MODELING EFFECTS OF PARTIAL ROOTZONE IRRIGATION ON STOMATAL CONDUCTANCE AND TRANSPIRATION OF YOUNG CITRUS TREES

Abstract

Partial rootzone irrigation has been proposed for decreasing the amount of water applied to citrus. The computermodel WATFLO was developed to investigate how environmental, plant, and soil factors influence water use of young citrustrees with all or only part of their root systems irrigated. Solar radiation, drybulb temperature, dewpoint temperature, leafarea, root length density, soil volume, and soil waterholding characteristics are specified. Soil water movement to roots iscomputed using microscopicscale radial flow to individual roots. Leaf stomatal conductance regulates actual transpirationin response to solar radiation, atmospheric evaporative demand, and soil water potential in the immediate vicinity of the roots,which is compatible with concepts of roottoshoot abscisic acid signals that control leaf stomatal conductance. Threemethods of calculating the effective soil water potential for governing leaf stomatal conductance were investigated. Partialrootzone irrigation is accomplished by dividing the rooting volume into four compartments and specifying the initial soil watercontent of each compartment. Predicted transpiration rates of young citrus trees with all or threequarters of thecompartments irrigated were in good agreement with measured values for young irrigated citrus trees in controlledenvironment chambers. Predicted transpiration of trees with onehalf or onequarter of the compartments irrigateddecreased during the afternoon due to reduced stomatal conductance in response to simulated depletion of soil water adjacentto the roots. The model indicates that partial rootzone irrigation may limit plant transpiration (with implicit plant waterstress), especially if rooting volumes are small and soils are sandy with low waterholding capacity.