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over the southern basin, and the larger peak occurs in about December, associated with rainfall over the northern basin. Historically, discharge has varied at Kinshasa‐Brazzaville between about 1960 and 1995, with a notable significant increase in the decade from 1960 to 1970, and is only now returning to its long‐term average (Laraque et al., 2001, 2020).

      Remote sensing is important for measuring wetland and lake areas and for measuring water elevations. Bwangoy et al. (2010) used visible band satellite imagery, synthetic aperture radar (SAR) mosaics, and a digital elevation model to find that wetlands in the Congo Basin cover 360,000 km². Lee et al. (2011) used radar altimetry to measure water surface elevations of the Congo River and the immediately adjacent waters in the Cuvette Centrale. They found that the wetland water levels were consistently higher in elevation, suggesting that locally the water exchange was always from the wetland to the river. Jung et al. (2010) used interferometric SAR to measure changes in water levels in the Cuvette Centrale and suggested that flows were diffusive, not channelized at the 100 m resolutions of the measurements.

      Since Richey et al. (2002) first published their findings regarding CO₂ evasion in the Amazon Basin, understanding gaseous carbon evasion from tropical wetlands into the atmosphere has been recognized as a key part of the global carbon cycle. However, while the Amazon Basin has experienced numerous carbon evasion studies, the Congo Basin has relatively fewer. Notably, Borges et al. (2015) measured pCO₂ to estimate a carbon flux of 0.5 PgCO₂ equivalents/yr from the Cuvette Centrale. But this is only a beginning, and more studies are needed to better understand the carbon evasion from the world’s second largest basin.

      In the review of the Congo Basin by Alsdorf et al. (2016), the findings of many previous researchers were summarized to suggest seven testable hypotheses. These hypotheses became the foundation for an AGU Chapman Conference held in Washington D.C. in September 2018 (Beighley et al., 2019) and helped to frame the content of this monograph. These hypotheses are:

      1 The water in the Cuvette Centrale is supplied mostly by rainfall.

      2 The water empties from the Cuvette Centrale mostly by evapotranspiration.

      3 Despite known variations in the discharges of the Congo and Oubangui Rivers, previous rainfall amounts have varied comparatively less across the basin.

      4 Because of its location beneath the “tropical rainbelt,” the Congo Basin will experience significant changes in both rainfall amounts and geographic locations from climate change.

      5 Deforestation of 30% of the headwater sub‐basins will significantly increase headwater flows and hence increase downstream discharge.

      6 Future hydroelectric power generation will not impact waters flowing in rivers.

      7 The annual average amount of CO2 and CH4 evasion from all Congo Basin waters is more than 480 Tg C/yr, i.e., more than a value comparable to that of the Amazon per unit area.

      Note that these are not statements of fact; rather, these are testable ideas that will be proven true or proven false. Indeed, many of the chapters in the monograph address these statements and provide insights regarding their true or false nature. Authors met at the Chapman Conference to discuss the hypotheses and to build new collaborations while strengthening existing research programs. These interactions became the launching point for this monograph. Contributions to the monograph were broadly invited, including authors who participated in the Chapman as well as researchers who were not able to attend. As a guideline, but not a requirement, authors were encouraged to address any of the hypotheses.

1.3. THE HISTORY AND THE FUTURE OF THE CONGO BASIN

      Many contributors to this book have dedicated their lives to studying sub‐Saharan Africa and the Congo Basin in particular. We asked four esteemed colleagues for their thoughts on the historic past and hopeful future in the Congo and its surrounding basins.

      1.3.1. Comments from Alain Laraque

       How has research in the Congo or in sub‐Saharan Africa changed over your career and what do you think will be an exciting new research opportunity?

The golden age of hydrology in the Congo Basin extended through several decades during the 20th century. This “rising stage” of exploratory work, including multiple data collections, the setting up, management, and maintenance of the hydro‐pluviometric networks, were carried out under very difficult conditions. Then came a “falling stage” of field work, as severe as it was worrisome. In more than 30 years that I have been studying this basin, I have witnessed the abandonment of the hydro‐pluviometric networks from the colonial era (from more than 400 hydrological stations in the middle of the 20th century to about a dozen actually operational today!). Yet it is still this in‐situ knowledge base that serves as a reference for current studies, often indirectly via remote sensing.

      This immense basin is blessed by nature, since it abounds in natural resources (wood, minerals, hydroelectricity, etc.) as well as being preserved from natural calamities. In the heart of the African continent, the world’s second‐largest tropical forest, still little impacted by man, is in fact a vast carbon sink and one of the world's last great reserves of biodiversity. Its role is primordial in the balance and transfer of energy and materials, which are the major issues that need to be better understood in order to refine the modelling of the functioning of the Earth’s surface.

      It faces gigantic challenges (commensurate with its flows!), torn between the development aspirations of its demographic bomb and the need to preserve the environment, on which the quality of life of its population depends.

      This basin is also that of the “white elephants” with huge technical and financial projects such as the “Grand” Inga dam, which will have the largest hydroelectric potential on the planet, and the development of river navigation or the Ubangi‐Chari canal to link it to Lake Chad. Interests and social and environmental impacts are still debatable in the absence of scientific studies on their impacts.

      This is what has happened in the Amazon Basin, whose socioenvironmental future is very problematic. Thus, strengthened by the mistakes made by industrialized countries and other emerging countries, regional decision‐makers have a heavy responsibility to find the best formula for sustainable and harmonious development between these antagonistic constraints, relying in particular on the insights of scientific research. Their First Peoples, such as the Indians of the Amazon or the Pygmies of the Congo, have successfully crossed the millenaries without ransacking their habitat! So, it is perhaps by turning respectfully and humbly to the past that the present will know how to best manage its future.

      Finally, the emergence of a regional hydrological awareness seems imperative to ensure the best integrated water resource management of the basin. If not, the Congo will remain for a long time to come a long, quiet river, for better or worse.

      1.3.2. Comments from Sharon Nicholson

       How has research in the Congo or in sub‐Saharan Africa changed over your career and what do you think will be an exciting new research opportunity?

      The dearth of studies of the Congo Basin, as mentioned earlier, clearly applies to meteorology. Until recently this region was probably the world's least understood in terms of the physical processes governing meteorology and climate. There are several possible explanations for that. For one, in contrast to other areas of Africa, there was relatively little interest in the storm systems affecting the region. This may have been because the rainfall was quite localized, leading to the assumption that rainfall was linked to local thunderstorms and not to large‐scale meteorological processes. In fact, that was the general assumption concerning tropical rainfall. However, by the 1960s and 1970s, large‐scale systems were being recognized in West and Southern Africa, leading to extensive meteorological study. In contrast, meteorological interest in the Congo was historically linked to agriculture and the rubber plantations. Consequently, extensive climatological data on rainfall, evapotranspiration, and related parameters were gathered and published in the Bultot atlas (e.g.,

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