Modeling effects of partial rootzone irrigation on stomatal conductance and transpiration of young citrus trees

Partial rootzone irrigation has been proposed for decreasing the amount of water applied to citrus. The computer model WATFLO was developed to investigate how environmental, plant, and soil factors influence water use of young citrus trees with all or only part of their root systems irrigated. Solar radiation, dry-bulb temperature, dewpoint temperature, leaf area, root length density, soil volume, and soil water-holding characteristics are specified. Soil water movement to roots is computed using microscopic-scale radial flow to individual roots. Leaf stomatal conductance regulates actual transpiration in response to solar radiation, atmospheric evaporative demand, and soil water potential in the immediate vicinity of the roots, which is compatible with concepts of root-to-shoot abscisic acid signals that control leaf stomatal conductance. Three methods of calculating the effective soil water potential for governing leaf stomatal conductance were investigated. Partial rootzone irrigation is accomplished by dividing the rooting volume into four compartments and specifying the initial soil water content of each compartment. Predicted transpiration rates of young citrus trees with all or three-quarters of the compartments irrigated were in good agreement with measured values for young irrigated citrus trees in controlled-environment chambers. Predicted transpiration of trees with one-half or one-quarter of the compartments irrigated decreased during the afternoon due to reduced stomatal conductance in response to simulated depletion of soil water adjacent to the roots. The model indicates that partial rootzone irrigation may limit plant transpiration (with implicit plant water stress), especially if rooting volumes are small and soils are sandy with low water-holding capacity.