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Drawdown meaning hydropowe7/1/2023 ![]() ![]() The most obvious consequence of water-level changes, however, is a changing water surface area (Fig. Water-level fluctuations have several consequences for reservoir limnology and biogeochemical cycling 8, 9. They are driven both by natural hydrological dynamics and water management. Water-level changes are a typical feature of reservoirs and a major discriminator to natural lakes 7. Globally, rates of organic carbon (OC) burial in reservoirs have been estimated to exceed rates of C emissions from reservoirs 1, 6. At the same time, reservoirs act as a sediment trap, accumulating organic material from the catchment and in-lake primary production, and they have previously been shown to bury C at higher rates than natural lakes 6. The exact quantification of this source is also important because GHG emissions from reservoirs may affect the carbon (C) footprint of irrigation 2 and hydropower production and thus its perception as C-neutral energy 3, 4, 5. The quantification of this GHG source has been subject to intensive research for at least two decades, and current assessments estimate reservoirs to annually emit 800 Tg carbon dioxide equivalent (CO 2e) to the atmosphere 1. Reservoirs, like inland aquatic ecosystems in general, are recognized as a globally relevant source of greenhouse gases (GHGs) 1. Thus, consideration of drawdown areas overturns our conception of the role of reservoirs in the carbon cycle. This suggests that reservoirs emit more carbon than they bury, challenging the current understanding that reservoirs are net carbon sinks. Taking into account drawdown areas, the ratio between carbon emissions and carbon burial in sediments is 2.02 (1.04–4.26). The new estimate assigns 26.2 (15–40) (95% confidence interval) TgCO 2-C yr −1 to drawdown areas, and increases current global CO 2 emissions from reservoirs by 53% (60.3 (43.2–79.5) TgCO 2-C yr −1). We re-assessed the global carbon emissions from reservoirs by apportioning CO 2 and methane emissions to water surfaces and drawdown areas using published areal emission rates. ![]() ![]() Exposure of drawdown areas was most pronounced in reservoirs close to the tropics and shows a complex dependence on climatic (precipitation, temperature) and anthropogenic (water use) drivers. Here we show, on the basis of satellite observations of 6,794 reservoirs between 19, that 15% of the global reservoir area was dry. However, the global extent of drawdown areas is unknown, precluding an accurate assessment of the carbon budget of reservoirs. Reservoir drawdown areas-where sediment is exposed to the atmosphere due to water-level fluctuations-are hotspots for carbon dioxide (CO 2) emissions. ![]()
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