Greenhouse gas fluxes from reservoirs determined by watershed lithology, morphometry, and anthropogenic pressure

Human population growth has increased the demand for water and clean energy, leading to the massive construction of reservoirs. Reservoirs can emit greenhouse gases (GHG) affecting the atmospheric radiative budget. The radiative forcing due to CO2, CH4, and N2O emissions and the relative contribution of each GHG in terms of CO2 equivalents to the total forcing is practically unknown. We determined simultaneously the CO2, CH4, and N2O fluxes in reservoirs from diverse watersheds and under variable human pressure to cover the vast idiosyncrasy of temperate Mediterranean reservoirs. We obtained that GHG fluxes ranged more than three orders of magnitude. The reservoirs were sources of CO2 and N2O when the watershed lithology was mostly calcareous, and the crops and the urban areas dominated the landscape. By contrast, reservoirs were sinks of CO2 and N2O when the watershed lithology was predominantly siliceous, and the landscape had more than 40% of forestal coverage. All reservoirs were sources of CH4, and emissions were determined mostly by reservoir mean depth and water temperature. The radiative forcing was substantially higher during the stratification than during the mixing. During the stratification the radiative forcings ranged from 125 mg CO2 equivalents m(-2) d(-1) to 31 884 mg CO2 equivalents m(-2) d(-1) and were dominated by the CH4 emissions; whereas during the mixing the radiative forcings ranged from 29 mg CO2 equivalents m(-2) d(-1) to 722 mg CO2 equivalents m(-2) d(-1) and were dominated by CO2 emissions. The N2O contribution to the radiative forcing was minor except in one reservoir with a landscape dominated by crops and urban areas. Future construction of reservoirs should consider that siliceous bedrocks, forestal landscapes, and deep canyons could minimize their radiative forcings.