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An assessment of the human impact on the global water cycle requires estimating the volume of water withdrawn for irrigated agriculture. A key parameter in this calculation is the irrigation efficiency, which corrects for the fraction of water lost between irrigation withdrawals and the crop due to management, distribution or conveyance losses. Here we show that the irrigation efficiency used in global irrigation models is flawed for it overlooks key ambiguities in partial efficiencies, irrigation technologies, the definition of 'large-scale' irrigated areas or managerial factors. Once accounted for, these uncertainties can make irrigation withdrawal estimates fluctuate by more than one order of magnitude at the country level. Such variability is larger and leads to more extreme values than that caused by the uncertainties related with climate change. Our results highlight the need to embrace deep uncertainties in irrigation efficiency to prevent the design of shortsighted policies at the river basin-water-agricultural interface.

Hg ∕ SO2, Hg ∕ CO, NOx ∕ SO2 (NOx being the sum of NO and NO2) emission ratios (ERs) in the plume of the coal-fired power plant (CFPP), Lippendorf, near Leipzig, Germany, were determined within the European Tropospheric Mercury Experiment (ETMEP) aircraft campaign in August 2013. The gaseous oxidized mercury (GOM) fraction of mercury emissions was also assessed. Measured Hg ∕ SO2 and Hg ∕ CO ERs were within the measurement uncertainties consistent with the ratios calculated from annual emissions in 2013 reported by the CFPP operator, while the NOx ∕ SO2 ER was somewhat lower. The GOM fraction of total mercury emissions, estimated using three independent methods, was below ∼ 25 %. This result is consistent with other findings and suggests that GOM fractions of ∼ 40 % of CFPP mercury emissions in current emission inventories are overestimated.