A water flux model with daily resolution is described which permits one to assess how changes in the rooting volume, amount of sapwood, leaf area and conductance properties interact to affect water uptake, internal storage, and transpiration. A root zone water compartment is defined for a particular tree on the basis of root depth, lateral extension and moisture holding characteristics of the soil. Water is taken up from different subcompartments of the root zone as a function of vertical position, soil water content, and water deficit within the sapwood. Excess water entering the root zone is channeled into runoff or seepage. The sapwood compartment of the model is restricted to the main stem of the tree and does not include sapwood in the branches or roots. The model assumes whatever water deficit is built up in the sapwood during the day will be replenished at night if the root zone water supply/capacity ratio exceeds 20%. A complex exponential equation describes the amount of water extractable from 20% to 0 capacity when no uptake is possible. The maximum change in volume of water in the sapwood of a large Douglas-fir is estimated to represent more than a 10 day supply for transpiration. Water loss through transpiration is predicted as a function of the mean daily absolute humidity deficit, leaf area, leaf conductance and daylength. Leaf conductance is controlled by predawn plant moisture stress which in turn is a function of the rooting zone water supply. The model incorporates two special constraints upon water uptake and transpiration. The first accounts for the effect of cold soil temperatures reducing the possible uptake by Douglas-fir to half at 2°C and to 0 at-2°C. The second represents a critical absolute humidity deficit sufficient to cause stomatal closure which results in leaf conductance being reduced to a minimum. The model is employed to compare trees of different sizes and those with different stomatal behavior. From this experience, it is suggested that future studies include, at a minimum, simultaneous measurements of: absolute humidity deficit, leaf area, sapwood volume and change in water content, predawn stress and leaf conductance.