A combined use of in situ and satellite-derived observations to characterize surface hydrology and its variability in the Congo River Basin

Abstract

Abstract. 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 stations) and surface water extent (SWE) from a multi-satellite technique to better characterize CRB surface hydrology and its variability. Firstly, we show that SWH from radar altimetry multi-missions agree well with in situ water stage at various locations, with root mean square deviation varying from 10 cm (with Sentinel-3A) to 75 cm (with European Remote Sensing-2). SWE from multi-satellite observations also shows a good behavior over a ~25-year period against in situ observations from sub-basin to basin scale. Both datasets help to better characterize the large spatial and temporal variability of hydrological patterns across the basin, with SWH exhibiting annual amplitude of more than 5 m in the northern sub-basins while Congo main-stream and Cuvette Centrale tributaries vary in smaller proportions (1.5 m to 4.5 m). Furthermore, SWH and SWE help better illustrate the spatial distribution and different timings of the CRB annual flood dynamic and how each sub-basin and tributary contribute to the hydrological regime at the outlet of the basin (the Brazzaville/Kinshasa station), including its peculiar bi-modal pattern. Across the basin, we jointly use SWH and SWE to estimate time lag and water travel time to reach the Brazzaville/Kinshasa station, ranging from 0–1 month in its vicinity downstream the basin up to 3 months in remote areas and small tributaries. Northern sub-basins and the central Congo region highly contribute to the large peak in December–January while the southern part of the basin supplies water to both hydrological peaks, in particular to the moderate one in April–May. The results are supported using in situ observations at various locations in the basin. Our results contribute to a better characterization of the hydrological variability in the CRB and represent an unprecedented source of information for hydrological modeling and to study hydrological processes over the region.

Hydrometeorological Extreme Events in Africa: The Role of Satellite Observations for Monitoring Pluvial and Fluvial Flood Risk

“…The dense spatial information provided by nadir altimetry is very valuable for monitoring or studying ungauged or partially ungauged basins (Bogning et al 2018, Andriambelosom et al 2020Kitambo et al 2021) and also for assessing the quality of hydrological and hydrodynamic models (Paris et al 2020), or calibrate their parameters (O'Loughlin et al 2019). This type of observation is extremely useful for the management of transboundary basins when data sharing is an issue.…”

Water Resources in Africa: The Role of Earth Observation Data and Hydrodynamic Modeling to Derive River Discharge

“…Despite the huge contribution provided by multi-mission approaches, the combination of different altimetry measurements represents still a challenge because of the inter-satellite biases (Normandin et al 2018) which impede a straightforward combination of water level measurements (Tourian et al 2022). The inter-satellite bias depends not only on the mission but also on the atmospheric corrections (Fernandes et al 2014) or the retracker algorithm (Kitambo et al 2021), and therefore it cannot be considered global, but it needs to be estimated at regional scale. For rivers, a hydraulically and statistically merging (Tourian et al 2016) can be considered a solution to solve the inter-satellite bias and to improve the temporal resolution even if accuracy may be compromised.…”

“…However, the use of radar altimetry represents a very important resource for the evaluation of the dynamic of fresh water in African rivers (Becker et al 2018). For example in the study of Kitambo et al (2021) the intense activities of monitoring carried out by the collaboration with the regional partners of the Congo Basin Water Resources Research Center (CRREBaC) and the Environmental Observation and Research project (ORE HYBAM, https:// hybam. obs-mip.…”

Current availability and distribution of Congo basin's freshwater resources

“…The presence of lakes and wetlands, with massive storage capacities like the Bangwelu swamp, the Upemba depression and Lake Tanganyika, greatly influences the flow regime of the downstream sub-basins (41). In the Lualaba-North, the discharge reaches its maximum in November, while in the Lualaba-South and Lualaba-Lukuga the maximum discharge is reached in April-May (18). This is mainly driven by the difference in precipitation patterns over these three sub-basins (figures S7 and 4).…”

“…Therefore, understanding the major factors controlling the basin's freshwater variability at proper space and time scales remains a significant challenge. The lack of data has became less pronounced in the last two decades due to the availability of spaceborne observations on surface water characteristics through satellite altimetry and imagery (18,4). Additionally, GRACE and GRACE Follow-On satellite gravimetry missions provide unique estimates of continental water storage variability (35,13).…”