The research, led by Malak Sadki of Magellium in France with colleagues from CLS, Météo-France, the European Space Agency Climate Office, and the University of Toulouse, centers on products from ESA’s Climate Change Initiative River Discharge project. These datasets span the years 2000 to 2020 and come in three flavors. The first is water surface elevation, measured directly by radar altimetry from a fleet of missions including ERS-1 and -2, Envisat, SARAL, the Topex-Poseidon and Jason series, Sentinel-3, Sentinel-6A, and CryoSat-2. The second is discharge derived from those altimetry measurements through rating curves calibrated against historical gauge data. The third, and in some ways the most intriguing, is discharge estimated from multispectral imagery, exploiting the fact that the reflectance of water differs sharply from that of land, so the wetted area near a river channel tracks how much water the river is carrying.

To test these products rigorously, the team assimilated them into two fundamentally different modeling frameworks. The first is CTRIP, the river routing component of the ISBA land surface system used in the CNRM-CM6 climate model that contributed to the IPCC Sixth Assessment Report. CTRIP runs on a global grid at one-twelfth of a degree resolution and, crucially, is uncalibrated: its parameters come from physical reasoning and global datasets rather than fitting to observations, which makes it consistent everywhere but vulnerable to structural errors. The second is MGB, a semi-distributed, calibrated hydrological model designed for regional applications and operational flood forecasting, which explicitly simulates floodplains, wetlands, and the evaporation that occurs when floodwaters spread across them. Each model carries its own ensemble Kalman filter data assimilation system, HyDAS for CTRIP and HYFAA for MGB, giving the researchers two independent laboratories in which to evaluate the same satellite data.

The experimental stage was deliberately chosen for contrast. The Niger Basin, draining roughly 1.5 million square kilometers across nine countries, is dominated by strong monsoon-driven seasonality and the Inner Niger Delta, a vast floodplain in Mali that dissipates enormous volumes of water through absorption and evaporation. The Congo Basin, the second largest on Earth at over 3.7 million square kilometers, behaves almost oppositely: its mean annual flow of around 40,500 cubic meters per second is remarkably stable from year to year, buffered by an immense network of tributaries, lakes, and wetlands. Crucially, the Congo has lost most of its in-situ gauging stations since the 1960s, making it an ideal proving ground for satellite-only approaches, while the Niger retains a comparatively dense gauge network for validation.

The headline result is that assimilating discharge data generally outperformed assimilating water surface elevation anomalies, because discharge is the variable the models actually compute. When observations speak the model’s native language, corrections propagate smoothly and physically consistently through the river network. In the Niger Basin, altimetry-derived discharge assimilation into MGB produced the strongest gains of the entire study, lifting the median Nash-Sutcliffe Efficiency to 0.83 and the correlation coefficient to 0.94. Water surface elevation assimilation, by contrast, sometimes introduced noise, with visible jumps in simulated discharge after each update, a phenomenon the authors call the hashed effect. This occurs when corrections are applied intermittently and the model drifts back toward its biased state between updates, and it worsens when the relationship between water level and discharge, the rating curve, differs between the model and the observations.

That rating curve mismatch proved to be the crux of the elevation-versus-discharge debate. At Kinshasa on the lower Congo, the MGB model produces a nearly flat rating curve, while the altimetry observations show water level swings of up to seven meters for the same range of flows. Converting observed elevation anomalies into discharge corrections under these conditions introduced serious errors, at times driving the filter to diverge and even produce physically impossible negative discharge values. Yet the story is not one-sided. In the uncalibrated CTRIP model, whose open-loop simulation over the Niger overestimates high flows by up to 2.3 times because it lacks evaporation from the Inner Delta, elevation anomaly assimilation actually delivered better statistical scores, largely because its lower uncertainty gave those observations more weight in the Kalman filter when the model’s own rating curves happened to align well with the satellite data.

Perhaps the most striking finding concerns temporal density rather than accuracy. The multispectral discharge product is noisier than its altimetry-based counterpart, but it is available far more frequently, and that frequency transformed its value. In the Niger Basin, assimilating multispectral discharge cut the median bias ratio from 1.2 to nearly 1 in MGB and from 2.3 to 1.78 in CTRIP, and pushed the Kling-Gupta variability component toward 1.0, meaning the models finally captured the short-term ebb and flow of the rivers. The mechanism is intuitive: frequent updates prevent the model from reverting to its biased state, steadily correcting both high-flow and low-flow errors throughout the hydrological year. At Koulikoro on the upper Niger, this dense sampling reduced bias by 17 to 20 percent compared with the open loop. For climate studies concerned with floods, droughts, and the evolution of water resources, capturing that internal variability may matter as much as getting the average right.

The Congo Basin, however, delivered a sobering counterpoint. Improvements there were modest and heavily dependent on product quality. The available CCI stations provide weak spatial coverage of a basin whose outlet discharge integrates contributions from four major tributary systems, and at Bangui the multispectral product failed to reproduce the hydrological signal at all, with low correlation and no meaningful seasonality. Assimilating that noisy, phase-shifted data degraded model performance rather than improving it, a reminder that temporal density is only an asset when the underlying observations are trustworthy. The authors also note that uncertainty handling matters in subtle ways: because discharge uncertainty is proportional to flow magnitude, high-flow outliers are naturally down-weighted by the filter, but during low flows the absolute uncertainty shrinks and the filter may overfit isolated erroneous observations.

The dual-basin, dual-model design allowed the team to separate what is universal from what is context-dependent. Discharge assimilation is more physically consistent than elevation assimilation in models whose purpose is to simulate flow, and high-frequency data reliably reduces bias and improves variability when quality permits. But the magnitude of improvement depends on the interplay between model structure, observation sampling, and basin hydrology. In the calibrated MGB, assimilation refined already decent simulations; in the uncalibrated CTRIP, it reduced bias but could not conjure missing physics such as floodplain evaporation. In the strongly seasonal, well-observed Niger, gains were clear and diagnosable; in the stable, sparsely gauged Congo, they were harder to detect and more fragile. The researchers also caution that Congo validation scores deserve careful interpretation, since the same long-term records served both product generation and model evaluation.

Looking forward, the team outlines a roadmap for the next phase of the CCI Discharge project: merging altimetry and multispectral discharge streams to combine spatial reach with temporal density, exploiting the SWOT mission’s detailed measurements of water surface slope, refining localization schemes that currently rely on 20-year spatial correlation statistics, and exploring joint state-and-parameter estimation using river width and slope observations. They also plan smoother Kalman filter methods to suppress the hashed effects that plague intermittent updates, and dynamic localization distances that could adapt to human interventions like dams and irrigation. For a planet where climate change is reshaping river regimes faster than gauges can be installed, the message of this study is quietly revolutionary: the satellites already overhead, properly assimilated, can give the world’s water models the memory and the fidelity they have long lacked.

Subject of Research: Assimilation of ESA Climate Change Initiative satellite discharge and water surface elevation data into large-scale river routing models over the Niger and Congo basins

Article Title: Improving large-scale river routing models with ESA long-term CCI discharge data assimilation: advancing accuracy for climate studies

Article References: Sadki, M., Noual, G., Munier, S., Pedinotti, V., Verma, K., Albergel, C., Biancamaria, S., & Andral, A. (2026). Improving large-scale river routing models with ESA long-term CCI discharge data assimilation: advancing accuracy for climate studies. Hydrology and Earth System Sciences, 30(19), 6159-6187. https://doi.org/10.5194/hess-30-6159-2026

Image Credits: AI Generated

DOI: 10.5194/hess-30-6159-2026

Keywords: data assimilation, river discharge, ESA Climate Change Initiative, satellite altimetry, hydrological modeling, Niger Basin, Congo Basin, ensemble Kalman filter, CTRIP, MGB model, climate studies, remote sensing