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We simulate the effect of climate change on water requirements of cold season wheat in various climatic zones of Iran. The research considers both observed climate (temperature and precipitation) changes during recent decades (1960-2009) based on instrumental records and projected future changes to 2100 based on the MAGICC/SCENGEN 5.3 compound model. 20 General Circulation models are used based on a single scenario known as P50, which is the average of SRES or emission scenarios. Results indicate that whilst winter precipitation has marginally increased across the country as a whole, a significant decline in mean spring precipitation is recorded between 1960 and 2009. However, considerable variability in trends is measured across various climatic regions of Iran. Mean annual temperature / rainfall changes in the various climate zones of Iran for the period 1960-2009 follow: +0.1 degrees C, / decade / +0.43 mm / decade in arid regions, -0.1 degrees C / decade / -1.7 mm / decade in semi-arid regions, +0.1 degrees C / decade / -1.33 mm / decade in Mediterranean / semi-humid regions and -0.01 degrees C / decade / -0.04 mm / decade in humid / hyper humid regions. Temperature projections to 2100 indicate an overall temperature rise of ca. 4.25 degrees C relative to that for 1961-1990, with increases projected for all climatic regions of Iran. Despite an overall projected mean precipitation increase of 36% for the year 2100, relative to that for 1961-1990, these are insufficient to compensate for temperature increases. Consequently, calculated water deficits during the growing season (autumn to spring) in Iran’s wheat producing areas are expected to increase from 5.2% in 1980 to over 23% by 2050 and 38% by 2100.