A comprehensive review of automation of irrigation based on volumetric soil water content (VSWC) in the framework of IoT (Internet of Things) is presented. The fundamentals of electromagnetic sensors based on soil dielectric permittivity and the techniques used for measuring the VSWC are briefly described. Factors affecting sensor performance that have to be considered for selecting the appropriate sensor along with a SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis are outlined. Special attention must be paid to the small soil volume explored by these sensors, installation accuracy, calibration, power supply and consumption and the effects of salinity on the soil water content. Since it is connected to a telemetry system, a wireless sensor network should include robust transmission units, energy-efficient processor, flexible configuration of the I/O ports, long-life battery, and a friendly software platform. A bidirectional wireless network layout allows sensor activity to be monitored, acts on solenoid valves to trigger irrigation (based either on direct VSWC values or algorithms) and provides real-time feed-back information of the soil-plant-atmosphere continuum. As revealed by the field research studies, significantly higher water, energy and labour savings were possible using automated irrigation based on VSWC sensors than with conventional irrigation scheduling based on computed evapotranspiration.

Physiological plant water status indicators are useful for managing precision irrigation in regions with limited water resources. The aim of this work was to evaluate the effect of shade netting on the diurnal and seasonal variations of several plant water status indicators in young lime trees (Citrus latifolia Tan., cv. Bearss), grown at the CEBAS-CSIC experimental station in Murcia, Spain. Stem water potential (Ψ_stem), leaf gas exchange (net photosynthesis (P_n) and stomatal conductance (g_s)), and canopy temperature (T_c) were measured on representative days of winter and summer. The Ψ_stem daily pattern was quite similar in both seasons under both conditions. However, the circadian rhythm of leaf gas exchange was affected by shade conditions, especially in summer, when shaded leaves showed maximum g_s values for a longer time, allowing higher net photosynthesis (37%). Canopy temperature behaved similarly in both conditions, nevertheless, lower values were recorded in open-air than in shaded trees in the two seasons. The canopy-to-air temperature difference (T_c − T_a), however, was lower in shaded trees during the daylight hours, indicating the higher degree of leaf cooling that was facilitated by high g_s values. The possibility of continuously recording T_c makes it (or the proposed canopy thermal index, CTI) a promising index for precise irrigation scheduling. Shade netting was seen to favour gas exchange, suggesting that it may be considered alternative to open-air for use in semi-arid areas threatened by climate change.

The aim of this study was to test the combined effect of water stress and cropping system on yield and fruit quality in Bearss lime trees. For this purpose, two irrigation treatments were applied during stage II of fruit growth: control (well irrigated, automatically managed by soil water content sensors) and stress (non-irrigated), both under open-field and shaded conditions. Soil water status was assessed by determining soil water content and plant water status by measuring stem water potential (Ψ_stem), stomatal conductance (g_s), and net photosynthesis (P_n). Yield parameters (kg and the number of fruits per tree and fresh mass per fruit) and fruit quality were assessed on two harvest dates. In addition, on the second harvest date, the content of metabolites and nutrients in the lime juice was analyzed. The results showed that soil water deficit induced 35% lower g_s values in open-field than in shaded conditions. The highest kg and the number of fruits per tree were observed in the shaded system, especially on the first harvest date. The lowest yield was observed in stressed trees grown without netting. Slightly higher fresh mass and equatorial diameter were observed in shaded fruits than in open-field fruit. Soil water deficit increased fruit total soluble solids and decreased juice content, especially in open-field trees. Shaded conditions made the lime trees more resilient to soil water deficit, which led to higher yields and better external fruit quality traits. In addition, fruit precocity was significantly higher in the shaded system.

This study evaluated the effect of (i) irrigation water source: transfer (TW) and reclaimed water (RW), and (ii) crop phenological stage: winter rest (WR), flowering-sprouting (FS), and fruit growth (FG), on NO₃⁻ and K⁺ dynamics in soil and leaf of a citrus crop. The experiment was carried out during the 2018 and 2019 growing seasons on adult ‘Star Ruby’ grapefruit trees (Citrus paradisi Macf.). The concentration of both nutrients was periodically measured in soil and leaf samples and continuously monitored in the soil soluble fraction with nutrient sensors. Moreover, soil NO₃⁻ leaching was indirectly estimated by the periodic measurement of the leaf enrichment in ¹⁵N isotope (¹⁵N_leaf). The two water sources showed a different nutrient loading. Thus, NO₃⁻ and K⁺, were approximately 5 and 7 times higher, respectively, in the RW. Furthermore, the average contents of NO₃⁻ and K⁺ in the soil samples from the RW treatment were 10.1 and 19.7%, respectively, higher than in TW, with the highest soil NO₃⁻ leaching observed in RW treatment. In line with this, the mean contents of NO₃⁻ and K⁺ in the leaves from the RW treatment were 106.9 and 30.4% higher than the TW ones. As for the different phenological stages, in the FG stage, the lowest concentrations of NO₃⁻ in the soil samples and the highest in the leaf tissue were observed after a high soil leaching event. In this study the nutrient sensors measurements varied according to the dynamic of NO₃⁻ and K⁺ in the soil samples. The use of RW promoted an accumulation of NO₃⁻ and K⁺ in the soil and leaves of grapefruit trees, but also enhanced soil NO₃⁻ leaching, indicating that the proper management of this water source is necessary to avoid soil contamination. The mobilization of NO₃⁻ and K⁺ from soil to leaf was the highest in the FG stage, to ensure fruit development and vegetative growth.

This field experiment focuses on the effects of a heavy rainfall event (DANA, depresión aislada en niveles altos) that occurred on 12–14 September 2019 (DOY, Day of the year, 255–257), in southern Spain on plant water status and the thermal response of nectarine trees. Two irrigation treatments were applied during the summer–autumn postharvest period (DOY 158–329): full-irrigated (CTL) and non-irrigated (DRY). Volumetric soil water content (θ_v), air temperature (Ta) and canopy temperature (Tc) were monitored in real-time and the crop water stress index (CWSI) was calculated. The difference in Tc between the DRY and CTL treatments (Tc’ − Tc) is proposed as a new thermal indicator. Stem water potential (Ψ_stem) and leaf gas exchange measurements were recorded on representative days. During the DANA event, only the Tc measured by the infrared radiometer sensors could be monitored. Therefore, the effects of the DANA forced the soil water content sensors to be switched off, which prevented Ψ_stem and leaf gas exchange determinations from DOY 255 to 275. Before the DANA event, withholding irrigation caused a gradual decrease in the soil and plant water status in the DRY treatment. Significant differences appeared between treatments in the studied thermal indexes. Moreover, Tc’ − Tc was more sensitive than Tc − Ta in assessing nectarine water stress. The effects of the DANA reduced these differences, suggesting different baselines for the calculation of CWSI. In this respect, the relationship Tc − Ta vs. VPD improved the coefficient of determination after the DANA event in full-irrigated trees. Similar values of Ψ_stem and leaf gas exchange were found in both treatments after the DANA event, even though thermal indexes showed some significant differences. In addition, the strong relationship found between Tc − Ta and CWSI vs. Ψ_stem worsened after DANA occurred, revealing a lower sensitivity of Ψ_stem compared to canopy temperature to accurately assess nectarine water status in these saturated soil conditions. This research underlined the robustness of infrared thermography to continuously monitor plant water status under these extreme weather conditions.

In this work was evaluated the capability to detect water stress of different water status indicators in young lime trees (Citrus latifolia Tan., cv. Bearss) cultivated in ridges and without ridges at the field station of CEBAS-CSIC in Murcia. Two irrigation treatments were applied: Control (well irrigated) and Drought (withholding irrigation). Soil water status was determined continuously by measure soil water content (SWC) and soil matric potential (SMP). Plant water status was assessed by measuring the stem water potential (Ψstem) and gas exchange. Plant and soil water status not showed significant differences between the crop systems evaluated. Trees subjected to drought showed a progressive decrease of the SWC, SWP and Ψstem. Cultivation on ridges did not significantly affect the behavior of the indicators studied. Real time soil water status allows identifying the beginning and the recovery of the plant water stress. The combined use of soil water content and soil matric potential allows to manage the irrigation in precision.