4. Discussion and conclusion
The Congo basin is one of the most hydrologically active and pristine
locations with limited understanding of how precipitation changes
impacts on stream flow dynamics and variations in catchment stores. This
poor understanding in the hydro-climatic processes of the basin is most
often the results of gaps in hydrological information and insufficient
observational networks for hydrological applications (e.g., Ndehedehe,
2019; Tshimanga and Hughes, 2014; Conway et al., 2009). Due to limited
hydro-meteorological observation network in the basin, Munzimi et al.
(2015) mentioned that much is still unknown about the Congo river
basin’s hydrological behaviour. The comparative assessment of
precipitation estimates from GPCC and CRU data over the Congo basin was
therefore undertaken to improve quantitative knowledge of Congo basin
precipitation and its influence on river discharge. There are still some
ongoing debates regarding the uncertainties and ambiguities in
precipitation estimates from various products over the Congo basin.
While that is not the focus of this chapter, results indicate both
products (GPCC and CRU) are generally consistent with high correlation
values and statistically insignificant difference in their means.
However, the poor association between
these data in the Congo Cuvette central during the period could suggest
dynamics in the availability of gauged stations. The GPCC is a widely
used observational reference data unlike the CRU precipitation. The
distribution of gauges is expected to vary in time and space and the
GPCC-based precipitation product has more gauged observations available
globally compared to the CRU-based precipitation.
The spatial distribution of rainfall in the Congo basin takes the form
of a north–south dipole patterns. These patterns result in wet and dry
seasons with opposite phase in the south and north of the basin. That
is, the wet season in the south (December-February) coincides with dry
season in the north and vice versa. The dipole patterns are caused by
the difference in seasons, the more reason why the northern section is
wet between May and October. The south on the other hand, remains
relatively dry during the same period. The dipoles and this alternating
seasonal rainfall patterns have been attributed to the position of the
Congo basin across the equator (Munzimi et al., 2015). Be it trends,
annual, or semi-annual amplitudes, rainfall during different
climatological periods (1901-1930, 1931-1960, 1961-1990 and 1991-2014)
over the Congo basin has undergone significant changes. These changes
were also echoed by Samba and Nganga (2012) who reported on considerable
declines in Congo rainfall between 1980 and 1990. The surrounding
oceans, especially the Indian Ocean have been identified as an important
source of observed precipitation experienced in wet years in the Congo
basin (Dyer et al., 2017). Generally, the Congo basin and much of
central Africa are hydrologic hotspots or web of climatic influence. For
instance, the SST anomalies along the Benguela Coast, and warming or
cooling of the three oceans (Atlantic, Indian, and Pacific) and indices
of oceanic variability have been identified as key factors that govern
the complex changes in precipitation patterns over West Central Africa
(e.g., Ndehedehe et al., 2019; Balas et al., 2007).
As opposed to West Africa where 3-4 significant modes of rainfall
variability have been identified (e.g., Ndehedehe et al., 2016a; Sanogo
et al., 2015), there are actually more than five statistically
significant modes of orthogonal rainfall modes of variability over the
Congo basin. Physical interpretation and attribution could be
challenging for some of these modes. However, we found that rainfall in
the Congo basin is characterised by annual, multi-annual (or bimodal),
trends, short term seasonal signals and those resulting from regional
factors and atmospheric-ocean interactions and large-scale processes.
Locally generated moisture and the interactions of the nearby oceans
(Sorí et al., 2017; Dyer et al., 2017) with land surface processes in
the basin could also contribute to the magnitude of seasonal variations
observed in the Congo rainfall. Sorí et al. (2017) argued that the
various sources of moisture for the Congo basin makes nourishment
possible during extreme climate events (e.g., flood) thus modulating
water balance within the region. On the one hand, this could be the
reason for significant amount of rainfall in the Congo basin all through
the year except during the June-July period in the southern catchment.
On the other hand, the observed multiple significant modes of the Congo
basin rainfall can be attributed to signals emanating from the combined
influence of climate, natural
variability, ocean interactions and the influence of land surface
processes. As an example of the influence of land surface conditions on
Congo precipitation, locally generated heat low and reduced
precipitation were identified as the direct consequence of decreased
evaporation over deforested area in Central Africa (Nogherotto et al.,
2013). Arguably, this reassert the important role of land surface
processes in the Congo basin (Koster et al., 2004). In this era of
anthropogenic-induced climate change, the skills of global and regional
climate models in reproducing the Congo basin climatology (spatial
patterns, seasonality, and magnitude of precipitation) could be
restricted due to uncertainties (Aloysius et al., 2016). However, the
question of how the interactions of increasing anthropogenic pressure on
the Congo forest and important processes of inter-annual variability
impact surface water hydrology are therefore interesting future research
directions.
So, has rainfall amounts varied comparatively less across the Congo
basin despite known variations in the Congo river flow? In the analysis
of historical space-time variability of rainfall (1901- 2014) over
the Congo basin we found that previous rainfall (1931- 1990)
varied more compared to the 1991-2014 period. As opposed to the two
climatological periods between 1931 and 1990, rainfall varied less
between 1991 and 2014. It is not clear if the time slice during this
last climatological period contributed to this as it was not up to 30
years. However, there were more dry spells and drought events during
this period (1991 and 2014) in the Congo basin compared to the last two
climatological periods prior to 1991 (Ndehedehe et al., 2019; Hua et
al., 2016). The strong positive anomalies of rainfall in extreme wet
years are usually captured in the annual component and show strong
spatial distributions (loadings) in the northern and southern sections.
Although the analysis for the latter climatological period used only 23
years (1991-2014), the spatial distribution and amplitudes of rainfall
is consistent and similar to other periods with relatively higher modes
of variability. Generally, there seems to be a shift in the hydrological
regimes of the Congo river, (especially after 1994). This change can be
attributed to the rather pervasive influence of extreme droughts, which
fluctuated between 50% (i.e., affected areas) in the early years of the
century and 40% during the 1994-2014 period (Ndehedehe et al., 2019).
To further support this argument, the rainfall analysis by Samba and
Nganga (2012) in Congo-Brazzaville (Congo) during 1932–2007 period
indicates that the largest rainfall deficits were observed in the 1980s
and 1990s. And since 1985, Zhou et al. (2014) observed a consistent
drying of the Congolese forest, which was attributed to gradual decline
in precipitation. The change in rainfall trajectory in the post 1994
period is consistent with observed drought and drying in the region. In
view of inconsistent trends between rainfall and discharge as evidenced
in their cumulative departures, this suggests other key secondary
drivers of hydrologic variability that are non-climatic exist in the
basin.
The considerable association of discharge with rainfall in catchments
characterised by strong annual and seasonal amplitudes in rainfall
implies that the wetland hydrology of the basin is largely nourished by
rainfall, in addition to possible exchange of fluxes within the Congo
floodplain wetlands. Notably, a significant proportion of changes in the
dominant rainfall patterns is still not explained by those of river
discharge. This information signals the threshold of complex
hydrological processes in the region. Importantly, it also suggests the
influence of anthropogenic contributions (e.g., deforestation, changes
in land cover states, surface water developments, etc.) and strong
multi-scale ocean-atmosphere phenomena as key secondary drivers of
hydrologic variability. Ultimately, it is obvious nonetheless, that
several African regions have been identified as a hot spot with
considerable influence of several climate teleconnections (e.g., El-Niño
Southern Oscillation and Atlantic Multi-decadal Oscillation, Indian
Ocean Dipole, etc.). Given the impacts of these multi-scaled climate
indices
on rainfall variability and extreme climatic events (droughts and
floods) in Africa (see, e.g., Ndehedehe et al., 2020; Gizaw and Gan,
2017; Diatta and Fink, 2014; Paeth et al., 2012), the combined influence
of climate and human activities on hydrological variability is thus
palpable and multi-faceted.