et al. 2023, link for review: https://dataservices.gfz-potsdam.de/panmetaworks/review/923c14519deb04f83815ce108b48dd2581d57b90ce069bec9c948361028b8c85/).</p

Variabilité du cycle hydrologique des grands bassins fluviaux

Les eaux à surface libre sur les continents interagissent avec l’atmosphère, les eaux stockées dans le sol et les océans. Leurs variabilités spatio-temporelles couvrent donc des échelles très diverses et jouent un rôle complexe au sein du cycle de l’eau et de l’énergie. Elles sont aussi une importante source d’eau douce pour les sociétés humaines. Cependant, aux échelles régionales/globale, de grandes incertitudes existent encore sur leurs dynamiques du fait de l’hétérogénéité en temps et en espace des réseaux de mesures in situ. C’est pourquoi, pour compléter ces réseaux, l’utilisation de la télédétection s’est fortement développée depuis les dernières décennies et est au cœur de mon projet de recherche. Celui-ci aborde les questions scientifiques suivantes :- Comment améliorer l’observation de la partie continentale du cycle de l’eau ?- Quelles en sont les variabilités saisonnières à décennales ?- Cette variabilité subit-elle des changements notables depuis les dernières décennies ?- Quels mécanismes physiques essentiels sont à considérer pour modéliser les grands bassins versants et les interactions avec l’atmosphère et les océans ?- Quelle est la part du forçage climatique et la part du forçage anthropique « direct » ?Dans ce manuscrit d'HDR, je présente mes activités de recherche menées depuis dix ans au LEGOS en lien avec ces questions, en me focalisant essentiellement sur trois grands bassins tropicaux : l’Amazone, le Gange/Brahmapoutre et le Mékong.Dans un premier temps, je décris les données de télédétection utilisées et les traitements développés au cours de mes recherches, en collaboration avec de nombreux collègues. En particulier, je me suis beaucoup investi sur le traitement de mesures d'altimètres nadirs, pour augmenter le nombre d’observations sur les eaux à surface libre. Comme les données satellitaires actuelles ont certaines limitations, je me suis aussi investi sur de futures missions spatiales (notamment la mission « Surface Water and Ocean Topography », SWOT, qui devrait être lancée fin 2021/début 2022 et sur l’apport de constellations d’altimètres) pour observer encore plus finement les eaux continentales.Dans un deuxième temps, je montre l’utilisation de ces données d’altimétrie pour améliorer des estimations et des prévisions du débit fluvial. En particulier, nous avons pu réaliser des réanalyses journalières du débit par assimilation dans un modèle hydrologique (ISBA-CTRIP) sur l’ensemble du bassin Amazonien (travail de thèse de la première étudiante que j’ai eu à encadrer, Charlotte EMERY). D’autre part, il a aussi été possible d’utiliser l’altimétrie pour aider la prévision des débits du Gange et du Brahmapoutre au Bangladesh.Enfin, j'ai participé à la combinaison d’observations de plusieurs missions spatiales pour estimer la variabilité des stocks d’eau de surface sur le bas Mékong (en particulier sur le lac Tonle Sap). Nous avons exploré les liens entre les variations de ces stocks d'eau et le forçage climatique, ainsi qu'avec le débit du Mékong proche de son exutoire.En guise d’ouverture, je conclus ce manuscrit par mes principales perspectives de recherche pour les années à venir