This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come.

Since 1970 the West African Sahel has experienced a significant drought which, according to some authors, corresponds to a discontinuity (abrupt change) in the rainfall series. An annual rainfall index was calculated over the period from 1896 to 2000, with a selection of 21 synoptic stations updated regularly by the Agrhymet Regional Centre in Niamey. Several statistical analyses of the index confirmed the previous descriptions, particularly the significance of the drought since 1970 and a nonstationarity, with abrupt change in the series between 1969 and 1970. Although the two recent wet years, 1994 and 1999, gave some hope of an end to the drought, the statistical results and the temporal distribution of the dry and wet years showed that the drought was not over at the end of 2000.

We review the published results on the Congo Basin hydrology and summarize the historic and ongoing research. Annual rainfall is ~1900 mm/yr along an east-west trend across the basin, decreasing northward and southward to ~1100 mm/yr. Historic studies using lysimeters, pans, and models suggest that the annual potential evapotranspiration varies little across the basin at 1100 to 1200 mm/yr. Over the past century, river discharge data have been collected at hundreds of stream gauges with historic and recent data at 96 locations now publicly available. Congo River discharge at Kinshasa-Brazzaville experienced an increase of 21% during the 1960–1970 decade in comparison to most other decades. Satellite altimetry measurements of high and low flows show that water levels in the “Cuvette Centrale” wetland are 0.5 m to 3.0 m higher in elevation than the immediately adjacent Congo River levels. Wetland water depths are shallow at about a meter and there does not appear to be many sizable channels across the “Cuvette”; thus, wetland flows are diffusive. Cuvette waters alone are estimated to emit about 0.5 Pg CH₄ and CO₂ equivalents/yr, an amount that is significant compared to global carbon evasions. Using these results, we suggest seven hypotheses that focus on the source of the Cuvette waters and how these leave the wetland, on the river discharge generated by historic rainfall, on the connection between climate change and the rainfall-runoff generated by the migrating “tropical rainbelt,” on deforestation and hydroelectric power generation, and on the amount of carbon emitted from Congo waters.

River basin rainfall series and extensive river flow records are used to characterize and improve understanding of spatial and temporal variability in sub-Saharan African water resources during the last century. Nine major international river basins were chosen for examination primarily for their extensive, good quality flow records. A range of statistical descriptors highlight the substantial variability in rainfall and river flows [e.g., differences in rainfall (flows) of up to −14% (−51%) between 1931–60 and 1961–90 in West Africa], the marked regional differences, and the modest intraregional differences. On decadal time scales, sub-Saharan Africa exhibits drying across the Sahel after the early 1970s, relative stability punctuated by extreme wet years in East Africa, and periodic behavior underlying high interannual variability in southern Africa. Central Africa shows very modest decadal variability, with some similarities to the Sahel in the adjoining basins. No consistent signals in rainfall and river flows emerge across the whole of the region. An analysis of rainfall–runoff relationships reveals varying behavior including strong but nonstationary relationships (particularly in West Africa); many basins with marked variations (temporal and spatial) in strength; weak, almost random behavior (particularly in southern Africa); and very few strong, temporally stable relationships. Twenty-year running correlations between rainfall and river flow tend to be higher during periods of greater rainfall station density; however, there are situations in which weak (strong) relationships exist even with reasonable (poor) station coverage. The authors conclude for sub-Saharan Africa that robust identification and attribution of hydrological change is severely limited by data availability, conflicting behavior across basins/regions, low signal-to-noise ratios, sometimes weak rainfall–runoff relationships, and limited quantification of the magnitude and effects of land use change.