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5 Understanding the Influence of Climate Variability on Surface Water Hydrology in the Congo Basin

       Christopher E. Ndehedehe1, Vagner G. Ferreira2, Augusto Getirana3,4, and Nathan O. Agutu5

       1 Australian Rivers Institute, and Griffith School of Environment and Science, Griffith University, Nathan, Australia

       2 School of Earth Sciences and Engineering, Hohai University, Nanjing, China

       3 Hydrological Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

       4 Science Applications International Corporation, Reston, Virginia, USA

       5 School of Civil, Environment, and Geospatial Engineering, Jomo Kenyatta University of Agriculture and Technology, Juja, Kenya

      ABSTRACT

      Understanding the impacts of climate on surface water hydrology is required to predict consequences and implications on freshwater habitats, ecological assets, and wetland functions. Although the Congo basin is considered a freshwater‐rich region, largely characterized by numerous water resources after the similitude of the Amazon basin, recent accounts of droughts in the basin are indications that even the most humid regions of the world can be affected by droughts and its impacts. Given the scarcity and limited availability of hydrological data in the region, GRACE (Gravity Recovery and Climate Experiment) observations are combined with model and SPEI (standardized precipitation evapotranspiration index) data to investigate the likelihood of such impacts on the Congo Basin’s surface water hydrology. By integrating multivariate analysis with support vector machine regression (SVMR), this study provides some highlights on the characteristics (intensity and variability) of drought events and GRACE‐derived terrestrial water storage (TWS) and the influence of global climate on the Congo river discharge. The southern section of the basin shows considerable variability in the spatial and temporal patterns of SPEI and extreme droughts over the Congo Basin appear to have persisted with more than 40% coverage in 1994. However, there has been a considerable fall in drought intensities since 2007 and this coincides with periods of strong positive anomalies in discharge (i.e., 2007‐010). GRACE‐derived TWS over the Congo basin is driven by annual fluctuations in rainfall (r = 0.81 at three months phase lag) and strong interannual variations of river discharge (r = 0.88, α= 0.05). Generally, results show that changes in the surface water variations (from gauge and model output) of the Congo basin is a key component of the GRACE water column. The outputs of the SVMR scheme indicate that global climate through sea surface temperature anomalies of the Atlantic (r = 0.79, α= 0.05), Pacific (r = 0.79, α= 0.05), and Indian (r = 0.74, α= 0.05) oceans are associated with fluctuations in the Congo river discharge, and confirm the importance of climatic influence on surface water hydrology in the Congo basin.

5.1. INTRODUCTION

The knowledge of global climate influence on drought evolutions and freshwater availability is vital to drought risk mitigation, and evaluation of the cascading impacts of droughts on hydrological stores and agriculture (e.g., Agutu et al., 2019; Ndehedehe et al., 2019; Thomas et al., 2017). Drought events are increasingly becoming complex due to the combined effects of unmitigated climate change/climate variability, perceived human factors, and other non‐climatic factors such as the interference of water abstraction from underground reservoirs with the propagation process of drought characteristics and intensity (e.g., Kubiak‐Wójcicka & Bąk, 2018; Ndehedehe, 2019; Ndehedehe et al., 2020a; Thomas et al., 2017; Van Loon et al., 2016). Understanding the impacts of climate on surface water hydrology is therefore required to predict consequences and implications on several freshwater habitats, ecological assets, and wetlands functions such as floodwater storage, drought relief for wildlife, provision of shelter for fish, and support for aquatic biodiversity, among others (e.g., Chen et al., 2014; Gidley, 2009; Ozesmi & Bauer, 2002; Tockner et al., 2010).

      Furthermore, increased competition for freshwater, as is now the case in some semi‐arid African regions, are some challenges that have been associated with its highly limited and shared water resources, which are considerably variable in time and space (e.g., Freitas, 2013; Ndehedehe, 2019; Okewu et al., 2019). The high variability of freshwater in these regions, laced with considerable and disproportionate trans‐boundary water sharing due to increased demand for freshwater, creates the propensity for inter‐state tensions and rivalry. These conditions nonetheless can be amplified by extreme and prolonged drought events, thus increasing the vulnerability of rural agro‐communities to poverty and famine. While a broad range of socioecological impacts are imminent during such times, even distant populations that indirectly depend on the water resources of Africa could be subjected to far‐reaching impacts of limited freshwater caused by extreme drought (FAO, 2016; Ndehedehe, 2019).

      Moreover, the impacts of climate variability and/or climate change on agriculture and freshwater availability create several risks and key challenges for hydro‐power production, water security, and a broad range of ecosystem services (see, e.g., Agutu et al., 2017; Cenacchi, 2014; Ferreira et al., 2018; Hall et al., 2014; Ndehedehe et al., 2018a; Schroth et al., 2016; Shiferaw et al., 2014; Spinoni et al., 2014; Van Loon et al., 2017). Indeed, the myriad recent scientific reports on droughts and impacts of climate variability in the African subregion (e.g., Agutu et al., 2017, 2019; Epule et al., 2014; Hua et al., 2016; Ndehedehe et al., 2019; Nkiaka et al., 2017) only reinforce the notion of the continued influence of global climate on the continent. Although the Congo Basin is considered a freshwater‐rich region, largely characterized by numerous water resources after the similitude of the Amazon Basin, recent accounts of droughts in the basin (e.g., Hua et al., 2016; Ndehedehe et al., 2019; Zhou et al., 2014) are indications that even the most humid regions of the world can be affected by extreme droughts and its impacts. For example, the impacts of prolonged and frequent droughts on the tropical Congolese rainforest systems will have compositional and structural changes on the Congolese forest (Zhou et al., 2014).

      In line with the need to assess global freshwater change, pioneering hydrological studies over the Congo Basin found declines in Gravity Recovery and Climate Experiment (GRACE, Tapley et al., 2004) derived terrestrial water storage (TWS) while other reports have highlighted the key hydrological characteristics and uniqueness of the Congo basin’s surface water hydrology and hydrodynamics (e.g., Alsdorf et al., 2016;