Plant-Based Methods for Irrigation Scheduling of Woody Crops
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horticulturae Review Plant-Based Methods for Irrigation Scheduling of Woody Crops José Enrique Fernández Group on Irrigation and Crop Ecophysiology, Instituto de Recursos Naturales y Agrobiología (IRNAS, CSIC), Avenida de Reina Mercedes, 10, 41012 Sevilla, Spain; [email protected] Academic Editors: Arturo Alvino and Maria Isabel Freire Riberiro Ferreira Received: 12 January 2017; Accepted: 26 May 2017; Published: 1 June 2017 Abstract: The increasing world population and expected climate scenarios impel the agricultural sector towards a more efficient use of water. The scientific community is responding to that challenge by developing a variety of methods and technologies to increase crop water productivity. Precision irrigation is intended to achieve that purpose, through the wise choice of the irrigation system, the irrigation strategy, the method to schedule irrigation, and the production target. In this review, the relevance of precision irrigation for a rational use of water in agriculture, and methods related to the use of plant-based measurements for both the assessment of plant water stress and irrigation scheduling, are considered. These include non-automated, conventional methods based on manual records of plant water status and gas exchange, and automated methods where the related variable is recorded continuously and automatically. Thus, the use of methodologies based on the Scholander chamber and portable gas analysers, as well as those of systems for measuring sap flow, stem diameter variation and leaf turgor pressure, are reviewed. Other methods less used but with a potential to improve irrigation are also considered. These include those based on measurements related to the stem and leaf water content, and to changes in electrical potential within the plant. The use of measurements related to canopy temperature, both for direct assessment of water stress and for defining zones with different irrigation requirements, is also addressed. Finally, the importance of choosing the production target wisely, and the need for economic analyses to obtain maximum benefit of the technology related to precision irrigation, are outlined. Keywords: precision irrigation; precision agriculture; water stress; water use efficiency; water productivity 1. Precision Irrigation Precision agriculture began in the 1960s with the use of geographic information systems (GIS), probably the first precision farming tools developed. In the late 1980s and early 1990s, Global Positioning System (GPS) receivers became available for agricultural users interested in making sustainable decisions, and this was the origin of modern practices in precision farming. Within the context of precision agriculture, the aim of precision irrigation (PI) is to irrigate each plant with the right amount of water and at the right time, for optimal input use efficiency and minimum environmental impact [1]. Precision irrigation relies on an optimal combination of the irrigation system, the irrigation strategy and the irrigation scheduling method, as well as a wise choice of the production target. A huge variety of components are currently available on the market to adjust the irrigation system (furrow, sprinkling, localized ... ) to the conditions of any farm or orchard, such that the effective choice and implementation of the irrigation system is usually accomplished without major difficulties. In this work, therefore, the most demanding steps for PI, i.e., on the wise choice of the irrigation strategy, the method for the assessment of water stress and for irrigation scheduling, and the definition of the most appropriate production target, are considered. Horticulturae 2017, 3, 35; doi:10.3390/horticulturae3020035 www.mdpi.com/journal/horticulturae Horticulturae 2017, 3, 35 2 of 37 Horticulturae 2017, 3, 35 2 of 38 PrecisionPrecision irrigation irrigation usually usually implies implies thethe useuse of defi deficitcit irrigation irrigation (DI), (DI), because because of ofits itspotential potential to to increaseincrease crop crop water water productivity productivity (WP) (WP) [ 2[2].]. AchievingAchieving the greatest greatest WP WP in in a aparticular particular orchard orchard implies implies a wisea wise choice choice of theof DIthe strategy.DI strategy. A variety A variety of DI of strategies DI strategies have have been been defined, defined, according according to the to amount the andamount frequency and offrequency water supplyof water (Table supply1). (Table Regulated 1). Re deficitgulated irrigation deficit irrigation (RDI) is (RDI) especially is especially suitable forsuitable woody for crops woody [3,4], crops including [3,4], including those in hedgerowthose in hedgerow orchards orchards with high with plant high densities, plant densities, currently consideredcurrentlyamong considered the most among profitable the most management profitable management systems for fruitsystems tree crops.for fruit The tree beauty crops. of The RDI is beauty of RDI is that it takes into account changes in the sensitivity of the crop to water stress at that it takes into account changes in the sensitivity of the crop to water stress at different developmental different developmental stages (e.g., as in Figure 1). This allows for maximum benefits of the stages (e.g., as in Figure1). This allows for maximum benefits of the supplied irrigation amounts. supplied irrigation amounts. Regulated deficit irrigation, however, is challenging because it requires Regulated deficit irrigation, however, is challenging because it requires both a sound knowledge of the both a sound knowledge of the physiological response of the crop to water conditions and precise physiologicalassessment of response plant water of the stress crop levels to water along conditions the growing and cycle precise [5]. Thus, assessment precision of irrigation plant water has not stress levelsbeen along a suitable the growing approach cycle for farmers [5]. Thus, and precision orchardists irrigation until recently, has not when been enough a suitable knowledge approach on for farmersmajor andprocesses orchardists related until to recently,the impact when of envi enoughronmental knowledge conditions on major on the processes development related and to the impactproduction of environmental of crops has conditions become available, on the development and advances and in sensoring, production monitoring of crops has and become data transfer available, andhave advances been made. in sensoring, monitoring and data transfer have been made. FigureFigure 1. 1. Regulated deficit deficit irrigation irrigation strategy strategy for a forhedgerow a hedgerow olive orchard olive with orchard 1667 withtrees per 1667 ha treesin persouthwest ha in southwest Spain. The Spain. production The target production was to targetachieve was the most to achieve profitable the balance most between profitable oil balanceyield betweenand oil oil quality yield at and the oilsame quality time that at thea long same prod timeuctive that life a of long the productiveorchard was life ensured of the by orchard reducing was ensuredproblems by reducingderived from problems competition derived for fromlight competitionamong trees. SHD for light = super-high among trees. density; SHD IN == super-highseasonal irrigation needs; ETc = crop evapotranspiration; Pe = effective precipitation; IA = seasonal irrigation density; IN = seasonal irrigation needs; ETc = crop evapotranspiration; Pe = effective precipitation; amount; AW = available soil water; and i.e. = irrigation event. After Fernández et al. [6] and IA = seasonal irrigation amount; AW = available soil water; and i.e. = irrigation event. After Padilla-Díaz et al. [7]. Fernández et al. [6] and Padilla-Díaz et al. [7]. Horticulturae 2017, 3, 35 3 of 37 Table 1. Name, brief definition and key reference papers of the most-used deficit irrigation strategies. Strategy Definition and Remarks Reference Paper A single irrigation event is applied when a fixed threshold for water stress is achieved. Just 2 or 3 irrigation events in the irrigation season. Used when water Supplementary or Abd-Eel-Rahman and for irrigation is very scarce or in temperate regions with low complementary El-Sharkawi [8]; Lavee et al. [9]; evapotranspiration rates or high rainfall. It can lead to substantial increases in irrigation Proietti et al. [10] crop performance, as compared to dry-farming, if the irrigation events are applied at the right moments of the growing cycle. The soil is left to dry until the readily available water is consumed. Irrigation is Low frequency deficit then applied, until field capacity. This is repeated several times along the Lavee and Wodner [11] irrigation (LFDI) irrigation season. Goldhamer et al. [12]; Sustained deficit A fixed fraction of the crop water needs is applied all throughout the irrigation Grattan et al. [13]; irrigation (SDI) season. Irrigation is applied daily or 2–4 times per week. Ramos and Santos [14] Partial root zone Similar to SDI, but water is applied to half of the root zone, switching to the Dry et al. [15]; drying (PRD) other half every 2–3 weeks. Fernández et al. [16] Irrigation amounts equal or close to the crop water needs are supplied at the phenological stages most sensitive to drought. Irrigation in those periods is Chalmers et al. [17]; Regulated deficit applied daily, or at least several times per week. For the rest of the crop cycle, Goldhamer [18]; irrigation (RDI) irrigation is drastically reduced (not only the dose is reduced but also the Fernández et al. [6] frequency, to one or