Quantification of Carbopeaking and CO2 ${\text{CO}}_{2}$ Fluxes in a Regulated Alpine River

• Published in: Water resources research Vol. 61; no. 2
• Main Authors: Dolcetti, G., Piccolroaz, S., Bruno, M. C., Calamita, E., Larsen, S., Zolezzi, G., Siviglia, A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.02.2025
• Subjects: Anthropogenic factors; Atmosphere; Biogeochemistry; Biological activity; Buffers (chemistry); carbon; Carbon cycle; Carbon dioxide; Carbon dioxide exchange; Carbonates; carbopeaking; case studies; Climate and human activity; Climate change; CO2; Fluxes; Human influences; Hydraulics; hydrochemistry; Hydroelectric power; hydropeaking; hydropower; Mathematical models; Numerical models; Oceans; Outlets; Photosynthesis; Reservoirs; Residual flow; River regulations; riverine carbon flux; Rivers; Spatial distribution; Temporal distribution; water; Water chemistry; water power
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2024WR037834

Carbon dioxide (CO2 ${\text{CO}}_{2}$) fluxes in regulated Alpine rivers are driven by multiple biogeochemical and anthropogenic processes, acting on different spatiotemporal scales. We quantified the relative importance of these drivers and their effects on the dynamics of CO2 ${\text{CO}}_{2}$ concentration and atmospheric exchange fluxes in a representative Alpine river segment regulated by a cascading hydropower system with diversion, which includes two residual flow reaches and a reach subject to hydropeaking. We combined instantaneous and time‐resolved water chemistry and hydraulic measurements at different times of the year, and quantified the main CO2 ${\text{CO}}_{2}$ fluxes by calibrating a one‐dimensional transport‐reaction model with measured data. As a novelty compared to previous inverse modeling applications, the model also included carbonate buffering, which contributed significantly to the CO2 ${\text{CO}}_{2}$ budget of the case study. The spatiotemporal distribution and drivers of CO2 ${\text{CO}}_{2}$ fluxes depended on hydropower operations. Along the residual flow reaches, CO2 ${\text{CO}}_{2}$ fluxes were directly affected by the upstream dams only in the first ∼ ${\sim} $ 2.5 km, where the supply of supersaturated water from the reservoirs was predominant. Downstream of the hydropower diversion outlets, the CO2 ${\text{CO}}_{2}$ fluxes were dominated by systematic sub‐daily fluctuations in CO2 ${\text{CO}}_{2}$ transport and evasion fluxes (“carbopeaking”) driven by hydropeaking. Hydropower operational patterns and regulation approaches in Alpine rivers affect CO2 ${\text{CO}}_{2}$ fluxes and their response to biogeochemical drivers significantly across different temporal scales. Our findings highlight the importance of considering all scales of CO2 ${\text{CO}}_{2}$ variations for accurate quantification and understanding of these impacts, to clarify the role of natural and anthropogenic drivers in global carbon cycling. Plain Language Summary Rivers play a key role in the transport, processing and exchange of carbon and CO2 ${\text{CO}}_{2}$ between land, atmosphere, and oceans. CO2 ${\mathrm{C}\mathrm{O}}_{2}$ fluxes are governed by biological, chemical, and hydraulic processes which may be perturbed by human activities, such as hydropower. Understanding and quantifying these effects is challenging due to the various processes and temporal and spatial scales involved. We conducted an extensive measurement campaign over several days and across all seasons along an Alpine river where hydropower strongly affects hydraulic conditions, and used the data to calibrate a numerical model and estimate the main sources of CO2 ${\text{CO}}_{2}$ fluxes. CO2 ${\mathrm{C}\mathrm{O}}_{2}$ levels were high near the dam, where water with high CO2 ${\text{CO}}_{2}$ concentration was released from the hydropower reservoir, but decreased rapidly along the river as CO2 ${\text{CO}}_{2}$ was either released into the atmosphere or absorbed through photosynthesis. During peaks in hydropower activity, CO2 ${\text{CO}}_{2}$ concentration and emissions into the atmosphere increased rapidly, a phenomenon called “carbopeaking.” The study highlights the importance of considering both natural and human sources of river CO2 ${\text{CO}}_{2}$ fluxes to fully understand the role of rivers in the carbon cycle and to predict their response to human activities and climate change. Key Points Flow regulation in an Alpine river affects the spatiotemporal variability and drivers of CO2 fluxes Diel metabolism and carbonate buffering sustained by lateral inflows dominate CO2 dynamics in the residual flow reaches Intense and localized peaks in CO2 concentration and evasion rate are observed downstream of the hydropower outlets during hydropeaking