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• Seasonal and diel CO 2 emissions in a hard-water reservoir show temporal variability. • Inputs of dissolved inorganic carbon play a central role in sustaining the p CO 2. • Nocturnal fluxes are higher than daytime fluxes due largely to aquatic metabolism. • Episodic weather events reverse the diel pattern and cause higher daytime fluxes. • Using daytime measurements alone would underestimate the daily efflux by 9–25%. Reservoirs represent a key component of the global carbon cycle. However, estimates of carbon dioxide (CO 2) emissions from reservoirs remain poorly constrained due to the absence of spatially and temporally resolved measurements. We performed high-resolution monitoring of CO 2 emissions (F CO2) in a semiarid hard-water reservoir to examine its seasonal and diel variability. Our results suggest that dissolved inorganic carbon input plays a central role in sustaining the surface water CO 2 partial pressure (p CO 2), which varies from 1076 to 4587 μatm. Although the reservoir is moderately to highly productive throughout the year, it is a net CO 2 source with F CO2 values in the range of 308–1753 mg C m−2 d–1. This high CO 2 efflux indicates that productive waters are not necessarily CO 2 sinks. Both p CO 2 and F CO2 exhibit clear seasonal and diel patterns. Surface water p CO 2 is highest in March and presents a consistent diurnal/nocturnal pattern with the daytime p CO 2 6–13% lower than the nighttime p CO 2. High CO 2 efflux is observed during the ice-thaw period, indicating the release of CO 2 that was accumulated during the winter. CO 2 effluxes are typically higher during the nighttime driven by aquatic metabolism, but episodic weather events (e.g., rainfall and strong winds) can significantly enhance CO 2 emissions and even reverse the diel pattern. Our study also shows that using only daytime measurements to estimate daily CO 2 emissions would underestimate it by 9–25%. Hence, future global assessments should incorporate CO 2 emissions from hard-water reservoirs and account for their seasonal and diel variability. [ABSTRACT FROM AUTHOR]