Seasonal variation in methane ebullition in a temperate hydropower reservoir impacts annualized estimates

Abstract

Estimates of ebullitive methane (CH ₄ ) emissions from reservoirs are highly uncertain, often based on spatial and temporal upscaling from a limited number of sampling sites and periods. While some of these estimates may adjust for seasonal variation in temperature, they rarely account for drawdown and the resulting changes in site depth and the areal extent of the reservoir. In Douglas Reservoir (Tennessee, USA), we conducted monthly sampling of 20 sites over 1 year to capture temporal and spatial variation in CH ₄ ebullition. We then compared various approaches for obtaining annualized, whole‐reservoir ebullition estimates. CH ₄ ebullition fluxes were zero‐inflated and highly skewed, with fluxes rising appreciably above zero in July and peaking in autumn. Estimates that did not account for drawdown overestimated annualized rates by 20%, though with high uncertainty. Seasonal variability and hotspots strongly influence annualized ebullition estimates and are important to account for in inventories and global upscaling.