A whole-plant model of water movement consists of movement of water at two scales of distance. Short distance movement is governed by diffusion across membranes separating compartments within plant tissue. This process is rapid and tends to equilibrate water potential among compartments composed of: cytoplast, xylem, phloem, and apoplast. Long distance movement corresponds to flux of water in xylem and phloem between organs or tissues of a plant. This is driven by hydrostatic pressure, related to water potential in the xylem, and to turgor pressure in the phloem. The model plant is partitioned into root, stem, and leaf tissues, and a conductance is defined for movement in xylem and phloem between each tissue. The same proportions are used to divide each tissues into the various compartments; apoplast, cytoplast, xylem, and phloem. Both short and long distance water fluxes were calculated from linear relations defined by conductances and potential differences, and programmed as finite difference equations using the VENSIM visual dynamic simulation modeling tool. The model was used to examine effects of diurnal variation in transpiration, and effects of leaf to air and solution to root conductance, on movement of water in xylem and phloem, and on water content of various tissues in an idealized plant. The flow of water through the whole plant water took about one hour to reach a steady state after the transition from light to dark. The solute concentration and flow rate in phloem was relatively little affected by the diurnal variation in water status. An increase in transpiration resulted in an approximately linear decrease in water potential in all tissues. However, the decrease was much larger in leaves and stem than in roots. A decrease in the contact between roots and soil solution, lowered root water potential far more than did increasing transpiration.