Quantification of Carbopeaking and CO2 Fluxes in a Regulated Alpine River

Abstract

Carbon dioxide (${\text{CO}}_2$) 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 ${\text{CO}}_2$ 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 ${\text{CO}}_2$ 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 ${\text{CO}}_2$ budget of the case study. The spatiotemporal distribution and drivers of ${\text{CO}}_2$ fluxes depended on hydropower operations. Along the residual flow reaches, ${\text{CO}}_2$ 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 ${\text{CO}}_2$ fluxes were dominated by systematic sub-daily fluctuations in ${\text{CO}}_2$ transport and evasion fluxes (“carbopeaking”) driven by hydropeaking. Hydropower operational patterns and regulation approaches in Alpine rivers affect ${\text{CO}}_2$ fluxes and their response to biogeochemical drivers significantly across different temporal scales. Our findings highlight the importance of considering all scales of ${\text{CO}}_2$ variations for accurate quantification and understanding of these impacts, to clarify the role of natural and anthropogenic drivers in global carbon cycling.