Abstract.Two models of the relationship between diurnal variation in shoot water potential and transpiration in 14‐year‐oldPicea sitchensis(Bong.) Carr. were compared. The first model was a physiologically based resistance‐capacitance (R‐C) analogue with its associated differential equations. The second was a non‐physiological discrete‐difference (D‐D) or stochastic transfer function model. The RC model included only the effect of water storage in the phloem and bark while the D‐D model implicity included all storage mechanisms. The R‐C and D‐D models explained similar fractions (62% and 68% respectively) of the variation in shoot water potential due to diurnal changes in transpiration rate. However, the D‐D model had fewer parameters than the R‐C model. The results from the D‐D model showed that the resistance to flow from soil to shoots along the trunk, (RT), was 5 × 103MPa kg‐1s and the capacitance of the phloem and bark treated as a single store, (Cs), was 1.6 kg MPa‐1. It is suggested that the resistance to flow into storage (Rs) is much greater than RTand can be disregarded. A non‐linear version of the D‐D model suggested [hat resistance to flow in the trunk increases