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.