Modelling water uptake by a mature apple tree

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.