The latest work on the main African rivers on the Atlantic coast has made it possible to subdivide the multi-year streamflow records into several homogeneous phases. The year 1970 seems to mark both for West and Central Africa the major hydroclimatic event of the 20th century, heralding its main period of deficit flow. For the first time, this article presents a comparative study of the hydro-rainfall records of five drainage systems (those of the Congo River and its main tributaries Lualaba, Kasai, Sangha, Oubangui) based on field data, obtained on both the left and right banks of the Congo River. A reconstitution of the Cuvette Centrale regime is proposed. The 1970 hydro-rainfall disruption is common in most tributaries of the Congo River basin, with significant reductions in flows depending on various factors (geographical location, vegetation cover, surface conditions and land use, etc.). The Oubangui is the most fragile northern tributary that continues to suffer from flow deficits, with an increase in the duration and intensity of its low flows. Since 1995, flows of the Congo River at its main station in Brazzaville/Kinshasa seem to have returned to the interannual average since 1903. However, from the same year onwards, an increase in seasonal variability and a decrease in spring flood flows can also be observed for its bimodal tributaries. This article explains some of the hydrological paradoxes specific to this basin, which illustrate the complexity of its hydrological functioning. Finally, it shows that the period of excess flow in the 1960s is the major hydrological anomaly of the Congo River over a continuous 116-year history. For the whole basin, hydrological variations are attenuated compared to those of precipitation. Finally, the hydrometric regimes reconstructed by spatial altimetry and modelling are compared with those from in situ data.

This study intends to integrate heterogeneous remote sensing observations and hydrological modelling into a simple framework to monitor hydrological variables in the poorly gauged Congo River basin (CRB). It focuses on the possibility to retrieve effective channel depths and discharges all over the basin in near real time (NRT). First, this paper discusses the complexity of calibrating and validating a hydrologic–hydrodynamic model (namely the MGB model) in the CRB. Next, it provides a twofold methodology for inferring discharge at newly monitored virtual stations (VSs, crossings of a satellite ground track with a water body). It makes use of remotely sensed datasets together with in-situ data to constrain, calibrate and validate the model, and also to build a dataset of stage/discharge rating curves (RCs) at 709 VSs distributed all over the basin. The model was well calibrated at the four gages with recent data (Nash-Sutcliffe Efficiency, NSE> 0.77). The satisfactory quality of RCs basin-wide (mean NSE between simulated discharge and rated discharge at VSs, NSEmean = 0.67) is an indicator of the overall consistency of discharge simulations even in ungauged upstream sub-basins. This RC dataset provides an unprecedented possibility of NRT monitoring of CRB hydrological state from the current operational satellite altimetry constellation. The discharges estimated at newly monitored locations proved to be consistent with observations. They can be used to increase the temporal sampling of water surface elevation (WSE) monitoring from space with no need for new model runs. The RC located under the fast sampling orbit of the SWOT satellite, to be flown in 2022, will be used to infer daily discharge in major contributors and in the Cuvette Centrale, as soon as data is released.

Freshwater biodiversity is declining dramatically, and the current biodiversity crisis requires defining bold goals and mobilizing substantial resources to meet the challenges. While the reasons are varied, both research and conservation of freshwater biodiversity lag far behind efforts in the terrestrial and marine realms. We identify fifteen pressing global needs to support informed global freshwater biodiversity stewardship. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated action towards its sustainable management and conservation.