This chapter describes the variability of rainfall and river discharges in the Ogooué River basin (ORB) in recent decades (since 1940). Due to its location crossing the Equator, the ORB receives abundant precipitation that maintains one of the world’s best-preserved ecosystems. In contrast to neighboring forest basins that have been severely degraded because of deforestation, mining resources extraction, extensions of agricultural areas, and river transport, which is a crucial alternative to the cruel lack of road infrastructures, the ORB is experimenting with an exceptional conservation policy in the region. For example, the rural penetration rate in Gabon is about 1 inhabitant per km² and many studies report a deforestation rate close to 0%, with even full natural regeneration. However, the fluctuations of the standardized anomaly index of rainfall in the ORB show three main phases of variations: the first wet phase was characterized by abundant precipitations from 1940 to 1970, the second phase of the long-term mild drought was extended in the 1970s and 1980s and the final third phase presented a slight return of abundance in precipitation. Even though drought severity in the ORB was mainly weak, its effects in river discharges were very sensitive on seasonal and inter-annual scales. The pure equatorial regime of the ORB characterized by equal maximum floods in spring and autumn changed significantly from the difference between both maximum discharges of 13.5 % during the 1960s to 27.0 %, 38.4 %, 33.9 %, and 26.7 % for the 1970s, 1980s, 2000s and 2010s respectively. A brief comparison between the ORB and the Congo River basin showed that changes in the ORB are part of a regional process that Central Africa is undergoing with some spatial heterogeneities.

15 The increasing pressure on wetland resources continues to threaten the role wetlands play in 16 maintaining the ecological balance of watersheds. The Cuvette Centrale of the Congo is the 17 greatest intertropical peatland in the world. To fully understand its role in water resources and 18 ecological services linked to the quality of water and life in the basin, we first need to quantify 19 its role in the hydrological dynamics. To achieve this aim, we used the Soil and Water 20 Assessment Tool model (SWAT)-modified for tropical environments-in combination with 21 monthly discharge data. We analyzed water fluxes entering and flowing out of the Cuvette 22 Centrale of the Congo River Basin on a monthly time scale for the 2000-2012 period. The 23 model was calibrated, validated, and compared with discharge from gauging stations and 24 surface water elevation from radar altimetry. Results showed that upland runoff from the 25 Congo River was the highest contributor to the Cuvette Centrale (33 percent) followed closely 26 by efficient precipitation inside the Cuvette Centrale (31 percent) with right bank and left bank 27 tributaries contributing 25 percent and 11 percent respectively. We simulated monthly mean 28 interannual inflows of approximately 34,150 m 3 s-1 (88 billion m 3) with the main flood peaking 29 in November (45,310 m 3 s-1) and total outflows averaging around 39,860 m 3 s-1 (100 billion 30 m 3) peaking at 52,430 m 3 s-1 in December for the simulation period. We subsequently estimated 31 a negative monthly mean interannual variation of storage in the Cuvette Centrale wetlands in 32 the order of 5,700 m 3 s-1 suggesting that the Cuvette Centrale supplies the river during low 33 water periods. This highlights the important regulatory function of the Cuvette Centrale and 34 the need for protection of groundwater resources in order to maintain wetland water quantities 35 and quality. 36

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.