Use of thermal and visible imagery for estimating crop water status of irrigated grapevine

Achieving high quality wine grapes depends on the ability to maintain mild to moderate levels of water stress in the crop during the growing season. This study investigates the use of thermal imaging for monitoring water stress. Experiments were conducted on a wine-grape (Vitis vinifera cv. Merlot) vineyard in northern Israel. Irrigation treatments included mild, moderate, and severe stress. Thermal and visible (RGB) images of the crop were taken on four days at midday with a FLIR thermal imaging system and a digital camera, respectively, both mounted on a truck-crane 15 m above the canopy. Aluminium crosses were used to match visible and thermal images in post-processing and an artificial wet surface was used to estimate the reference wet temperature (T_wet). Monitored crop parameters included stem water potential (Ψ_stem), leaf conductance (g_L), and leaf area index (LAI). Meteorological parameters were measured at 2 m height. CWSI was highly correlated with g _L and moderately correlated with Ψ_stem. The CWSI-g_L relationship was very stable throughout the season, but for that of CWSI-Ψ_stem both intercept and slope varied considerably. The latter presumably reflects the non-direct nature of the physiological relationship between CWSI and Ψ_stem. The highest R² for the CWSI to g_L relationship, 0.91 (n=12), was obtained when CWSI was computed using temperatures from the centre of the canopy, T_wet from the artificial wet surface, and reference dry temperature from air temperature plus 5 °C. Using T_wet calculated from the inverted Penman-Monteith equation and estimated from an artificially wetted part of the canopy also yielded crop water-stress estimates highly correlated with g _L (R²=0.89 and 0.82, respectively), while a crop water-stress index using 'theoretical' reference temperatures computed from climate data showed significant deviations in the late season. Parameter variability and robustness of the different CWSI estimates are discussed. Future research should aim at developing thermal imaging into an irrigation scheduling tool applicable to different crops.