This study aimed to analyze the average air temperature data estimated by ERA5-Land reanalysis over 10 years (01/01/2011 to 12/31/2020), based on data from 12 automatic weather stations located in different cities in the state of Pernambuco, northeast of Brazil. For more careful evaluation, the average air temperature data were stratified by mesoregions of the state. ERA5-Land reanalysis data were validated using statistical indices that evaluated the accuracy. The average air temperature estimated by ERA5-Land reanalysis agree well with weather stations in almost the entire state of Pernambuco. The highest accuracy of the average air temperature estimated by ERA5-Land occurred in the city of Ibimirim (R2 = 0.98), while the lowest accuracy was measured in the city of Caruaru ( R2 = 0.57). Rootmean-square error (RMSE) generated by the ERA5-Land reanalysis was lower than 0.60 °C in most Pernambuco. The highest RMSE of ERA5-Land for average air temperature was calculated using data from the city of Caruaru (1.11 °C), whereas the lowest RMSE was obtained with data from the city of Recife (0.41 °C). According to mean bias errors, (MBE) the ERA5-Land underestimated the average air temperature data compared to automatic weather stations data, especially in Ouricuri, which had the highest underestimation (−0.80 °C). On the other hand, among the municipalities where ERA5-Land overestimated the temperature values, the highest overestimation was identified in Garanhuns (0.35 °C). Therefore, based on the results of this study, ERA5-Land reanalysis successfully estimated the average air temperature for the state of Pernambuco.

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Accurate estimations of actual crop evapotranspiration are of utmost importance to evaluate crop water requirements and to optimize water use efficiency. At this aim, coupling simple agro-hydrological models, such as the well-known FAO-56 model, with remote observations of the land surface could represent an easy-to-use tool to identify biophysical parameters of vegetation, such as the crop coefficient Kc under the actual field conditions and to estimate actual crop evapotranspiration. This paper intends, therefore, to propose an operational procedure to evaluate the spatio-temporal variability of Kc in a citrus orchard characterized by the sporadic presence of ground weeds, based on micro-meteorological measurements collected on-ground and vegetation indices (VIs) retrieved by the Sentinel-2 sensors. A non-linear Kc(VIs) relationship was identified after assuming that the sum of two VIs, such as the normalized difference vegetation index, NDVI, and the normalized difference water index, NDWI, is suitable to represent the spatio-temporal dynamics of the investigated environment, characterized by sparse vegetation and the sporadic presence of spontaneous but transpiring soil weeds, typical of winter seasons and/or periods following events wetting the soil surface. The Kc values obtained in each cell of the Sentinel-2 grid (10 m) were then used as input of the spatially distributed FAO-56 model to estimate the variability of actual evapotranspiration (ETa) and the other terms of water balance. The performance of the proposed procedure was finally evaluated by comparing the estimated average soil water content and actual crop evapotranspiration with the corresponding ones measured on-ground. The application of the FAO-56 model indicated that the estimated ETa were characterized by root-mean-square-error, RMSE, and mean bias-error, MBE, of 0.48 and -0.13 mm d−1 respectively, while the estimated soil water contents, SWC, were characterized by RMSE equal to 0.01 cm3 cm−3 and the absence of bias, then confirming that the suggested procedure can produce highly accurate results in terms of dynamics of soil water content and actual crop evapotranspiration under the investigated field conditions.

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<p>Optical and thermal sensors installed on Unmanned Aircraft Systems (UAS) can be considered a technological innovation for precision farming. The visible and thermal regions of the electromagnetic (EM) spectrum provide useful information to assess the quality of crop growth and monitor plant water status. Accurate measurements of plant water status with high-resolution thermal images associated with high-efficiency irrigation systems can be a suitable solution to improve energy and water saving.</p> <p>The objective of this work was to estimate and compare the Crop Water Stress Index (CWSI) obtained in a citrus orchard irrigated with two different irrigation systems, by using a UAS equipped with a thermal camera.</p> <p>The experiment was carried out in a commercial citrus orchard located in the Northwest of Sicily, Italy, during the irrigation season of 2022. Optical and thermal high-resolution images were acquired at noon on August 23 and 25, and September 2 over two plots, the first of which was irrigated with a subsurface drip irrigation (SDI) and the second with a micro-sprinkler (MSI). Hourly crop reference evapotranspiration, ETo, and Vapour Pressure Deficit (VPD) were calculated by using the weather variables measured by a standard weather station installed in the field, while the plant water status was monitored at an hourly time scale, through three microtensiometers (FloraPulse, Davis, CA) embedded into the woody tissue of trees considered representative of the two irrigation systems. For each thermal image, characterized by a thermal spatial resolution of 15 cm,  soil pixels were initially removed; then, the dry and wet reference temperatures, T<sub>dry</sub> and T<sub>wet</sub>, were estimated as the 0.5 and 99.5 percentiles of the canopy temperature. The values of CWSI were finally calculated based on the maximum T<sub>dry</sub> and minimum T<sub>wet</sub> obtained in the two plots during the examined days.</p> <p>Vapor pressure deficit and crop reference evapotranspiration resulted in quite similar values in the three days, with hourly VPD and ETo at noon ranging between 1.49 and 1.65 kPa, and between 0.50 and 0.62 mm, respectively. Irrigation heights provided in the examined period resulted equal to 65 mm in a single application in the MSI plot and 48 mm, equally distributed in eight irrigation events, in the SDI plot. In the latter plot, the values of daily stem water potential ranged between -0.5 and -1.1 MPa during the entire period with values of the corresponding CWSI between 0.22 and 0.28; on the other hand, in the plot irrigated with the MSI system the values tended to decline to a daily range between -1.1 and -1.3 MPa as a consequence of the soil drying between consecutive waterings with values of CWSI ranging between 0.30 and 0.34. The analysis showed that both plots were characterized by low water stress levels. However, despite the lower irrigation volume supplied by the SDI system, the values of CWSI resulted always lower than those obtained under the MSI system, confirming the potential of the SDI system to improve water use efficiency. </p>