Transpiration of an apple orchard in a cool humid climate: Measurement and modeling

For irrigation design and scheduling, water use of crops is commonly estimated from grass reference evapo-transpiration (ETo) multiplied by published crop coefficients (Kc). This method is assumed to adjust crop values in different climates. However, the simple application of Kc may not be accurate in cool, humid climate, especially for tall crops well coupled to bulk air. The aim of this work was to measure actual transpiration in an apple orchard in New York, and to test the values against published Kc values. Measurements of water use in dwarf apple trees were made with heat pulse sap flow gauges calibrated with whole-tree gas exchange chambers (to check or correct potentials errors related to the velocity-to-flow assumptions). Daily ETo was estimated from meteorological data acquired nearby. Published Kc values generally overestimated the measured water use rates, suggesting the inadequacy of using Kc values directly from arid climates in our cool and humid climate. Also, the crop coefficient concept implies that grass and orchards behave the same in all climates; this does not seem to be true in cool humid climates. Grass-to-tree differences in advective flows, boundary layer conditions and stomatal regulation me be the reason for variation in ET ratios. Modifications of the Penman-Montieth equation were made to take into account the specific characteristics of apple tree canopy (like light interception and coupling to the bulk air), and stomata regulation by light and VPD. Results are in good agreement with field measures.