Low and variable atmospheric coupling in irrigated Almond (Prunus dulcis) canopies indicates a limited influence of stomata on orchard evapotranspiration

The degree of coupling to the environment of almond (Prunus dulcis) orchards during periods of transient water stress was investigated in a two-year study in California. Plant water status was monitored weekly, before and/or after irrigation, measuring midday stem water potential (Ψstem) that ranged from −0.5 to −2 MPa, while actual evapotranspiration (ETa) was measured with an eddy covariance tower. Irrigation was applied weekly following common commercial practice, resulting in weekly cycles of Ψstem. Despite Ψstem reaching levels shown to induce substantial stomatal closure, the ratio actual to reference evapotranspiration (ETa/ETo = Ka) did not show a decrease during weekly periods of low Ψstem in the two years of the study. Midday average canopy surface resistance (rcmid), computed by reversing the Penman-Monteith equation from eddy covariance ET data, yielded a statistically significant increase with a decrease in Ψstem, but just in the first year of the study. However, rcmid did not show a significant relationship with stomatal resistance measured at the leaf level with porometry and scaled-up to the canopy level. In the first year, rcmid showed a sharp increase after harvest, when Ka also decreased, perhaps produced by the composite effect of defoliation associated with harvest and stomatal closure associated with water stress. During the growing season, rcmid ranged from 0 to 100 s m−1 and midday average aerodynamic resistance (ramid) ranged between 0 and 50 s m−1. Despite rcmid being generally larger than ra, the midday average decoupling factor (Ω) averaged 0.7 during the irrigation season, indicating decoupled conditions. However, there was a large day to day fluctuation of midday Ω ranging from 0.16 to 0.98 mostly associated with rcmid and wind speed. This study indicated that tall and rough canopies can be relatively decoupled depending on the effect of wind speed and canopy resistance on the decoupling factor. From a water management point of view, this result suggests that inducing transient mild to moderate water stress may not produce substantial water savings in areas having low to moderate winds.