Water movement in response to gradients of water potential in the soil-plant-atmosphere catena is considered. An approximate solution is presented of the differential equation describing soil moisture flow towards a plant root which is absorbing water at a periodically varying rate. This is combined with hypothetical plant characteristics to form a model of the hydraulic behaviour of a crop. The characteristics specified are the depth of penetration of roots into the soil, the density of rooting (length of root per cm3 of soil volume), the internal resistance of the crop to the flow of water and the critical value of leaf water potential which is associated with stomatal closure. The level of leaf water potential in relation to environmental conditions, and the rate of transpiration from the crop when leaf water potential falls to the critical value, are determined by the `supply function' which is a measure of the ability of the soil-crop system to supply water to the evaporating surfaces within the leaves. The variation of this function with the water content of the soil is examined for different values of the parameters involved, and the behaviour of the soil-crop model under constraints representing those imposed by the external environment is discussed.