The principal objective for models of water flow through the soil-plant-atmosphere system is the accurate prediction of leaf water potential (Ψleaf) and water uptake by roots, for a given soil water potential (Ψsoil) and transpiration rate. Steady-state models of water flow through plants, which include only resistances, are sufficient to predict total daily water uptake by roots. Non-steady-state models, which use both water flow resistances and capacitances, are necessary for the prediction of Ψleaf and instantaneous rate of water uptake for diurnal variations of transpiration rate. Potential difference resistances and capacitances are defined for water flow (volume/time) and are best used for individual plant models; resistivities and capacitivities are based on volume flux density ((volume/land surface area)/time) and should be used for plant stands. Prediction of Ψleaf may not be necessary for general circulation models and global climate models (GCM) because stomatal conductance (necessary for the prediction of transpiration rate) is probably controlled by the vapor pressure difference at the leaf surface and Ψsoil and not by Ψleaf. If liquid water flow models through plants are necessary for GCM in order to account for diurnal variations of land-surface energy partitioning, then perhaps an ecosystem time constant for water flow through vegetation of each biome type should be used.