Comment on egusphere-2023-422

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Reservoirs can emit substantial amounts of the greenhouse gas methane (CH₄) via different emission pathways. In some reservoirs, reservoir flushing is employed as a sediment management strategy to counteract growing sediment deposits that threaten reservoir capacity. Reservoir flushing utilizes the eroding force of water currents during water level drawdown to mobilize and transport sediment deposits through the dam outlet into the downstream river. During this process, CH₄ that is stored in the sediment can be released into the water and degas to the atmosphere resulting in CH₄ emissions. Here, we assess the significance of this CH₄ emission pathway and compare it to other CH₄ emission pathways from reservoirs. We measured seasonal and spatial CH₄ concentrations in the sediment of Schwarzenbach Reservoir, providing one of the largest datasets on CH₄ pore water concentrations in freshwater systems. Based on this dataset we determined CH₄ fluxes from the sediment and estimated potential CH₄ emissions due to reservoir flushing. CH₄ emissions due to one flushing operation can constitute 7&ndash;14 % of the typical annual CH₄ emissions from Schwarzenbach Reservoir, whereby the amount of released CH₄ depends on the timing of the flushing operation within the season. The larger the thickness of the sediment layer mobilized during the flushing operation the larger the average CH₄ concentration per unit volume of flushed sediment. This suggests that regular flushing of smaller sediment layers releases less CH₄ than removal of the same sediment volume in fewer flushing events of thicker sediment layers. In other reservoirs with higher sediment loadings than Schwarzenbach Reservoir, reservoir flushing could cause substantial CH₄ emissions, especially when flushing operations are conducted frequently. Therefore, CH₄ emissions due to reservoir flushing must be included in estimates of annual overall greenhouse gas emissions from reservoirs that are subject to regular flushing operations.