Agricultural Water Management 196 (2018) 57–65 Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat Research Paper Low and variable atmospheric coupling in irrigated Almond (Prunus dulcis) canopies indicates a limited influence of stomata on orchard evapotranspiration Gerardo M. Spinelli a,∗ , Richard L. Snyder b , Blake L. Sanden c , Matthew Gilbert a , Ken A. Shackel a a University of California, Davis, Dept. of Plant Sciences, One Shields Ave, Davis, CA 95616-8683, USA b University of California, Davis, Dept. of Land, Air and Water Resources, One Shields Ave, Davis, CA 95616-8683, USA c University of California Cooperative Extension, 1031 S. Mt. Vernon Ave., Bakersfield, CA 93307, USA a r t i c l e i n f o Article history: Received 31 December 2016 Received in revised form 11 October 2017 Accepted 24 October 2017 Keywords: Omega factor Decoupling Aerodynamic resistance Eddy covariance Water stress Midday stem water potential a b s t r a c t The degree of coupling to the environment of almond (Prunus dulcis) orchards during periods of tran- sient 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 ( stem ) that ranged from −0.5 to −2 MPa, while actual evapotranspiration (ET a ) was measured with an eddy covariance tower. Irri- gation was applied weekly following common commercial practice, resulting in weekly cycles of  stem . Despite  stem reaching levels shown to induce substantial stomatal closure, the ratio actual to refer- ence evapotranspiration (ET a /ET o = K a ) did not show a decrease during weekly periods of low  stem in the two years of the study. Midday average canopy surface resistance (r cmid ), computed by reversing the Penman-Monteith equation from eddy covariance ET data, yielded a statistically significant increase with a decrease in  stem , but just in the first year of the study. However, r cmid did not show a significant rela- tionship with stomatal resistance measured at the leaf level with porometry and scaled-up to the canopy level. In the first year, r cmid showed a sharp increase after harvest, when K a also decreased, perhaps pro- duced by the composite effect of defoliation associated with harvest and stomatal closure associated with water stress. During the growing season, r cmid ranged from 0 to 100 s m −1 and midday average aerody- namic resistance (r amid ) ranged between 0 and 50 s m −1 . Despite r cmid being generally larger than r a , the midday average decoupling factor () averaged 0.7 during the irrigation season, indicating decoupled conditions. However, there was a large day to day fluctuation of midday  ranging from 0.16 to 0.98 mostly associated with r cmid and wind speed. This study indicated that tall and rough canopies can be rel- atively 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. © 2017 Published by Elsevier B.V. Abbreviations: Cp, air heat capacity at constant pressure; ea, air vapor pressure; e s(Ta) , saturated vapor pressure at air temperature; e s(Ts) , saturated vapor pressure at surface temperature; ETa, actual evapotranspiration; ETo, reference evapotranspira- tion; G, ground heat flux; H, sensible heat flux; Ka, crop coefficient ratio of actual to reference ET; LAI, leaf area index; LE, latent heat flux; r cmid , midday average canopy surface resistance; r amid , midday average aerodynamic resistance; R i , input radia- tion; Rn, net radiation; Ta, air temperature; uz, wind speed at height z; u*, friction velocity; , psychrometric constant; , slope of the vapor pressure and temperature relation; , surface emissivity; , air density; , stefan-boltzmann constant; stem, midday stem water potential; , midday average decoupling factor. ∗ Corresponding author. E-mail addresses: gspinelli@ucdavis.edu (G.M. Spinelli), rlsnyder@ucdavis.edu (R.L. Snyder), blsanden@ucdavis.edu (B.L. Sanden), megilbert@ucdavis.edu (M. Gilbert), kashackel@ucdavis.edu (K.A. Shackel). 1. Introduction Almonds are the second largest crop of California by acreage with 328 thousand hectares (810 thousand acres) estimated in 2013, and produced a value of $3.387 billion in 2012 (Almond Board of California). Essentially all commercial almond orchards are irri- gated, making almonds the second largest crop of the State in terms of water use (California Department of Water Resources). Water stress occurs in commercial orchards as a result of water shortage, but short periods of water stress can also be induced as a man- agement tool to control vegetative growth, reduce fungal diseases at hull split, and facilitate harvest operations. In a previous study (Spinelli, 2016) we showed that a substantial reduction (about https://doi.org/10.1016/j.agwat.2017.10.019 0378-3774/© 2017 Published by Elsevier B.V.