Plant and Soil 206: 61–77, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 61 The root zone dynamics of water uptake by a mature apple tree Steve Green * and Brent Clothier Environment and Risk Management Group HortResearch Institute Private Bag 11-030 Palmerston North New Zealand Received 12 May 1998. Accepted in revised form 29 September 1998 Key words: sap-flow, time-domain reflectometry (TDR), transpiration, soil water, modelling 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 (TDR) 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. We also measured the tree’s distribution of root-length density and obtained supporting data to characterize the soil’s hydraulic properties. The experimental data were used to examine the output of the WAVE-model (Vanclooster et al, 1995; Ecol. Model. 81, 183–185)in which soil water transport is predicted using Richards’ equation, and where root uptake is represented by a distributed macroscopic sink term. When the surface soil layers were uniformly wet, 70% of the trees water uptake occurred in the top 0.4 m of the root zone, in which approximately 70% of the tree’s fine roots were located. When a partial irrigation was applied to just one side of the root zone, the apple tree quickly shifted its pattern of water uptake with an almost two- fold increase in uptake from the wetter soil parts and a corresponding reduction in uptake from the drier parts. The response of root-sap flow to irrigation was almost immediate (i.e. root flow increased within hours of the irrigation). Following subsequent irrigations over the whole soil surface, TDR measurements revealed a surface-ward shift in the pattern of water extraction, and root flow measurements revealed a recovery in the uptake function of seemingly inactive roots located in the previously-dry soil. Via our root sap flow measurements, we observed two roots on the same tree locally responding quite differently to similar events of soil wetting. This observation suggests that there may be considerable functional variability across the apple root system. Our measurement-model calculations yielded similar results and stress the prime role played by the plant in modifying the root zone balance of water. Following an irrigation or rainfall event, root uptake by apple appears to be more dependent upon the near-surface availability of water than it is related to the distribution of fine roots. Introduction Water is essential for all living plants. It is the vehicle that carries nutrients, such as fertilizer, from the sur- face through the soil to the roots and it is the medium through which other transport processes (i.e. diffu- sion) take place. With both fertigation and the more traditional methods of fertilizer application, the prac- tical goal is to maintain adequate water and nutrients within the root zone. For environmental protection, it is meanwhile imperative that percolation losses of wa- * FAX No: 6 354-6731. E-mail: greens@hort.cri.nz ter and chemical below the root zone are minimized. If application rates or methods are poorly matched to either the soil type, or the plant’s requirements, surface-applied chemicals that are water-borne may eventually bypass the root zone, and proceed onward to pollute the groundwater. Understanding the physical processes in the soil which govern water and chemical entry into the root zone, is a necessary step towards developing more efficient and environmentally sustainable strategies of root zone management. However, it is also important to understand the biological processes that operate in