*1.1. Crop Water Status Estimation for Precision Irrigation Management*

Crop water stress index (CWSI) is an indirect measurement of crop water status derived from a thermal image. Absolute canopy temperature is a function of stomata opening and cooling by subsequent crop transpiration and is affected by meteorological factors, including ambient temperature, vapor pressure, wind speed, and radiation [3]. To compare thermal images and eliminate the need to measure all of the meteorological parameters, normalization of canopy temperature via CWSI was proposed as a proxy of crop water status [4,5]:

$$\text{CWSI} = \frac{T\_{canopy} - T\_{\text{wet}}}{T\_{dry} - T\_{\text{wet}}} \tag{1}$$

where *Tcanopy* is the temperature of the canopy, *Twet* is the temperature of a fully transpiring canopy, and *Tdry* is the temperature of a non-transpiring (stressed) canopy. CWSI ranges from zero to one, where higher values indicate higher water stress. The difference between *Tcanopy* ( ◦C) and *Tair* ( ◦C) is dependent on vapor pressure deficit (VPD) [4,5]. *Tdry* is typically calculated using an empirical method [6,7], while *Twet* is determined by employing empirical, theoretical, and statistical methods [6,8] or, for commercial plot scale, by calculating the average temperature of the coolest 5–10% of canopy pixels of each individual thermal image [7–9].

The calculation of *Tcanopy* involves two steps. First, canopy pixels need to be extracted from the image and separated from non-canopy pixels, including "mixed pixels" (combinations of canopy, soil, weeds, foreign objects, and shade). The second step is the calculation of canopy temperature. A common approach for calculating *Tcanopy* of an area of interest (a whole plot or a management zone) is by using the mean [10] or the median [7] temperature of extracted canopy pixels. Meron et al. [6] proposed using the coldest 33% of canopy pixels for the calculation. Cohen et al. [8] reported an over-estimation of water stress in cotton with the mean of all canopy extracted pixels. When the mean of the coldest 33% was used, water status was better estimated. No such comparison between the approaches used for calculating canopy temperature was found for orchards.
