Stéphane Calmant

• PhD geophysics
• Senior Researcher at Institute of Research for Development

In late 2023, the Amazon River Basin experienced its most extreme drought, putting its population and ecosystem at major risks. Gauges that were still functioning measured the lowest river water levels (RWL) on record. Here, satellite observations, including Surface Water Ocean Topography (SWOT), reveal the spread and timing of extremely low RWL ac...

Satellite altimetry data are useful for monitoring water surface dynamics, evaluating and calibrating hydrodynamic models, and enhancing river-related variables through optimization or assimilation approaches. However, comparing simulated water surface elevations (WSEs) using satellite altimetry data is challenging due to the difficulty of correctl...

Satellite altimetry has revolutionized river monitoring, particularly for hydrologists working on river flow monitoring in sparsely or ungauged areas. Despite this, there's a lack of a comprehensive evaluation of radar and lidar altimeters with varying sensor specifications for river water level retrieval, seasonal change characterization, and wate...

Study region: The Congo River basin (CRB), the world's second-largest river system, is subject to extreme hydrological events that strongly impact its ecosystems and population. Study focus: Here we present an improved 40-year (1981-2020) hydrological reanalysis of daily CRB discharge and analyze the spatiotemporal dynamics of recent major CRB floo...

In recent decades, water availability, water use, water sharing and freshwater supply for basic human and economic needs have become central scientific and humanitarian issues. With increasing water scarcity in many regions and increasing frequency of extreme flooding in other regions, there is a need to improve predictive capacity, to collect a la...

Inicialmente, na década de 70, os satélites altimétricos trabalhavam apenas com a determinação da topografia e da superfície dos oceanos. No entanto, devido à sua cobertura global, foram iniciadas medições de águas continentais superficiais e, atualmente, as medições altimétricas também são obtidas para os rios da bacia amazônica, cujas informações...

The spatio-temporal variation of surface water storage (SWS) in the Congo River basin (CRB), the second-largest watershed in the world, remains widely unknown. In this study, satellite-derived observations are combined to estimate SWS dynamics at the CRB and sub-basin scales over 1992–2015. Two methods are employed. The first one combines surface w...

The Congo Basin is of global significance for biodiversity and the water and carbon cycles. However, its freshwater availability and distribution remain relatively unknown. Using satellite data, here we show that currently the Congo Basin's Total Drainable Water Storage lies within a range of 476 km3 to 502 km3 , unevenly distributed throughout the...

The spatio-temporal variation of surface water storage (SWS) in the Congo River basin (CRB), the second largest watershed in the world, remains widely unknown. In this study, satellite-derived observations are combined to estimate SWS dynamics at the CRB and sub-basin scales over 1992–2015. Two methods are employed. The first one combines surface w...

The African continent hosts some of the largest freshwater systems worldwide, characterized by a large distribution and variability of surface waters that play a key role in the water, energy and carbon cycles and are of major importance to the global climate and water resources. Freshwater availability in Africa has now become of major concern und...

The Congo River basin (CRB) is the second largest river system in the world, but its hydroclimatic characteristics remain relatively poorly known. Here, we jointly analyse a large record of in situ and satellite-derived observations, including a long-term time series of surface water height (SWH) from radar altimetry (a total of 2311 virtual statio...

The Congo Basin is of global significance for biodiversity and the water and carbon cycles. However, its freshwater availability remains highly unknown. Here, we leverage a new method to characterize the relationship between drainable water storage and river discharge across the entire basin. We estimate that the Congo Basin currently holds 481 ± 2...

This study intends to integrate heterogeneous remote sensing observations and hydrological modeling into a simple framework to monitor hydrological variables in the poorly gauged Congo River Basin (CRB). It focuses on the possibility of retrieving effective channel depths and discharges all over the basin in near real time (NRT). First, this paper...

Cette étude a pour but d'intégrer des observations hétérogènes de télédétection et la modélisation hydrologique dans un cadre simple pour le suivi des variables hydrologiques dans le bassin du fleuve Congo (CRB), qui est peu jaugé. Elle se concentre sur la possibilité d'estimer les profondeurs et les débits effectifs des rivières sur l'ensemble du...

Deep convolutional neural networks have proven their efficiency for image processing and are routinely used for image classification. In this article, we use them to convert radar measurements into water distance and ultimately into water levels of inland waterbodies. The measurements used are the successive echoes of the spaceborne radar altimeter...

As the largest river basin on Earth, the Amazon is of major importance to the world's climate and water resources. Over the past decades, advances in satellite‐based remote sensing (RS) have brought our understanding of its terrestrial water cycle and the associated hydrological processes to a new era. Here, we review major studies and the various...

Satellite radar altimetry has immense potential for monitoring fresh surface water resources and predicting the intra-seasonal, seasonal, and inter-annual variability of inundated surface water over large river basins. As part of the preparation for the Surface Water and Ocean Topography mission scheduled for launch in mid-2022, the present study a...

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and mo...

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and mo...

The Congo River Basin (CRB) is the second largest river system in the world, but its hydroclimatic characteristics remain relatively poorly known. Here, we jointly analyze a large record of in situ and satellite-derived observations, including long term time series of Surface Water Height (SWH) from radar altimetry (a total of 2,311 virtual station...

As the largest river basin on Earth, the Amazon is of major importance to the world's climate and water resources. Over the past decades, advances in satellite-based remote sensing (RS) have brought our understanding of its terrestrial water cycle and the associated hydrological processes to a new era. Here, we review major studies and the various...

Lake water height is a key variable in water cycle and climate change studies, which is achievable using satellite altimetry constellation. A method based on data processing of altimetry from several satellites has been developed to interpolate mean lake surface (MLS) over a set of 22 big lakes distributed on the Earth. It has been applied on nadir...

This work presents a practical approach to reconstructing past and present discharge and water depth time series for operational monitoring of small-sized ungauged watersheds using remotely sensed and freely accessible datasets in conjunction with hydrological models. The methodology was applied to the Tsiribihina watershed in Madagascar. Mostly, s...

River flows are an essential component of the water cycle and are directly accessible for human consumption and activities. River water flux (i.e., river discharge) not only can be measured locally at in situ gauges but also can be estimated at larger scales with the river routing models. However, the number of in situ gauges is declining world...

With the upcoming SWOT satellite mission, which should provide spatially dense river surface elevation, width and slope observations globally, comes the opportunity to assimilate such data into hydrodynamic models, from the reach scale to the hydrographic network scale.Based on the HiVDI (Hierarchical Variational Discharge Inversion) modeling strat...

Most of the part of India is already under water-stressed condition. In this regard, the continuous monitoring of the water levels (WL) and storage capacity of reservoirs, lakes, and rivers is very important for the estimation and utilization of water resources effectively. The long term ground observed WL of many of the water bodies is not easily...

Flooding dynamics across a medium‐size (Janauacá Lake, 786 km2) floodplain system along the Amazon/Solimões River over a 9‐years period (2006–2015) is studied through integration of remote sensing and limited in situ data in hydrologic‐hydrodynamic modelling based on Telemac‐2D model. We firstly detail the methodological approach and the modelling...

The capabilities of radar altimetry to measure inland water bodies are well established, and several river altimetry datasets are available. Here we produced a globally distributed dataset, the Global River Radar Altimeter Time Series (GRRATS), using Envisat and Ocean Surface Topography Mission (OSTM)/Jason-2 radar altimeter data spanning the time...

Significance This work provides a robust estimate of water-level (WL) changes in the Ganges–Brahmaputra–Meghna delta, driven by continental freshwater dynamics, vertical land motion, and sea-level rise. Through an unprecedented set of 101 gauges, we reconstruct WL variations since the 1970s and show that the WL across the delta increased slightly f...

Dans le cadre du projet 'Niger, fleuve pilote en hydrologie spatiale' un collectif de scientifiques et d'experts des techniques spatiales pour l'hydrologie (IRD, CNES) a développé, en collaboration avec l'Autorité du Bassin du Niger (ABN), une plateforme de modélisation hydrologique intégrant des mesures spatiales. Cette étude est basée sur un modè...

The intertidal zones are well recognized for their dynamic nature and role in near-shore hydrodynamics. The intertidal topography is poorly mapped worldwide due to the high cost of associated field campaigns. Here we present a combination of remote-sensing and hydrodynamic modeling to overcome the lack of in situ measurements. We derive a digital e...

Multi-satellite sensing of continental water surfaces (WS) represents an unprecedented and increasing potential for studying ungauged hydrological and hydraulic processes from their signatures, especially on complex flow zones such as anabranching rivers. However the estimation of discharge from WS observations only is a very challenging, ill-posed...

The subsidence of the Ganges‐Brahmaputra‐Meghna Delta (GBMD) drastically increases the adverse impacts of coastal flooding and exacerbates the vulnerability of populations from ongoing rapid sea level rise. We focus here on estimating the present‐day subsidence rates induced by the loading of sediments continuously deposited within the GBMD over th...

The capabilities of radar altimetry to measure inland water bodies are well established and several river altimetry datasets are available. Here we produced a globally-distributed dataset, the Global River Radar Altimeter Time Series (GRRATS), using Envisat and Ocean Surface Topography Mission (OSTM)/Jason-2 radar altimeter data spanning the time p...

River flow models that consider satellite observations of the water surface elevation (WSE) require meaningful segmentation into reaches. Segmentation methods involve a trade-off between the spatial sampling and error characteristics of satellite observations, both of which impact the flow model realism/accuracy. This paper investigates the spatial...

Comment, depuis plusieurs décennies, la recherche scientifique contribue-t-elle au développement des pays du Sud ? À travers plus de 100 succès emblématiques de la recherche en partenariat, cet ouvrage nous plonge au coeur des grandes questions de développement : oeuvrer pour des sociétés plus justes, lutter contre les maladies, faire face aux risq...

Bathymetric data relative to mean sea level for the northern Bay of Bengal. Details can be found in Krien et al. 2016. Data in rivers as well as DEM are copyrighted and cannot be distributed for the time being. They are thus not included here.

Calibration/Validation (C/V) studies using sites in the oceans have a long history and protocols are well established. Over lakes, C/V allows addressing problems such as the performance of the various retracking algorithms and evaluating the accuracy of the geophysical corrections for continental waters. This is achievable when measurements of spec...

Providing reliable estimates of streamflow and hydrological fluxes is a major challenge for water resources management over national and transnational basins in South America. Global hydrological models and land surface models are a possible solution to simulate the terrestrial water cycle at the continental scale, but issues about parameterization...

Across the oceans shorelines, monitoring the topography of the intertidal zone is generally challenging. The present study is motivated by the recognized role of the intertidal topography in the near-shore hydrodynamics. We consider the region of Chittagong (northern Bay of Bengal) because of its propensity to powerful cyclone surges and associated...

The influence of SST (Sea Surface Temperature) of adjacent oceans on the variability of water levels in the Amazon basin was investigated by using radar altimetry from the ENVISAT and Jason-2 missions. Data from the in situ network was used to compare the correlations of water level and SST anomalies in the sub-basins of the Amazonas-Peru, Solimões...

Providing reliable estimates of streamflow and hydrological fluxes is a major challenge for water resources management over national and transnational basins in South America. Global hydrological models and land surface models are a possible solution to simulate the terrestrial water cycle at the continental scale, but issues on parameterization an...

Land surface models (LSMs) are widely used to study the continental part of the water cycle. However, even though their accuracy is increasing, inherent model uncertainties can not be avoided. In the meantime, remotely sensed observations of the continental water cycle variables such as soil moisture, lakes and river elevations are more frequent an...

Abstract In the Congo River Basin (CRB), due to the lack of contemporary in situ observations, there is a limited understanding of the large-scale variability of its present-day hydrologic components and their link with climate. In this context, remote sensing observations provide a unique opportunity to better characterize those dynamics. Analyzin...

Hydrological and hydrodynamic models are core tools for simulation of large basins and complex river systems associated to wetlands. Recent studies have pointed towards the importance of online coupling strategies, representing feedbacks between floodplain inundation and vertical hydrology. Especially across semi-arid regions, soil-floodplain inter...

The contribution of subsidence to relative sea-level rise in the Ganges-Brahmaputra delta (GBD) is largely unknown and may considerably enhance exposure of the Bengal basin populations to sea level rise and storm surges. This paper focuses on estimating the present-day subsidence induced by Holocene sediment in the Bengal basin and by oceanic loadi...

The India–France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated primarily to oceanography. The mission objectives were firstly the observation of the oceanic mesoscales but also global and regional sea level monitoring, including the coastal zone, data assimilation, and operational oceanography. SARAL/AltiKa proved also to b...

In the world richest biotope, wetlands, and specially floodplains, remain particularly active zones in terms of nutrients, biodiversity, flows spreading, sediment transfer and human activities. Today, these sensitive zones are facing increasing human pressures and augmentation in frequency and intensity of extreme events. The impact of such extreme...

In the world’s richest biotope, wetlands, and especially floodplains, remain particularly active zones in terms of nutrients, biodiversity, flows spreading, sediment transfer, and human activities. Today, these sensitive zones are facing increasing human pressures and augmentation in frequency and intensity of extreme hydrological events. The impac...

Real-time monitoring of discharges in the Congo River basin is not straightforward given the strong geomorphological diversity, the relatively low level of knowledge of hydrological processes and the size of the basin, spreading on several countries. In this study, we built stage/discharge rating curves based on: 1) the discharges given by the dist...

The Poyang Lake has an important influence on the Yangsé River basin hydrology. However, estimating the discharge input in the river is not straightforward. Indeed, as the cycles of both water bodies are not synchronous, the stage/discharge relationship is ambiguous. At a given period of the hydrological year, the conditions may even inverse and pa...

Land Surface Models (LSM) are widely used to study the continental part of the water cycle. Yet, even though their accuracy is increasing, inherent model uncertainties can not be avoided. In the meantime, remotely-sensed observations of the continental water cycle variables such as soil moisture, lakes and rivers elevations are more frequent and ac...

Sentinel-3A, launched in February 2016, is part of ESA's long-term commitment to climate monitoring from space. Its suite of instruments for measuring surface topography includes a Microwave Radiometer (MWR) and SRAL, the first delay-Doppler instrument to provide global coverage. SRAL promises fine spatial resolution and reduced noise levels that s...

Poster briefly summarising the activities to assess the performance of SRAL (the Sentinel-3A altimeter)

Inland surface waters in tropical environments play a major role in the water and carbon cycle. Remote sensing techniques based on passive, active microwave or optical wavelengths are commonly used to provide quantitative estimates of surface water extent from regional to global scales. However, some of these estimates are unable to detect water un...

The Upper Niger Basin is located in Western Africa, flowing from Guinea Highlands towards the Sahel region. In this area lies the seasonally inundated Niger Inland Delta, which supports important environmental services such as habitats for wildlife, climate and flood regulation, as well as large fishery and agricultural areas. In this study, we pre...

In 2015, an emergency state was declared in Bolivia when Poopó Lake dried up. Climate variability and the increasing need for water are potential factors responsible for this situation. Because field data are missing over the region, no statements are possible about the influence of mentioned factors. This study is a preliminary step toward the und...

Altimetry was originally designed for oceanography in the 1970s. It was used to study favorable continental surfaces, especially in hydrology and glaciology, two themes for which monitoring the height of water or ice surfaces is crucial. The principle of the device has not changed but the heterogeneity of the target or the presence of slopes has co...

Fluctuations in water surface elevation (WSE) along rivers have important implications for water resources, flood hazards, and biogeochemical cycling. However, current in situ and remote sensing methods exhibit key limitations in characterizing spatiotemporal hydraulics of many of the world's river systems. Here, we analyze new measurements of rive...

In the low Amazonian basin, interactions between floodplains and river channels are important in terms of water exchanges, sediments, or nutrients. These floodplains and wetlands are considered as hotspot of biodiversity.They are among the most productive in the world. However, they are threatened by climatic changes and anthropic activities. Hence...

Satellite altimetry has demonstrated a strong potential for the continuous monitoring of ocean, ice sheets and land surfaces over the last 25 years. While instrument processing and retracking techniques of the radar echoes improved over the years for ocean applications to produce homogeneous products for all satellite altimetry missions, less effor...

Continental waters have a major impact on life on Earth as well as domestic, industrial and agricultural needs, but they also play an important role in climate variability. Nearly 73% of the worldwide demand for water is met by surface reservoirs (rivers, lakes and artificial reservoirs) and 19% is met by underground reservoirs. The rest is provide...

In large hydrological basin, water mass loading can produce significant crustal deformation. We compare the monthly vertical component of 18 GPS sites located in the Amazon basin, with the deflection models derived from GRACE observations in the one hand, and derived from HYDL, a global hydrological model, in the other hand. The GPS dataset include...

What hydraulic information can be gained from remotely sensed observations of a river's surface? In this study, we analyze the relationship between river bed undulations and water surfaces for an ungauged reach of the Xingu River, a first-order tributary of the Amazon river.This braided reach is crosscut more than 10 times by a single ENVISAT track...

The Bengal Delta is a highly complex and vulnerable environment where key dynamical features such as tides, storm surges, salinity or sediment transport strongly depend on accurate bottom topography representation. To curb the lack of accuracy of widely used global bathymetry databases in this shallow region, we produced an improved coastal bathyme...

In this project we aim at studying the synergistic potential of the use of SWOT futur data over wetlands with L-Band data from radiometers namely SMOS (ESA/CNES) and SMAP (Soil Moisture Active Passive – NASA).

In this study, rating curves (RCs) were determined by applying satellite altimetry to a poorly gauged basin. This study demonstrates the synergistic application of remote sensing and watershed modeling to capture the dynamics and quantity of flow in the Amazon River Basin, respectively. Three major advancements for estimating basin-scale patterns i...

The transfer of energy and water, as well as gaseous components, in the soil–plant–atmosphere system plays a crucial role in many processes involved in climate change. The surface energy balance drives both evapotranspiration (closely linked to the CO2 flux) and sensible heat flux. By providing direct information on heat dissipation efficiency, sur...

New Caledonia is located in the South West Pacific Ocean, on the Australian Plate just before its subduction under the North Fiji Basin. Because it is on the subducting side of the plate interface, New Caledonia is considered to be stable to first order and not to undergo rapid deformation. However, moderate seismicity is recorded close to the plat...

In the Amazon basin, the recently released SRTM Global 1 arc-second (SRTMGL1) remains the best topographic information for hydrological and hydrodynamic modeling purposes. However, its accuracy is hindered by errors, partly due to vegetation, leading to erroneous simulations. Previous efforts to remove the vegetation signal either did not account f...

Resumo – Devido à carência de informações fluviométricas e pluviométricas na bacia do rio Japurá, as estimativas de precipitação se tornaram uma ferramenta extremamente útil para fornecer a distribuição espacial das chuvas. Para o presente estudo foi avaliado a aptidão do Modelo de Grandes Bacias (MGB-IPH), utilizando três métodos de estimativas de...

Resumo. Modelos Digitais de Elevação (MDEs) são atualmente utilizados em modelagem hidrológica e hidrodinâmica. Na bacia Amazônica, o SRTM-GL1 ainda é o único e a melhor informação topográfica disponível em escala adequada para estudos de várzea. Entretanto, sua precisão é dificultada por vários erros em parte devido ao dossel florestal. No caso de...

Digital Elevation Models (DEMs) are currently used for hydrological and hydrodynamic modeling purposes. In the Amazon basin, the SRTM-GL1 is still the only and best available topographic information at a scale suitable for floodplain studies. Its accuracy is however hindered by various errors partly due to vegetation canopy. In the case of floodpla...

Satellite radar altimetry, initially designed for studying ocean surface topography, demonstrated a strong potential for the continuous monitoring of ice sheets and land surfaces over the last 25 years. If radar altimetry is mostly used for its capacity to determine surface height, the backscattering coefficients provide information on the surface...