<p>Climate change is expected to cause ever greater reductions to the yield of durum wheat due to declining rainfall in the durum growing areas of the world. Improving water use efficiency of this crop is crucial. This PhD study investigated the impact of deficit irrigation on the growth, yield and water use efficiency of eight durum wheat varieties, based on greenhouse and field experiments. The greenhouse experiment consisted of three levels of water replacement, 50, 75 and 100% of full point, under two CO₂ concentrations (350 and 750 ppm). The field experiment imposed four irrigation water replacements, 0, 50, 75 and 100% of fully irrigated crop water use, in 2017, and repeated in 2018. The field experiment indicated that irrigation replacement at 50% increased aboveground biomass, grain yield, water use and water use efficiency by 18, 33, 14 and 20% respectively, when compared with the 0% irrigation water replacement in 2017. Reducing irrigation from 100 to 0% generally reduced biomass linearly from 15 to 12 tonnes per hectare and this was true for Caparoi, Jandaroi, DBA Aurora, DBA Lillaroi and EGA Bellaroi in 2018. The interaction between irrigation replacement and varieties showed that Hyperno, DBA Lillaroi and DBA Aurora showed increases of 31, 18 and 16% respectively for water use efficiency under the 50% water replacement over the dryland treatment, while Jandaroi appeared unaffected. The remaining varieties declined in water use efficiency ranging from Caparoi (-3%) and DBA Bindaroi (-13%). The greenhouse experiment demonstrated that the levels of irrigation and CO₂ significantly affected water use efficiency of biomass production (WUE_b). The interaction between irrigation and CO₂ levels shows that the greater WUE_b achieved at 750 ppm CO₂ increased more as irrigations were reduced than for the ambient CO₂ level (5.1 to 6.0 verses 3.6 to 4.1 g kg^-1 ). In all cases varieties produced their highest WUE_b at 50% irrigation and lowest at 100%, however for the 75% treatment WUE_b sometimes equalled that for the 50% and for other varieties the 100% irrigation.</p>