Browsing by Author "Luus, J."

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    Investigation of low-cost infrared thermal sensors for monitoring water stress in grapevines.
    (Stellenbosch : Stellenbosch University, 2021-12) Luus, J.; Els, D. N. J.; Poblete, C. A.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Infrared (IR) measurement of canopy temperature is an effective method of de- termining plant water stress. The crop water stress index (CWSI) is a normalised plant water stress value that can be calculated from vineyard IR measurements. Normalising reference temperature measurements, uncertainty of IR sensor re- quirements and unstandardised physical measurement parameters are, however, a hindrance to the adoption of IR temperature measurement for plant water stress detection in vineyards. To improve the CWSI reference temperatures, physical measurements in a commercial Cabernet Sauvignon vineyard were used to develop a heat transfer (HT) model and simple empirical (EMP) model for determining these temper- atures by using optimisation. The HT model is derived from fundamental heat transfer principles and uses data from a commercial weather station, while the EMP model requires only ambient temperature and humidity data. The HT and EMP models could calculate reference temperatures to within 1.0 K and 1.5 K of physical measurements, respectively. Low-cost IR array and single-area sensor canopy temperature measurements were compared to image analysed commercial IR camera measurements. The investigated low-cost IR array sensor was found to be unsuitable for CWSI mea- surements, while single-area sensors could measure temperatures similar to those of the IR camera. All low-cost sensors were, however, unable to accurately mea- sure CWSI reference temperatures. It was determined that a sensors’ spectral ranges have a significant effect on the measured canopy temperature, especially when measuring sunlit canopies. Finally, the best physical conditions for measuring CWSI using different sen- sor types were investigated. It was found that low-cost single-area sensors could measure CWSI accurately, as long as the sensor’s field of view was taken into ac- count. Furthermore, it was found that the shaded side of the canopy, measured one hour after solar noon, provided the best midday plant temperature measure- ments for all investigated sensors as it is more photosynthetically active from the morning sun. Using a single-area IR sensor for canopy temperature measure- ments along with the EMP method to determine the reference temperatures was found to be an effective low-cost and low-effort method of measuring CWSI in vineyards.