Detecting crop water requirements indicators in irrigated agro-ecosystems from soil water content profiles: an application for a citrus orchard

For annual cropping systems sensitive to water stress, such as citrus, efficient water management can allow facing their large water consumption and enhancing crop sustainability. However, to apply water-saving strategies it is necessary to monitoring soil and/or plant water status. In the last decade, a wide number of sensors providing indirect measurements of volumetric soil water content based on soil physical properties, such as dielectric permittivity or matric potential, have been developed. Among the sensors using the frequency domain reflectometry technique, the “drill and drop” (Sentek, Inc., Stepney, Australia) multi-sensor probes allow continuous acquisition of soil moisture dynamic every 10 cm starting from the soil surface; these data hide important information on root water uptake and actual crop evapotranspiration.

The objective of the paper was to analyze the temporal dynamics of soil water content profiles detected with multi-sensor probes during three years of field observations (July 2017- August 2020) in a citrus orchard, to estimate root water uptake and crop transpiration by three methodologies. Simultaneous measurements of sap fluxes and climate variables also allowed estimating the basal crop coefficient, K_cb, often considered for estimating crop water requirement.

The experiments were carried out in a 30 years-old citrus orchard (C. reticulata Blanco cv. Tardivo di Ciaculli) with trees spaced 5.0x5.0 m. The field is irrigated with a subsurface drip system installed in 2018, with two lateral pipes per plant row at 30 depth and distance of 1.1 m from the trunk. Integrated sensing methodologies supported by the Internet of Things and cloud computing technologies (Agrinet/Tuctronics, Walla Walla, WA, USA), linked with a suitable communication infrastructure, were used to acquire continuously, in real-time and from remote soil water contents and climate variables. Four soil moisture profiles corresponding to as many plants were monitored with 120 cm long drill and drop sensors installed at a distance of 30 cm from one emitter. A standard weather station (Spectrum Technologies, Inc) was also installed to acquire, once every half hour, wind speed and direction at 2 m height, solar radiation, air temperature, relative air humidity and precipitation. In both years, sap fluxes were also measured hourly on two citrus trees, by using two Granier’s thermal dissipation probes (TDP) per tree. Each hour, the difference of temperature between the upper heated and lower un-heated needles, combined with the temperature difference at night allowed to estimate the sap velocity and then the hourly sap fluxes.

The analysis evidenced the characteristic declines of soil water content after rainfall events, from which it was possible verifying that the hourly dynamic of root water uptake followed that of the corresponding sap flow sensors. Moreover, the knowledge of daily root water uptake, associated with the simultaneous values of reference evapotranspiration allowed obtaining suitable estimations of the basal crop coefficient. The proposed approach provided interesting insights into the dynamic of root water uptake of citrus trees and can represent a promising tool for precise irrigation scheduling.