Using unmanned aircraft system to estimate crop water stress index in a citrus orchard under different irrigation systems

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.

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.

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_dry and T_wet, 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_dry and minimum T_wet obtained in the two plots during the examined days.

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.