Increased drip irrigation during heatwaves does not alter vineyard land surface temperature and evapotranspiration

The frequency, duration, and intensity of heatwaves are threatening the stability of perennial crops, including winegrapes. Winegrapes are becoming increasingly irrigated with low-pressure, low-flow micro-irrigation systems (e.g., drip, micro-sprinklers) to address water scarcity. Heatwaves cause both heat and water stress in wine grapes, so growers commonly increase irrigation to cope with heatwaves. However, the effects of this practice on vineyard responses have not been extensively studied. We characterized biometeorological and physiological responses to different drip irrigation treatments surrounding three heatwaves in 2021 from the plant to vineyard scale using a combination of remote and proximal sensing approaches – satellite, drone, and tower-based methods – to quantify land surface temperature (LST) and actual evapotranspiration (ET a ). We further separated LST into canopy and soil components using a segmentation approach. Additionally, we measured soil water content and leaf-level vine responses including stomatal conductance, net photosynthesis, transpiration, stem and leaf water potentials, and radiometric leaf temperatures. Reflecting standard viticultural practices, supplemental drip irrigation treatments were only implemented surrounding heatwaves and included 60 % (i.e., maintaining standard deficit irrigation), 90 %, and 120 % of the estimated crop evapotranspiration. Our key findings were: (1) moderate increases in drip irrigation increased transpiration, stomatal conductance, CO 2 assimilation, and yield with maximum daily ranges between treatments of 2.21 −4.00 mmol m −2 s −1 , 0.06 −0.11 mol m −2 s −1 , 5.82 −7.27 µmol m −2 s −1 , and 16.3 −19.0 kg vine −1 , respectively. However, these physiological responses did not translate to differences in LST and ET a , which did not vary between treatments by more than 1.5 °C or 1 mm day −1 , respectively; (2) LST and ET a patterns followed the same ranking by treatment throughout the season, which did not align with or respond to differential drip irrigation treatments; (3) Soil surface temperatures also did not align with or respond to differential drip irrigation treatments. Findings from this study suggest that increased drip irrigation addresses vine water stress and increases leaf transpiration during heatwaves but does not provide the evaporative cooling benefits demonstrated by high-pressure, high-flow irrigation systems (e.g., center pivot irrigation). These findings can be used to inform optimal irrigation and heat management practices for agriculture during heatwaves. • Increased irrigation during heatwaves did not alter land surface temperature or ET a . • Vineyard evaporative cooling was not affected by heatwave irrigation treatments. • Additional heatwave irrigation may mitigate water stress but not heat stress. • Grapevine physiology measurements, yield, and berry quality varied with irrigation. • There are diminishing returns with increasing irrigation during heatwaves.