Scheduling deficit irrigation of fruit trees for optimizing water use efficiency

Regulated deficit irrigation (RDI) of fruit trees in the Goulburn Valley of southeastern Australia has increased water use efficiency by approximately 60 percent with no loss in yield or substantial reductions in vegetative vigour. Original techniques to schedule RDI were based on a 12.5 percent (peach) and 20 percent (pear) replacement of US Class A pan evaporation. Subsequent research into soil moisture measurement led to a recommended soil suction of 400 kPa to trigger irrigation. To extend the application of RDI to other environments and fruit crops, practical scheduling steps have been developed. Firstly, fruit growth is measured to determine when to apply RDI. Secondly, an irrigation plan is developed to estimate irrigation run time and interval based on soil type, root distribution, wetting pattern and average daily water use. Thirdly, soil moisture sensors are installed and irrigation is applied when soil suction reaches 200 kPa. Irrigation run time is adjusted by measuring soil moisture immediately following irrigation. Finally, US Class A pan evaporation is measured or reference crop evapotranspiration is calculated to estimate irrigation interval for scheduling in later years. (author)