*2.2. Agro-Meteorological Measurements*

Agro-meteorological measurements of the reference evapotranspiration (ET0) and actual evapotranspiration (ETc) were performed to derive the crop coefficient (Kc) as Kc = ETc/ET0. Kc is an important variable used to determine the irrigation dose [69]. Daily ETc was derived from water vapor flux measurements by eddy covariance systems [6,70]. The daily ET0 was calculated according to the Penman–Monteith method [33] based on meteorological measurements of air temperature, relative humidity, wind speed, and solar irradiance at the meteorological station closest to the field or at the flux tower itself (Table 2). Meteorological station data are publicly available at http://www.meteo.co.il/ (accessed on 24 June 2021) and http://www.mop-zafon.net/ (accessed on 24 June 2021). The Kc data used for developing the processing tomato models were smoothed in Python

with the SciPy library using the cubic and the second-order splines. Smoothing spline is a non-parametric regression technique, which was previously used in various remote sensing applications [15,71–73]. ETc was measured throughout the growing seasons of wheat and processing tomatoes and from the middle of the cotton growing seasons. ETc data collected during the Gadash processing tomato experiment in 2018 and the Yavne 2019 wheat experiment were not used for the Kc model development because more than half the data were lost due to technical difficulties that emerged during the experiments.

**Figure 1.** Locations of analysis polygons in the experiments conducted in Israel in 2016–2019.
