Department Surface Waters - Research and Management

Lake Kivu is a 485 m deep, Central-East African rift lake with huge amounts of carbon dioxide and methane dissolved in its stably stratified deep waters. In view of future large-scale methane extraction, one-dimensional numerical modelling is an important and computationally inexpensive tool to analyze the evolution of stratification and the content of gases in Lake Kivu. For this purpose, we coupled the physical lake model Simstrat to the biogeochemical library AED2. Compared to an earlier modelling approach, this coupled approach offers several key improvements, most importantly the dynamic evaluation of mixing processes over the whole water column, including a parameterization for double-diffusive transport, and the density-dependent stratification of groundwater inflows. The coupled model successfully reproduces today's near steady-state of Lake Kivu, and we demonstrate that a complete mixing event ∼2000 years ago is compatible with today's physical and biogeochemical state.

Water quality in tropical rivers is changing rapidly. The ongoing boom of dam construction for hydropower is one of the drivers for this change. In particular, the stratification in tropical reservoirs induces oxygen deficits in their deep waters and warmer surface water temperatures, which often translate into altered thermal and oxygen regimes of downstream river systems, with cascading consequences for the entire aquatic ecosystem. Operation rules of reservoirs, involving water intakes at different levels, could mitigate the consequences for downstream water quality. However, optimized water management of deep reservoirs relies on predictive models for water quality, but such predictive capability is often lacking for tropical dams. Here we focus on the Zambezi River Basin (southern Africa) to address this gap. Using the one-dimensional General Lake Model, we reproduced the internal dynamics of the transboundary Lake Kariba, the world’s largest artificial lake by volume, created by damming the Zambezi River at the border between Zambia and Zimbabwe. Through this modeling approach, we assessed and quantified the thermal and oxygen alteration in the Zambezi River downstream of the reservoir. Results suggest that these alterations depend directly on Kariba’s stratification dynamics, its water level and the transboundary policies for water withdrawal from the reservoir. Scenario calculations indicate a large potential for mitigating downstream water quality alterations by implementing a hypothetical selective withdrawal technology. However, we show that a different and cooperative management of the existing infrastructure of Kariba Dam has the potential to mitigate most of the actual water quality alterations.

Pumped-storage (PS) hydropower plants are expected to make an important contribution to energy storage in the next decades with growing market shares of new renewable electricity. PS operations affect the water quality of the connected water bodies by exchanging water between them but also by deep water withdrawal from the upper water body. Here, we assess the importance of these two processes in the context of recommissioning a PS hydropower plant by simulating different scenarios with the numerical hydrodynamic and water quality model CE-QUAL-W2. For extended PS operations, the results show significant impacts of the water exchange between the two water bodies on the seasonal dynamics of temperatures, stratification, nutrients, and ice cover, especially in the smaller upper reservoir. Deep water withdrawal was shown to strongly decrease the strength of summer stratification in the upper reservoir, shortening its duration by ~1.5 months, consequently improving oxygen availability, and reducing the accumulation of nutrients in the hypolimnion. These findings highlight the importance of assessing the effects of different options for water withdrawal depths in the design of PS hydropower plants, as well as the relevance of defining a reference state when a PS facility is to be recommissioned.

We analyzed the processes affecting the methane (CH₄) budget in Lake Kinneret, a deep subtropical lake, using a suite of three models: (1) a bubble model to determine the fate of CH₄ bubbles released from the sediment; (2) the one-dimensional physical lake model Simstrat to calculate the mixing dynamics; and (3) a biogeochemical model implemented in Aquasim to quantify the CH₄ sources and sinks. The key pathways modeled include diffusive and bubble release of CH₄ from the sediment, aerobic CH₄ oxidation, and atmospheric gas exchange. The temporal and spatial dynamics of dissolved CH₄ concentrations observed in the lake during 3 years could be well represented by the combined models. Remarkably, the relative contributions of ebullition and diffusive transport to the accumulation of CH₄ in the hypolimnion during the stratified period could not be accurately constrained based only on the observed evolution of CH₄ concentrations in the water column. Importantly, however, our analysis showed that most (∼99%) of the CH₄ supplied to the water column by bubble dissolution and diffusive transport from the sediment is aerobically oxidized, whereas a substantial fraction (∼60%) of the sediment-released bubble CH₄ is directly transported to the atmosphere. Ebullition is thus responsible for the bulk of the emissions from Lake Kinneret to the atmosphere. Therefore, as in all freshwaters, ebullition quantification is crucial for accurately assessing CH₄ emissions to the atmosphere. This task remains challenging due to high spatio-temporal variability, but combining in situ measurements with a process-based modeling can help to better constrain flux estimates.