Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 81

Although the Congo Basin is one of the most humid regions of the world, similar to the Amazon Basin, droughts and its impacts are unavoidable. Drought variability and frequency tend to be higher in the southern part of the Congo Basin, where seasonal rainfall amount is highest during the December–March period in the basin areas. Although extreme droughts affected more than 40% of the basin between 1992 and 2001, drought episodes and their intensities diminished over the Congo Basin after late 2006 when the basin became extremely wet because of strong changes in rainfall. Generally, there is consistency between the results here and the global scale analysis by Spinoni et al. (2014), who showed prolonged and severe droughts during the same period (1991–2010) over the Congo Basin. While the degree of intensity or impacts of extreme droughts might be different due to catchment characteristics, land cover change, topography, and land surface conditions, water deficits caused by prolonged climate‐induced, below average rainfall could have implications on freshwater variability and availability. For example, evolutionary patterns of standardized precipitation index and discharge show that these variables have considerable linear relationships in the Congo Basin (e.g., Ndehedehe et al., 2018c, 2019). Consistent with this study, we have noticed a rise in SWS of the basin in areas below the equator during wet periods. Similarly, a fall in SWS was observed during the 2004 drought period, confirming the critical role of climate variability on changes in surface water hydrology.

Figure 5.7 Modeling the temporal dynamics of Congo River discharge (1980–2010) using dominant patterns of (a) Atlantic, (b) Pacific, and (c) Indian SST anomalies in the SVM regression scheme.

Land surface conditions and human‐induced climate change are other possible drivers of surface water hydrology in the Congo Basin. Evapotranspiration losses caused by significant declines in soil moisture and droughts (e.g., Jung et al., 2010; Ndehedehe et al., 2018c) can alter hydrological regimes in the Congo Basin. Strong land‐atmosphere interactions and feedbacks and the importance of the Congo forest to its local hydrology and precipitation (Bell et al., 2015; Koster et al., 2004), could indeed induce considerable changes in hydrological regimes of the Congo Basin. Even though the physiographic characteristics of rivers connecting to the Congo River do have complex drainage systems that could create a non‐stationary relationship between surface water flow and rainfall (e.g., Ndehedehe et al., 2019), the terrestrial hydrology of the Congo Basin is directly regulated by the prolonged seasonal rainfall within the Congo Basin. For example, rainfall patterns over the Congo Basin are linearly correlated with the Congo River discharge (e.g., Conway et al., 2009). But from a sub‐regional analysis that included West Africa, the river discharge explained a considerable proportion of GRACE‐terrestrial hydrological signal in the Congo Basin (Ndehedehe et al., 2018b). Arguably, this relationship gives the notion that sink terms (runoff and evapotranspiration) in the basin are also key drivers of surface water hydrology other than rainfall. Locally recycled precipitation caused by the combined influence of the nearby ocean and evaporation from the Congo Basin (Dyer et al., 2017; Sorí et al., 2017) are further evidence supporting the argument of other hydrological drivers in the basin. Moreover, the observed change in hydrological response of the Congo river to strong deviations in rainfall suggests non‐linear interactions and complex hydrological processes in the basin. For instance, changes in the temporal series of discharge do not completely reflect those of observed land water storage. Although it is less debated that the waters of the Congo Basin are directly supplied by rainfall, changes in the surface water of the basin contribute significantly to variations in GRACE‐hydrological signals. Multi‐satellite assessments of the Congo terrestrial hydrology from recent studies (Becker et al., 2018; Ndehedehe et al., 2018b) agree that this is the case.

Figure 5.8 Surface water storage over the Congo Basin during the extreme drought period of 2004 (a, c, e, and g) and the 2007 wet episodes (b, d, f, and h).

Extreme negative anomalies in rainfall impacts surface water hydrology through a trickle‐down effect that culminates in soil moisture and hydrological droughts. While processes such as seasonality effects, catchment, and climate characteristics tend to influence drought propagation, strong precipitation deficits in tropical climates would normally result in reduced alimentation and temporary decrease in stream flows, storage reservoirs, and freshwater stocks (e.g., Kiem et al., 2016; Ndehedehe, 2019; Van Loon et al., 2014). However, it has recently been shown that this was not the case in the Congo Basin (1995–2010) as most drought episodes were inconsistent with discharge anomalies during the period (Ndehedehe et al., 2019). One wonders if there are known physical and ecological processes that play key roles in drought propagation in the Congo Basin. But the basin’s catchment stores (e.g., swamps, lakes, reservoirs, soil column, groundwater, etc.), which could create a prolonged reservoir memory in the hydrological system, could be a determinant in the delayed propagation of drought signals or even its absence in the discharge anomalies. It has been reported that the Congo Basin is the only river basin that seconds the Amazon River in terms of average yearly discharge (i.e., about 40,200 m³/s), and surface water storage (111 km³) (see Alsdorf et al., 2010; Lee et al., 2011). This storage capacity could increase catchment response time to drought events, and arguably create a non‐linear relationship that results in an asymmetric response of surface water dynamics to a drought signal (e.g., Loon, 2013; Ndehedehe et al., 2019). Although antecedent conditions could exist, this relationship can be disturbed or altered in the event of strong human footprints (e.g., deforestation), land surface conditions, and increased frequency in drought events triggered by changes in atmospheric circulation patterns. In other words, rainfall may not be the only driver of hydrological conditions and fluxes in the Congo Basin. Earlier studies have recognized rainfall as a key indicator regulating the hydrology of the region. However, river basin physiography and properties (e.g., topography, streamflow characteristics, etc.) and several ongoing human actions such as the effects of land use change and deforestation in the Congo Basin drive variability in river flows and surface water availability.