UC Davis

Freshwater reservoirs are hotspots for methane (CH4) and carbon dioxide (CO2) fluxes released into the atmosphere; however, there is little knowledge about CO2 flux rates from reservoirs in Mediterranean climate regions. Studies show that reservoirs can either emit or absorb CO2, depending on the season and time of day. The goal of this study was to analyze CO2 and gain knowledge of the temporal variability of CO2 fluxes at Uvas Reservoir, Santa Clara County, California, in a Mediterranean climate zone. Results showed a strong diel variability of CO2, with negative flux rates (CO2 sinks) generally occurring during the nighttime. For example, in June 2021 at Uvas Reservoir, CO2 emissions decreased from -0.27 mg m-2 h-1 to -0.48 mg m-2 h-1 at night. These fluxes were also variable seasonally, where positive values (CO2 sources) in CO2 flux rates occurred during fall turnover, with a peak flux rate calculated at +1.57 mg m-2 h-1 in the daytime. This example from the fall season occurred after the reservoir had used up all the nutrients and when algae were decomposing. Lastly, the CO2 surface gas flux results from Uvas Reservoir were compared to other freshwater reservoirs within the same climatic region, different climatic regions, and other land classification types. This comparative analysis showed that Uvas Reservoir had 20% and 6.2% higher CO2 flux rates compared to a tropical climate and a temperate climate in a dry year, respectively.