Sixty years since the creation of Lake Kariba: Thermal and oxygen dynamics in the riverine and lacustrine sub-basins

Abstract

The current boom of dam construction at low latitudes endangers the integrity and function of major tropical river systems. A deeper understanding of the physical and chemical functioning of tropical reservoirs is essential to mitigate dam-related impacts. However, the development of predictive tools is hampered by a lack of consistent data on physical mixing and biogeochemistry of tropical reservoirs. In this study, we focus on Lake Kariba (Southern Africa), the largest artificial lake in the world by volume. Kariba Dam forms a transboundary reservoir between Zambia and Zimbabwe, and therefore its management represents a socio-politically sensitive issue because the Kariba Dam operation completely changed the downstream hydrological regime. Although Lake Kariba represents a unique and scientifically interesting case study, there is no consistent dataset documenting its physical and chemical behaviour over time. This limits the scope for quantitative studies of this reservoir and its downstream impacts. To address this research gap, we aggregated a consistent database of in situ measurements of temperature and oxygen depth profiles for the entire 60 years of Lake Kariba’s lifetime and performed a detailed statistical analysis of the thermal and oxygen regime of the artificial lake to classify the different behaviours of the lake’s sub-basins. We demonstrate that the seasonal stratification strongly depends on the depth of the water column and on the distance from the lake inflow. Satellite data confirm these spatiotemporal variations in surface temperature, and reveal a consistent longitudinal warming trend of the lake surface water temperature of about 1.5°C from the inflow to the dam. Finally, our results suggest that the stratification dynamics of the lacustrine sub-basins have the potential to alter the downstream Zambezi water quality. Future research should focus on assessing such alterations and developing strategies to mitigate them.

Lake Modeling Reveals Management Opportunities for Improving Water Quality Downstream of Transboundary Tropical Dams

“…However, the total oxygen consumption estimated by our calibrated model (4.5·10 7 mol day −1 ), agrees quite well with the recent data‐driven estimation provided by Calamita et al. (2019b; 3.8·10 7 mol day −1 ). The difference between the two can be attributed to the fact that we consider the entire lake volume instead of only the volume of Basin IV of Lake Kariba, resulting in a slight overestimation of the oxygen consumption (bigger relative hypolimnetic volume and hypolimnetic area).…”

“…In this case, we choose the solution that maximizes the consumption from the sediments (see Table 1) because it is more similar to the data‐driven oxygen consumption estimation from Calamita et al. (2019b). The estimated light attenuation is lower than indicated by observations (Begg, 1970).…”

“…The currents induced by the inflow are therefore of minor importance for creating horizontal variation in the reservoir. This is also supported by the densimetric Froude Number which is often used to assess the importance of advective horizontal flows in reservoirs, and which is <<1/π for the two lower basins of Lake Kariba (Calamita et al, 2019b;Winton et al, 2019). One-dimensional vertical models are typically well suited for simulating water quality dynamics in reservoirs with such small Froude numbers (Deas & Lowney, 2000;Orlob, 1983).…”

“…The latter one showed that the two oxygen depletion rates, for the water column and for the sediments, strongly compensate each other, and therefore multiple sets of optimal parameters exist (see Figure S2). In this case, we choose the solution that maximizes the consumption from the sediments (see Table 1) because it is more similar to the data-driven oxygen consumption estimation from Calamita et al (2019b). The estimated light attenuation is lower than indicated by observations (Begg, 1970).…”

Anthropogenic influences on Zambian water quality: hydropower and land-use change

“…Hotspots of hypoxia in the study area follow expected patterns governed by well-documented biogeochemical processes. In the deep water of lakes and reservoirs, hypoxia is driven by persistent thermal stratication that occurs during warmer months of the year in most tropical reservoirs 3 including Kariba 19 and Itezhi-Tezhi. 20 Floodplain dynamics drive hypoxia across associated reaches of the Kafue River.…”