Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 69

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; Becker et al., 2018; Conway et al., 2009; Crowley et al., 2006; Lee et al., 2014; Ndehedehe et al., 2018b; O’Loughlin et al., 2013). Although extreme hydro‐climatic events in Africa are generally dominated by natural variability and other important processes of interannual variability (Anyah et al., 2018; Bahaga et al., 2019; Ndehedehe et al., 2019; Nicholson et al., 2018), from a multi‐satellite approach, surface water hydrology of the Congo basin is influenced by indices of oceanic variability such as the El‐Niño Southern Oscillation (ENSO) (Becker et al., 2018; Ndehedehe et al., 2018b). However, recent changes in land water storage in some parts of the Congo Basin have been linked to deforestation (Ahmed & Wiese, 2019). As some reports on the negative trends in TWS over the Congo Basin converge, a broader perspective of surface‐water interactions with droughts could provide more understanding of the implications of extreme events (droughts, floods) on biodiversity and the hydro‐ecological assets of the Congo Basin.

Tropical rivers provide essential services and ecological functions for society and ecosystems such as regulating nutrient cycle, maintaining fishery production, water supply, recreation and tourism, generation of hydropower, and support for a range of terrestrial and aquatic biodiversity (e.g., Bunn et al., 2006; Gidley, 2009; Keddy et al., 2009; Kennard et al., 2010; Ndehedehe et al., 2020b, c; Tockner et al., 2010; Zhao et al., 2012). Process‐based knowledge of the cascading impacts of extreme events such as drought on hydrology is crucial and can directly feed into management and policy frameworks. Because large‐scale hydro‐climatic fluctuations and decadal‐scale droughts impact hydrological regimes, a key focus of this chapter is to improve understanding on the response of the freshwater ecosystem to extreme drought and the role of climate variability on the terrestrial hydrology of the Congo Basin. This knowledge is important to help highlight the contributions of human activities such as deforestation and land cover change on surface water hydrology.

In other large watersheds and river basins, multiple lines of evidence confirm significant large‐scale alteration of hydrological processes caused by several human activities, including surface water developments for agriculture and hydropower and water diversion (e.g., Ndehedehe et al., 2019; Wada et al., 2017). For instance, Lake Volta, the largest man‐made lake, contributed 41.6% to the observed increase in GRACE‐derived TWS over the Volta basin during the 2002–2014 period when there was an apparent fall in precipitation (see, Ndehedehe et al., 2016, 2017a). Lake Victoria is the largest lake in Africa and as recently demonstrated, its water storage variability is dam controlled, contributing about 64% of TWS variability to its basin (Getirana et al., 2020). Arguably, the water resources in several river basins in Africa are generally being disturbed by natural variability, large‐scale ocean‐atmosphere phenomena, and a combined human‐induced factors, e.g., land use changes and surface water schemes (e.g., Descroix et al., 2009; Moore and Williams, 2014; Ngom et al., 2016; Redelsperger & Lebel, 2009). The impacts of these interventions have always altered surface water hydrology, culminating in complex hydrological processes and or increased variability in these regions (e.g., Gal et al., 2017; Li et al., 2007; Mahé and Olivry, 1999; Mahé & Paturel, 2009).

Apparently, the Congo Basin contains some of the largest areas of the world’s tropical forests and wetlands, which are considerably important to global carbon and methane cycle (Achard et al., 2002; O’Loughlin et al., 2013). And within the context of global environmental change triggered by various human actions and climate variability, the Congo Basin, which is home to the largest river in Africa and contains about 18% of the world’s tropical forests (e.g., Achard et al., 2002; Becker et al., 2018; Ndehedehe et al., 2018b; Verhegghen et al., 2012) are also vulnerable to multiple influences of human actions and climate change. The main contribution of this study therefore is to improve contemporary understanding on the influence of climate variability on surface water hydrology in the Congo Basin. Specifically, this study (i) investigates the characteristics of extreme events and land water storage using GRACE observations and multi‐scaled indicators and (ii) predicts the influence of global climate on surface water hydrology by integrating multivariate analysis with support vector machine regression. Although in this era of the Anthropocene where combined climate and human actions are leading drivers of environmental change, global hydrological hotspots such as the Congo Basin will experience more climatic disturbance due to the influence of the tropical oceans, physical mechanisms, and climate teleconnections. These factors regulate precipitation and the transport of moisture and will be the vehicle by which climatic extremes will be delivered across the basin and its environs. This chapter will therefore focus on exploring the interactions and links between land water storage (surface water hydrology) and global climate using sea surface temperature, GRACE‐derived TWS, and standardized precipitation evapotranspiration index (SPEI) data. Further details on data, statistical analysis, and modeling employed in this chapter are highlighted in subsequent sections.