The Impact of Water Intake Scheduling on Cascade Reservoirs on the Carbon Balance of Reservoir Systems

The construction and operation of reservoirs disrupt the natural flow regime of rivers, reducing flow velocities and creating prolonged anaerobic conditions, particularly in steep-gradient, deeply incised river channels. These conditions facilitate microbial decomposition of organic matter—originating from terrestrial plants and soils—leading to greenhouse gas emissions, such as carbon dioxide (CO₂) and methane (CH₄). Cascade reservoir systems, composed of multiple reservoirs connected in an upstream-downstream configuration, introduce further complexities due to the interactions between upstream discharges and downstream reservoirs. These interactions influence water temperature, flow disturbance, and material transport, among other factors.

The study employed the CE-QUAL-W2 model to developed a coupled hydrodynamic and water quality model for analyzing carbon transmitting among the five cascade reservoirs along the Wujiang River in Southwest China (Figure 1). This two-dimensional model, uses x-z plane layered grids to simulate water flow, temperature, and carbon cycling dynamics under specific power station intake location scenarios. By simulating these scenarios, we assessed how changes in power station intake elevations influence the carbon balance of individual and cascade reservoir systems.

Figure 1 The Wujiang River basin and the spatial distribution of five cascade reservoirs

The results indicate that for individual reservoirs such as WJD Reservoir, raising the intake elevation of the power station enhances surface water disturbance, which enhanced CO₂ diffusion across the water-air interface near the dam (Figure 2). However, this adjustment significantly reduces carbon release to downstream areas, thereby increasing the reservoir’s overall carbon retention capacity.

Figure 2 Average CO₂ diffusion fluxes across the water-air interface in the WJD Reservoir under different scenarios (scenario A-G represent progressively higher intake elevations at the WJD power station.

When the intake elevation of upstream DF Reservoir was raising, its carbon retention capacity improved. However, the warmer discharged water inhibits vertical carbon sedimentation in downstream reservoirs. This led to the accumulation of Total Inorganic Carbon (TIC) and Total Organic Carbon (TOC) in shallow water layers of downstream reservoirs (Figure 3).

Figure 3 Vertical distribution of TOC (left) and TIC (right) at the WJD reservoir dam under different intake elevations of upstream hydropower stations (Scenarios I–V represent progressively higher intake elevations).

Consequently, carbon transport to downstream reservoirs increased, reducing the total carbon sink capacity of the cascade reservoir system. The findings highlight a trade-off between local and system-wide carbon retention in cascade reservoirs. While elevating intake locations at individual reservoirs can improve carbon retention locally, the downstream impacts—such as reduced vertical carbon sedimentation and increased carbon transport—diminish the overall carbon storage efficiency of the cascade reservoirs system. Future reservoir management strategies should consider these complex interactions to balance energy production with environmental sustainability.