A growing international human population and rising living standards are increasing the demand for agricultural products. Under higher pressure over natural resources, environmental concerns are increasing as well, challenging current water use decision-making processes in irrigated agriculture. Higher agricultural productivity means water should be applied more efficiently, which requires instant information on weather, soil, and plant conditions throughout the growing season. An information-based irrigation scheduling application tightened to the spatiotemporal variability of the fields is critical for enhancing the current irrigation system and making better irrigation scheduling decisions. The aim of this study is to review current irrigation scheduling methodologies based on two case studies (woody and field crops) located in semi-arid areas of Southeast Spain. We realize that optimal irrigation programming requires consistent investment in equipment, expenditure on operation and maintenance, and qualified technical and maintenance services. These technological approaches will be worthwhile in farms with low water availability, high profitability, and significant technical-economic capacity.

Patterns of root abscisic acid (ABA) accumulation ([ABA]_root), root water potential (Ψ_root), and root water uptake (RWU), and their impact on xylem sap ABA concentration ([X-ABA]) were measured under vertical partial root-zone drying (VPRD, upper compartment dry, lower compartment wet) and horizontal partial root-zone drying (HPRD, two lateral compartments: one dry, the other wet) of potato (Solanum tuberosum L.). When water was withheld from the dry compartment for 0–10 d, RWU and Ψ_root were similarly lower in the dry compartment when soil volumetric water content dropped below 0.22cm³ cm^–3 for both spatial distributions of soil moisture. However, [ABA]_root increased in response to decreasing Ψ_root in the dry compartment only for HPRD, resulting in much higher ABA accumulation than in VPRD. The position of the sampled roots (~4cm closer to the surface in the dry compartment of VPRD than in HPRD) might account for this difference, since older (upper) roots may accumulate less ABA in response to decreased Ψ_root than younger (deeper) roots. This would explain differences in root ABA accumulation patterns under vertical and horizontal soil moisture gradients reported in the literature. In our experiment, these differences in root ABA accumulation did not influence [X-ABA], since the RWU fraction (and thus ABA export to shoots) from the dry compartment dramatically decreased simultaneously with any increase in [ABA]_root. Thus, HPRD might better trigger a long-distance ABA signal than VPRD under conditions allowing simultaneous high [ABA]_root and relatively high RWU fraction.

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.