Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water‐limited ecosystems. However, the dynamics in soil water availability for tree transpiration (Tt) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because Tt is influenced by soil water potential (Ψs) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth‐dependent soil water retention curves, we quantified the ‘transpirable soil water content’ (tSWC) and its seasonal and inter‐annual variations in a semi‐arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season Tt was 57% of rain and 72% of the infiltrated SWC. In early winter, Tt was exclusively supported by soil moisture at the top 10 cm (tSWC = 11 mm), whereas in spring (tSWC > 18 mm) and throughout the dry season, source water for Tt shifted to 20–40 cm, where the maximum fine root density occurs. Simulation with the soil–plant–atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20–40 cm soil layer critically depended on limited water infiltration below 40 cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC > 0) in this semi‐arid ecosystem. Copyright © 2013 John Wiley & Sons, Ltd.