2.6.1. Establishment of the Relationship between SWP and CWSI

Linear regression models were developed based on a field experiment that took place during the fruit growth stage III of season 2018. Different irrigation levels were applied to the three plots in the orchard to create a range of soil water contents and respective plant water status. A campaign, including stem water potential (SWP) measurements of five trees per plot (n = 15) and thermal imaging, took place on 05 August 2018 during stage III. Plant water status was evaluated by measuring SWP using a Scholander-type pressure chamber (Arimad, MRC Ltd., Holon, Israel). Two shaded leaves were covered with an aluminum foil zip-lock bag 1.5 h before the measurement. Measurements were performed on each measurement tree between the hours of 12:30–15:15, and the results were averaged per tree.

For each canopy extraction method (2PE, ED, VS, and RGB-BM), CWSI\_T100% and CWSI\_T33% were calculated per measurement tree using the method described in the previous section. Linear regression models were created using the SWP measurements and the CWSI calculations and evaluated using the following parameters: correlation coefficient (R2), root-mean-square error (RMSE), *p*-value, and the lower and upper 95% confidence intervals of the intercept and slope using the JMP statistical software (JMP Inc., Cary, NC, USA).
