Rootzone processes, tree water-use, and the equitable allocation of irrigation water to olives

Abstract

John Philip's first job in 1947, at Griffith in Australia's Murrumbidgee Irrigation Area, was to develop means by which irrigation practices could become sustainable. Subsequently, through his analytical endeavors he created revolutionary new understanding of mass and energy transfers in the soil-plant-atmosphere continuum. Here we describe applications and modeling that have directly benefited from John Philip's insights and perspicacity. We have used a new means for determining the radiation interception by an isolated olive tree, and we have employed these results to interpret and model the measured rates of tree water-use from heat-pulse measures of sapflow. These parameters are used in a risk assessment framework, along with measures of the soil's hydraulic character to provide a basis for establishing guidelines for the equitable and sustainable allocation of water for the irrigation of olive trees in Marlborough, New Zealand. We find that small 2-year old olive trees use about 25 litres a week, whereas mature 8-year old trees transpire about 525 L/wk. Our model developed to establish irrigation allocations, SPASMO, used a 28-year sequence of local weather records. For the Fairhall stony silt loam, we find that an irrigation allocation of 230 mm will meet the needs of olives 9 years in 10. Average requirements would be met with just 140 mm. Only 35 mm would be required to meet the needs of olives 90% of the time on the Woodbourne deep silt loam. Apposite measurements and apt modeling are shown capable of guiding regulatory authorities in managing the complexity of allocating water to olive irrigationists.