Greater Water Surface Variability Revealed by New Congo River Field Data: Implications for Satellite Altimetry Measurements of Large Rivers

Abstract

Large river hydrodynamics studies inform global and regional issues pertaining to biogeochemical cycling, ecology, water availability, and flood risk. Such studies rely increasingly on satellite measurements, but these are limited by resolution, coverage, and uncertainty and their inability to directly measure bathymetry or discharge. We obtain new in situ data covering 650 km of the Congo's main stem, including elusive bathymetry and discharge measurements that complement space‐borne data sets. Our key findings relate to our water surface elevation measurements, which show that spatial coverage of existing satellite altimetry for deriving river water surface profiles may be adequate through the globally important Cuvette Centrale but is not at its outlet where our field data reveal significant spatial variability in water surface slope. The findings have implications for altimetry‐based hydrodynamics studies of other large rivers, such as those that involve estimating discharge or modeling multichannel river hydraulics.

Sediment Balance Estimation of the ‘Cuvette Centrale’ of the Congo River Basin Using the SWAT Hydrological Model

“…Therefore, for the period 2000 to 2012, we estimated that the Cuvette Centrale retained over 23 megatons of material or 75% of the sediment fluxes from 70% of the basin area above Brazzaville/Kinshasa (combined contributing areas of stations 1 to 8). The low velocities observed in this area of the wetlands [83,91] 2021) analysed the water balance of the Cuvette Centrale and verified its role as a sponge or buffer that accumulates water in the rainy season and releases it in the dry season. A running hypothesis suggests that this may be the main means by which supplies-if any-of suspended sediment is made to the main river [33] (Alsdorf et al, 2016).…”

“…Water surface slopes are higher here with a reduced velocity. This river stretch is multi-channel with constrictions that reduce the widths considerably [83] (Carr et al, 2019). The Kasai River then joins the main stem at Kwamouth.…”

“…It is important to note also the Cuvette central area, although located on low slopes with low erodible soils, yield relatively high sediment within its tributaries because of their high content in organic matter, which are mostly made up of fine particles that are transported easily even at low velocities. The coarser particles, most likely from stream bank erosion, will travel only shorter distances and be deposited, helping to create anastomosing structures observed along the middle reach [83,87,88]…”

“…(Laraque et al, 1998a;Carr et al, 2019) would ensure deposition takes place. This would imply a minimum exchange of material between the wetland and the main river.…”

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

“…Therefore, the large size of the basin, its spatial heterogeneity and the lack of in situ observations have made difficult the validation of long-term satellite-derived observations of surface hydrology components and 120 the proper set-up of large-scale hydrological models (Munzimi et al, 2019). Recent results call for the need of a comprehensive spatial coverage of CRB water surface elevation using satellite altimetry-derived observation to encompass the full range of variability across its rivers and wetlands up to its outlet (Carr et al, 2019). Additionally, even if recent efforts have been characterizing how water flows across the CRB, the basin-scale dynamics is still understudied, especially regarding the contributions of the different sub -basins to the entire basin hydrology (Alsdorf 125 et al, 2016;Laraque et al, 2020) and to the annual bimodal pattern in the CRB river discharge near to its mouth.…”