Investigation of low-cost infrared thermal sensors for monitoring water stress in grapevines.

Thesis (MEng)--Stellenbosch University, 2021.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.AFRIKAANSE OPSOMMING: Infrarooi (IR) meting van wingerd lower temperatuur is ’n effektiewe manier om plant waterspanning te bepaal. Die gewas waterspanningsindeks (GWSI), is ’n genormaliseerde waterspanning waarde wat vanuit IR metings van wingerd tem- peratuur bepaal kan word. Normaliserende verwysingstemperature, onseker- heid oor IR sensor vereistes en ongestandaardiseerde meting prosedures is be- lemmerend tot die aanneming van IR temperatuur meting vir die bepaling van plant waterspanning in wingerde. Om die GWSI verwysing temperature te verbeter, is fisiese metings in ’n kom- mersiële Cabernet Sauvignon wingerd gebruik om, deur middel van optimering, ’n hitte oordrag (HO) en empiriese (EMP) model te ontwikkel wat hierdie tempe- rature kan bepaal. Die HO model is afgelei vanaf fundamentele hitte oordrag beginsels en gebruik weerstasie data, terwyl die EMP model slegs omgewings temperatuur en humiditeit data benodig. Die HO en EMP modelle kon die ver- wysingstemperature tot onderskeidelik 1.0 K en 1.5 K van die fisies gemete tem- perature bepaal. Lower temperature gemeet deur lae koste veelvuldige- en enkel-area IR sen- sors is vergelyk met metings van ’n kommersiële IR kamera op veskillende af- stande. Die lae koste veelvuldige- area sensor wat ondersoek is, kon nie akkurate waardes vir die GWSI bepaal nie. Die enkel-area sensors kon wel temperature vergelykbaar met die van die kommersiële IR kamera meet. Nie een van die goed- koop sensors kon die GWSI verwysings temperature akkuraat genoeg meet nie. Dit is bepaal dat die spektrale gebied wat ’n IR sensor meet ’n merkwaardige effek het op die gemete lower temperture, veral as die wingerd in direkte sonlig is. Laastens is die beste fisiese toestande vir die meting van GWSI met IR sen- sors ondersoek. Daar is gevind enkel-area lae koste sensors die GWSI akkuraat kon bepaal solank die besigtigingsarea van die sensors in ag geneem word. Daar is verder bepaal dat die skadukant van die lower, een uur na sonmiddag, die beste geleentheid bied vir die meting van lower temperature vir die GWSI. Hierdie kant van die lower werk goed omdat dit vir die heel oggend aktief gefotosinteer het. Daar is gevind dat ’n enkel-area sensor gepaard met die EMP metode om verwy- sings temperature te bepaal ’n effektiewe, goedkoop en maklike metode is om GWSI in wingerde te bepaal.Master