Diurnal changes in trunk diameter depend mostly upon variations in water content. The admitted interpretation of this process is based on a reverse flux of water from the trunk in response to the transpirational pull from the foliage. A linear relation between water content and water potential is assumed. The aim of this study is to calculate xylem water potential using trunk diameter changes. Measurements of stem diameter variations (with LVDT sensors) and xylem water potential (with pressure chamber on covered leaves) were made in Tarragona (Spain) on irrigated and non-irrigated walnut trees in an orchard. The relation between these two variables during the day is loop-shaped. A simple model is presented, assuming the existence of a water reservoir (representing elastic tissues) and thus the loop-shaped curve. After calibration for one or few days, the variations of xylem water potential with a fine time scale (few minutes) and for a long period could be monitored from the trunk diameter measurements. Such predicted values, especially predawn and midday xylem water potential, could be very useful for irrigation scheduling. Moreover, this kinetics could improve the understanding of tree water physiology.