1. Introduction
Rainfall is a major source of freshwater availability for the functioning of ecosystems and sustainability of both hydrological and agricultural systems. Indeed, rainfall is an important aspect of the hydrological cycle that shows an increasing acceleration owing to climate change. As the climate warms, the hydrological cycle accelerates, causing an increase in the spatiotemporal variability of precipitation and also in the duration and intensity of extreme events such as droughts, storms, and floods (e.g., Ndehedehe, 2019). The need to assess the response of surface water hydrology to changes in climate is therefore crucial.
The rise in extreme events, especially drought frequency, duration and severity across several African domain in recent times (see, e.g., Ndehedehe et al., 2019; Haile et al., 2019; Mpelasoka et al., 2018; Agutu et al., 2017; Ndehedehe et al., 2016b,c; Hua et al., 2016; AghaKouchak, 2015; Shiferaw et al., 2014; Zhou et al., 2014) are strong indications of climatic disturbance and perhaps the increasing vulnerabilities of the African sub-region to the impacts of extreme climatic conditions. These extreme drought events have been the direct result of prolonged limited or below average rainfall. Among other factors, they have been attributed mostly to the perturbations of the surrounding oceans, natural variability, important processes of oceanic variability and human actions, e.g., land use change, deforestation, among others (e.g., Haile et al., 2019; Ndehedehe, 2019; Nicholson et al., 2018; Andam-Akorful et al., 2017; Ndehedehe et al., 2016c,b; Epule et al., 2014). Based on regional climate model simulation, deforestation in the Congo basin, for example, will locally produce a heat low and lead to reduced precipitation (Nogherotto et al., 2013). Assessing evolutionary changes in rainfall patterns is therefore essential to improve knowledge on the interactions between climate systems and globally significant hydro-ecological domains such as the Congo basin.
The critical hydrological features of the Congo basin make it one of those globally significant domains given it key roles in global climate and the huge ecosystem services it provides on mutli-scales (e.g., Ndehedehe et al., 2018; Bell et al., 2015; Verhegghen et al., 2012; Washington et al., 2013). For example, the sensitivity of climate to the loss of the Congo basin rainforest and other ecological disturbance through extensive land cover change and human interaction with the ecosystem is well known (e.g., Bell et al., 2015; Malhi et al., 2013; Verhegghen et al., 2012). Numerical simulations from climate models have demonstrated the importance of the Congo forest to its local hydrology. For instance, the study by Bell et al. (2015) show that deforestation in the Congo basin results in increase albedo and will lead to cooler and drier climate conditions over the entire basin. They noted that the absolute depletion of the Congo basin forest will decrease rainfall by 42% in the western part with a slight increase of 10% in the basin’s eastern section. This ecological disturbance of the African rainforest biome could have implications on regional hydrology, especially modulating local rainfall regimes. In fact, it is argued that more than 50% of the total atmospheric moisture contribution to local precipitation emanates from within the Congo basin (Sorí et al., 2017). Furthermore, the Indian
Ocean and evaporation from the Congo basin are perceived as important moisture sources for the basin. Dyer et al. (2017) argued that the Indian Ocean contributes about 21% of the moisture while 25% of the moisture is recycled within the Congo Basin. They also noted that much of the wet season Congo basin rainfall was derived from the Indian Ocean moisture, stressing the need to understand links between circulation patterns over the Indian Ocean and the local circulation over the basin to aid the optimisation of future climate projections.
The Congo basin (Figs. 1a-c) is one of the most hydrologically active and pristine locations in the world with limited understanding of how precipitation changes impact on stream flow dynamics and variations in catchment stores. Although rainfall variability in different climatic zones of Congo-Brazzaville over the common period 1932–2007 has been studied (Samba and Nganga, 2012), limited and lack of complete and/or continuous observational data is a key challenge to quantitative assessment and characterization of the Congo river basin hydrology (Munzimi et al., 2015; Conway et al., 2009). This was the basis for assessing the performance of satellite precipitation in the Democratic Republic of Congo (Munzimi et al., 2015) and several other studies that relied on multi-satellite and global reanalysis data to aid the characterization of surface water hydrology, hydro-climatic, and land surface conditions in the region (see, e.g., Ndehedehe et al., 2018, 2019; Zhou et al., 2014; Lee et al., 2011, 2014; Nogherotto et al., 2013; Asefi-Najafabady and Saatchi, 2013; O’Loughlin et al., 2013; Conway et al., 2009; Crowley et al., 2006).
While simulating discharge of the Congo River is challenging (Santini and Caporaso, 2018), semi-distributed rainfall-runoff models have been employed to understand processes of runoff generation and study the impacts of climate and human actions on water resources availability (e.g., Tshimanga and Hughes, 2014). However, the response of the Congo river to historical changes in rainfall patterns over different climatological periods (1903-2010) is not well known. Apart from the lack of sufficient gauge data for hydrological applications, Tshimanga and Hughes (2014) also argued that the complexity of natural processes limits our understanding of surface water hydrology. The connection between Congo spring discharge and Gulf of Guinea SST (Materia et al., 2012) and the coupled interactions of the nearby oceans with precipitation patterns, provide important considerations for the rainfall-discharge relationship over the Congo basin. A key hypothesis this study aims to address is that ‘despite known variations in the discharges of the Congo river, previous rainfall amounts have varied comparatively less across the Congo basin’. This important hypothesis requires an assessment of the historical relationship of changes in river discharge with those of precipitation. In light of the proposed water transfer from the Congo basin to nourish the Lake Chad (see, e.g., Ndehedehe et al., 2016b; Lemoalle et al., 2012), understanding key hydrological metrics related to rainfall-runoff relationship are important issues to be considered. In addition to environmental impact assessments of this project on the donor basin, addressing technological and socio-cultural constraints that may impede the actualization of this project are also crucial. Ultimately, the knowledge of evolutionary patterns of river flow and potential socio-hydrological problems that could be linked to climate change can be used to improve diplomacy and regional cooperation by stake holders and riparian countries of both river basins to adapt and seek for innovative solutions on this project.
Given that the basin is among the three prominent convective regions that dominates global rainfall climatology during transition seasons (e.g., Washington et al., 2013), historical space-time variability of rainfall (1901-2014) over the basin in relation to river discharge is analysed in order to understand significant hydro-climatic shift. As we still know little about the nature of climate and anthropogenic influence on the basin, outcomes from model simulation studies, though largely contrasting (e.g., White and Toumi, 2014; Materia et al., 2012), are insufficient, warranting more research. In fact, the skills of several global climate models to simulate historical precipitation over central Africa is considerably limited (Aloysius et al., 2016). Hence, assessing the unique interactions between rainfall and discharge during the last century is essential to improve our understanding of the likely threats of climate change on hydro-ecological assets and freshwater of the Congo basin.