www.publish.csiro.au/journals/ajsr Australian Journal of Soil Research, 2003, 41, 365–380 © CSIRO 2003 0004-9573/03/030365 10.1071/SR02129 CSIRO PUBLISHING Modelling water uptake by a mature apple tree S. R. Green A , I. Vogeler, B. E. Clothier, T . M. Mills, and C. van den Dijssel Environment and Risk Management Group, HortResearch Institute, Private Bag 11-030, Palmerston North, New Zealand. A Corresponding author; email: greens@hort.cri.nz Abstract We report the results from a field experiment in which we examined the spatial and temporal patterns of water uptake by a mature apple tree (Malus domestica Borkh., ‘Splendour’) in an orchard. Time domain reflectometry was used to measure changes in the soil’s volumetric water content, and heat-pulse was used to monitor locally the rates of sap flow in the trunk and roots of the tree. The tree’s distribution of root-length density and supporting data to characterise the soil’s hydraulic properties were determined for the purpose of modelling soil water movement in the root-zone under an apple tree. Experimental data are compared against the output from a numerical model of the soil water balance that uses Richards’ equation for water flow, and uses a distributed macroscopic sink term for root uptake. In general, there was a very good agreement between the measured and modelled results. The apple trees consumed some 70 L of water per day during the middle of summer. The daily water use declined to about 20 L per day with the onset of autumn, coinciding with a reduced evaporative demand and an increasing number of rain days. Water movement in the root-zone soil was dominated by the water uptake via surface roots. Large changes in soil water content were also associated with each irrigation event. Our experimental data support the contention that more frequent irrigation in smaller doses will result in less water percolating through the root-zone. Such an irrigation strategy should make more efficient use of water by minimising the leaching losses. It will also be helpful for environmental protection by reducing the percolation losses of water and solute beyond the grasp of the roots. SR02129 S.R. Green et al. Modelingwater uptake byanapple tree Additional keywords: sap-flow, time-domain reflectometry (TDR), transpiration, soil water, modelling. Introduction Water is the vehicle that carries essential nutrients, such as nitrogen and other trace elements, through the soil to the plant roots where they may be absorbed and utilised for growth, reproduction, and maintenance of plant organs. Water is also the medium through which dissolved carbon dioxide gas is exchanged between stomata on the leaf surfaces and the ambient air. In the long term, any water loss via transpiration from the leaves must be matched by water uptake from the plant root system. ‘Water moves in all parts of the complicated soil–plant–atmosphere system down a gradient of potential (Philip 1957)’ … this concept gives a useful first picture, but it must be emphasised that for real plants the 3-dimensional disposition of roots, leaves, and other plant parts makes for a more complicated problem (Philip 1977). The dynamics of water and solute transport in unsaturated soil affect many aspects of soil management, such as the timing and rate of irrigation and fertiliser applications, sustainable loadings of wastewater disposal, as well as effective strategies for remediation and soil decontamination. With the use of fertigation and the more traditional methods of fertiliser application, the goal is to maintain an adequate supply of water and nutrients within the root-zone. During the growing season, pesticides and other chemicals may also be applied to crops grown under intensive agricultural practices, to control a wide range of weeds, pests, and diseases that may affect crop production. For environmental protection, it