Crop canopy temperature as indicator of water stress: Application to grapevines

Two methods were investigated. Firstly the energy balance of wet canopies was solved for absorbed shortwave radiation and corresponding dry canopies were modelled. Secondly, wet canopies temperatures were used as wet references and dry canopies modelled using a simplified method (Moller et al., 2007). In all cases, CWSI, Ig and stomatal resistance were computed and physiological thresholds were applied. Thermal measures were carried out four times daily for nine successive days. All indices were evaluated against measures of leaf, xylem and pre-dawn water potentials.The most mechanistic method was inappropriate for irrigation scheduling purposes due to the difficulty in appropriately measuring wind speed. The CWSI was found to yield similar information to Ig but with a significantly reduced coefficient of variation. The differential of CWSI (ΔCWSI) between the exposed and shaded sides of the canopies was found to correlate best with midday stem water potential when thermal measures were carried out at 9 am (R2 = 0.76, p<0.05). A concurrent analysis of the thermal indices, midday stem water potential measurements and reference evapotranspiration yielded more information than any of the individual measures. In particular, the release from water stress observed on days 6 and 7 was explained in terms of optimisation of stomatal resistance whilst maintaining stem water potential within the isohydric threshold of Cabernet Sauvignon.Sources of errors inherent to the methods are analysed and recommendations for irrigation scheduling and further research are made.