Water content is a key variable in plant physiology, even during the winter period. To simulate stem water content (WC) during the dormant season, a series of experiments were carried out on walnut trees under controlled conditions. In the field, WC was significantly correlated with soil temperature at 50cm depth (R²=0.526). In the greenhouse, WC remained low as long as soil temperature was kept cold (<+5°C) and increased after soil temperature was warmed to +15°C, regardless of the date. Stem dehydration rate was significantly influenced by WC and evaporative demand. A parsimonious model with functions describing the main experimental results was calibrated and validated with field data from 13 independent winter dynamics in Juglans regia orchards. Three functions of water uptake were tested and gave equivalent accuracies (RMSE=0.127-8; RMSEP=0.116). However, only a sigmoid function describing the relationship between root water uptake and soil temperature gave values in agreement with the experimental results. Finally, the simulated WC provided similar accuracy in predicting frost hardiness compared to the measured WC (RMSE ca. 3°C) and was excellent in spring (RMSE ca. 2°C). This model may be a relevant tool for predicting the risk of spring frost in walnut trees. Its genericity should be tested in other fruit and forest tree species.