An afforestation field experiment studying Eucalyptus camaldulensis, E. torquata, and Casuarina obesa with five different irrigation intervals was set up in an arid region in Western Australia. The irrigation intervals used were 2 weeks, 1 month, 3 months, 6 months, and no irrigation, which resulted in water supplies of 1243, 895, 511, 423, and 313 mm/y averaged over the experimental period, respectively. The average growth rates of C. obesa in sections with 2-week and 1-month irrigation intervals were 1.9 and 1.3 kg/(m(2) . y), respectively, about double those of E. camaldulensis. However, the average growth rate of C. obesa was lower than E. camaldulensis in sections with less irrigation. For C. obesa, water use efficiency was highest with a 2-week irrigation interval and quickly decreased with decreasing water supply. For E. camaldulensis, it was highest with a 1-month irrigation interval and did not decrease as much as that of C. obesa with decreasing water supply. We concluded that C. obesa has a high water use efficiency with a water supply more than 890 mm/y. E. camaldulensis, however, can grow more efficiently than C. obesa when the water supply is less than 510 mm/y. E. torquata is unsuitable for arid land afforestation.