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

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.