20 - Estimating citrus orchard canopy resistance from measurements of actual and potential transpiration

Summary

SUMMARY

The relationship between orchard transpiration estimated by a meteorological model or by sap flow in the trunk and canopy conductance was studied in a 17-year-old ‘grapefruit’ orchard. Low values of canopy conductance during most times of the day are related to the low ratio of potential to actual transpiration in this orchard. High transpiration rate was sustained even when canopy conductance was low, suggesting an important role of atmospheric evaporative demand on transpiration. Under limited soil water availability canopy conductance reached extremely low values, which resulted in reduced transpiration rate. Hourly actual transpiration rates, computed by the model, were well correlated with hourly rates of sap flow in the trunk.

INTRODUCTION

The ratio of actual (Tr) to potential (PTr) transpiration is less than unity because the leaf epidermis offers a resistance to water vapour flow. In a “Marsh” grapefruit {Citrus paradisiMacf.) orchard, the ratio was found to be relatively small (0.3) when soil water availability was not limited but it dropped to 0.2 when the soil dried out to about -80 kPa in the main root zone (Cohen, 1991). In the computation of potential transpiration, using a meteorological model, the resistance of the canopy to vapour diffusion is considered zero and this explains the above ratio of measured to potential transpiration. Numerous field and laboratory measurements of stomata have shown the relationship between transpiration and stomatal conductance (Hall & Schulze, 1982). On the other hand, several micrometeorological models have been used successfully to predict evapotranspiration for non-water-stressed vegetation without taking into account the stomatal characteristics (Kanemasu, Stone & Powers, 1976; Ritchie & Jordan, 1972).