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For a 120-year period, the projected effects of climate change on annual, seasonal, and monthly potential evapotranspiration (ETo) and green water deficit (GWD) were analyzed involving the associated uncertainties for five climatic zones of Iran. Analysis was carried out using data obtained from 15 general circulation models (GCMs) under three SRES scenarios of A1B, A2, and B1 which were downscaled using LARS-WG for 52 synoptic stations up to 2100. The majority of GCMs as well as the median of the ensemble for each scenario project a positive change in both ETo and GWD. A total of 5.8-19.8 % increase in annual ETo, drier than normal wet seasons, as well as 2.3-56.4 % increase in ETo during December-March period well represent a probable increase in the hydrological water requirement in Iran under global warming. Regarding GWD, the country will experience more arid years requiring 113.7 x 10(3)-576.8 x 10(3) Mm(3) more water to supply annual atmospheric water demand. Semi-arid and Mediterranean regions, principal agricultural producer areas of Iran, will be the most vulnerable part of the country due to 1-38.6 % increase in annual GWD under climate change. In addition, water scarcity for irrigated agriculture will enhance in all climatic zones due to 0.9-41 % increase GWD in June-August. However, rain-fed agriculture might be less affected in the hyper-humid and Mediterranean regions because of 1.1-105.3 % reduction in GWD during wet season. Nevertheless, uncertainty analysis revealed that given results for monthly timescale as well as those for times and regions with lower ETo will be the most uncertain. Based on the results, suitable adaptation solutions are highly required to be undertaken to relieve the extra pressure on the decreased blue water resources in the future.