2022 IEEE International Conference on Automation/XXV Congress of the Chilean Association of Automatic Control (ICA-ACCA) | 978-1-6654-9408-3/22/$31.00 ©2022 IEEE | DOI: 10.1109/ICA-ACCA56767.2022.10006120 Remote sensing tools for monitoring water requirements and water stress in vineyards and fruit trees S. Ortega-Farías, F. Fuentes-Peñailillo, K. Gutter, R. Vega-Ibáñez Abstract— Chile's main agricultural areas will significantly reduce rainfall (between 20-40%) due to global climate change. Also, Chile is periodically affected by the climatic phenomenon of “La Niña” (ENSO) and the Pacific Decadal Oscillation (PDO), which have produced important droughts and economic losses in most of the agricultural areas. Under these conditions, sophisticated irrigation water management, such as regulated deficit irrigation (RDI), will be required to optimize water productivity (kg m-3) and to maintain sufficient levels of yield and quality of wine and fruits. In this regard, this paper aims to describe remote sensing tools for monitoring water requirements (or evapotranspiration, ET) and plant water stress in vineyards and fruit orchards. keywords——crop coefficients, evapotranspiration, irrigation, soil evaporation, transpiration I. INTRODUCTION M ANY factors are forcing the agricultural industry to produce food efficiently and with the least impact on the environment. For this, it is necessary to implement new methodologies in the management of the different tasks that are carried out in the field. This is how the concept of Remote Sensing arises, which involves the integration of a series of technological developments in support of productive tasks, such as sensors, satellites, automation, and robotics, among others. The aforementioned becomes of great importance in the current scenario of a growing population and limited resources, where food security stands out as an important issue to be managed considering sustainable production in the context of climate change [1]. In this sense, the Food and Agriculture Organization of the United Nations (FAO) estimates that the world population will reach 8.5 billion by 2030, 9.7 billion by 2050, and 10.4 billion by 2100 [2]. This, added to the economic growth of many nations, puts pressure on agriculture, which should increase its world production by 70% by 2050. Moreover, in the case of This study was supported by the Chilean government through the project REDES-ANID (No. REDES190072), AGCI-Chile/México (Incremento de la eficiencia y cuidado del agua mediante experiencia conjunta entre comunidades de Sonora-México y Linares-Chile). and PCI-ANID (No. NSFC190013). S. Ortega-Farías K. Gutter and R. Vega are with Facultad de Agronomía, Centro de Investigación y transferencia en Riego y Agroclimatología (CITRA), Universidad de Talca, Casilla 747, Talca 3460000, Chile (sortega@utalca.cl, kgutter@utalca.cl and rjvega@utalca.cl). F. Fuentes-Peñailillo is with Instituto de Investigación Interdisciplinaria (I3), Universidad de Talca, Talca, Chile & Faculty of Agricultural Sciences, Research and Extension Center for Irrigation and Agroclimatology (CITRA), Universidad de Talca, Talca, Chile (ffuentesp@utalca.cl). Chile, climate change will bring uncertainty regarding the behavior of rainfall, with prolonged periods of drought and a decrease in the flows of the main hydrographic basins of the central area of the country [3], [4]. Model-based projections indicate that a reduction in mean annual precipitation of up to 40% relative to current values are expected for the second half of this century under high emission scenarios [5]. Also, Chile is periodically affected by the climatic phenomenon of “La Niña” (ENSO), a cyclic oceanicatmospheric phenomenon that occurs every 3-7 years and considers a warming/cooling of the equatorial Pacific Ocean. The warming phase of the Pacific corresponds to the so-called “El Niño,” while the cooling phase corresponds to “La Niña.” ENSO is currently considered the largest source of seasonal and inter-annual climate variability over various regions of the planet. This is how it has been possible to associate the existence of a higher mean flow in various rivers in South America when there are Niño events [6]. It has also been established that a large part of Chile's winter and interannual precipitation variability is linked to the seasonal aggregation of El Niño/Southern Oscillation [7]. Another factor that modulates precipitation in Chile is the Pacific Decadal Oscillation (PDO), a phenomenon of natural origin that considers warm and cold phases, such as ENSO, but its effect can prevail for decades. Cold periods are characterized by an intensification of the South Pacific Anticyclone, which causes drought conditions in the central zone [8]. The South Pacific Anticyclone is an area where the vertical airflow that promotes cloud formation is inhibited, therefore, it acts as a kind of blockage of the systems associated with rain. So, a stronger anticyclone causes less rain in winter. Data from the Chilean Meteorological Directorate show that as of 2007, the anticyclone has become increasingly intense and coincides with the Niña events, which largely explains the prolonged drought experienced in recent years [9]. In arid and semi-arid regions where grapes and other fruits are grown, precipitations are scarce or non-existent during the growing season, so it is necessary to evaluate irrigation strategies that improve the efficiency in the use of water resources and their productivity [10]. One of these strategies is regulated deficit irrigation (RDI), which consists of maintaining the plant's water status within predefined deficit limits during certain phases of its development in which it is less sensitive to water restriction [11]. For this, it is necessary to have parameters that reveal the water status of the plant and serve as indicators of convenience and distribution of irrigation in order to reduce the risks in the development of the crop [12], [13]. 978-1-6654-9408-3/22/$31.00 ©2022 IEEE Authorized licensed use limited to: Amrita School Of Engineering - Kollam. Downloaded on January 11,2023 at 07:13:59 UTC from IEEE Xplore. Restrictions apply. Because of this, the main objective of this work is to highlight the main remote sensing technologies applied in agricultural production, particularly in vineyards and fruit trees, in the context of climate change. II. STATE OF THE ART Agriculture has faced many challenges in the past few decades, leading agriculture to embrace digitalization. However, this isn’t something new, it has been happening since the 80s with the concept of precision agriculture, which involves the use of the GPS in agriculture equipment for the mapping of yields and other variables of interest with the intention of spatialized management, and which has reached a peak with the development and use of a series of proxy and remote detection methodologies for field monitoring [14]. Therefore, growing demand for tools that allow precise monitoring of large extensions of crops is observed, where realtime monitoring becomes essential to generate management strategies according to the needs of the crops [15], [16], also highlighting the need for modernization and intensification of agricultural practices and the need for highly efficient use of water and other resources [17], [18]. In modern agriculture, irrigation monitoring is commonly carried out through traditional techniques that determine the soil water content and the atmospheric demand. Among them, we find water content monitoring in the soil through tensiometers, watermark probes, neutron probes, or TDR (Time Domain Reflectometry) equipment, and water balance modeling. Other approaches to quantify the water status of plants include obtaining the relative water content (RWC), fresh weight (FW), turgid weight (TW), dry weight (DW), and equivalent water thickness (EWT) [19]. However, currently, it is also preferred to evaluate the water status of the plant through different physiological indicators [20] because the plant considers all the conditions of its environment, that is, it integrates the demand of the atmosphere and the soil water content [21]. Usually, the evaluation of water requirements is carried out by estimating the evapotranspiration of the crop (ETa), whose methodology is standardized and defined in the FAO-56 Manual [22]. For this, the reference evapotranspiration values must be known (ET0), which is estimated from meteorological data obtained in a particular locality, a value that is adjusted by the crop coefficient (KC), which considers the growth cycle of the plant and its variation over time [23], [24]. ET0 is usually obtained through daily meteorological variables (wind speed, solar radiation, air temperature, and relative humidity) recorded by an automatic weather station (AWS) under reference conditions (2 meters over an extensive area of well-watered green grass) equipped with a thermometer, hygrometer, anemometer, and pyranometer (Allen, Pereira, Raes, Smith y W, 1998; Campos et al., 2016). Another key physiological indicator suitable for describing the water status of plants is the gas exchange. Gas exchange depends, in the first place, on the opening of the stomata, which can be affected by climate and soil conditions [27]. The opening of the stomata allows an exchange of carbon dioxide (CO2) and water vapor between the plant and the atmosphere, which allows the processes of photosynthesis and transpiration to be carried out. The water status of the plant can also be evaluated using physiological indicators such as basal water potential, leaf water potential (Ѱ ), stem water potential (Ѱ ), stomatal conductance, sap flow, and trunk diameter. These parameters have the advantage of integrating climatic and soil conditions [20], [28]. In viticulture, one of the most used is the water potential, which involves measuring the pressure of the sap inside the xylem through a pressure chamber [13], [29]. This physiological variable can be obtained through the leaf water potential , stem water potential, and the pre-dawn water potential before dawn (Ѱ )[30]. However, under irrigation, it is preferred to use the stem water potential since it presents less variation between individual vine canopies compared to the leaf potential. [31]. In this regard, [32] mention that the Ѱ is considered a more stable and integrating measure of the water status of the plant compared to the Ѱ . Table I summarizes the traditional TABLE I MAIN ADVANTAGES AND DISADVANTAGES OF MEASURING SOIL WATER CONTENT, WATER BALANCE, WATER STATUS OF THE PLANT, AND STOMATAL CONDUCTANCE FOR THE IRRIGATION PROGRAMMING OF GRAPEVINES. Methodology I. Soil water measurement Soil Water Content (Ej. TDR, FDR) II. Calculation of the water balance Advantages Easy to apply in the field; can be very precise; water content measurement indicates how much water to apply; many commercial systems available; some sensors already automated In principle easy to apply; indicates how much water to apply (Requires estimation of evaporation and precipitation) Disadvantages Soil heterogeneity requires many (sometimes expensive) sensors or extensive monitoring programs; Difficulty selecting positions that are representative of the root zone; the sensors generally do not measure the water status at the surface level of the roots (which depends on the evaporative demand) Not as accurate as direct measurements: it needs accurate information on local rainfall; the calculation of evapotranspiration requires a good estimation of the crop coefficients (which depend on the development of the crop, depth of roots, etc.); errors are cumulative, so regular calibration is required. In general, it does not indicate “how much water” to apply; requires calibration to determine "control thresholds"; still largely in the research / development stage and little used yet routinely in agronomy (except for thermal sensing) III. Detection of plant "stress" Directly measures the plant's response to stress; integrates environmental effects; potentially very sensitive (a) Water status of tissues (Ex. pressure chamber) Widely accepted reference technique: more useful if the stem water potential is estimated, using covered leaves Slow and laborious (therefore expensive, especially for pre-dawn measurements); unsuitable for automation (b) Physiological response (Ex. stomatal conductance) Potentially more sensitive than measurements of the water status of tissues (especially leaves) Usually requires sophisticated or complex equipment; requires calibration to determine "control thresholds" Generally, a very sensitive response, except in some anisohydric species Large leaf-to-leaf variation requires many replications to obtain reliable data Adapted from [21]. Authorized licensed use limited to: Amrita School Of Engineering - Kollam. Downloaded on January 11,2023 at 07:13:59 UTC from IEEE Xplore. Restrictions apply. methodologies to know the water status of crops, their advantages, and their disadvantages. However, as seen in Table I, there are several disadvantages regarding using physiological measurements to assess plant water status, mainly referred to as technical and economic difficulties when applied on a large scale. Because of this, the need for a tool to overcome these disadvantages is increasing. In this sense, remote sensing appears as a tool to overcome the difficulties in crop production, given that it has the advantage of monitoring crops in a non-destructive way [1], providing near real-time information depending on the scale at which this tool is applied, generating recurrent information of different scales systematically, enabling the characterization of the spatiotemporal variability within a given area, in ways that are impossible with traditional techniques [1]. Remote sensing considers proxy and teledetection techniques, which usually consist of the non-contact measurement of the radiation reflected or emitted from agricultural orchards [29], [33]–[37]. Matter interacts with this radiation, so through the use of instrumentation (sensors) capable of simultaneously capturing the spatial and spectral attributes of a scene, an object can be characterized [38]. This involves an instrument or a sensor mounted on a platform, such as a satellite, an aircraft, an UAV/UGV, or a probe [1]. Among the properties and processes of plants that can be studied using remote sensing techniques, at a single plant level, including changes in physiology (for example, photochemical and non-photochemical quenching or NPQ), leaf temperature changes due to stomatal conductance regulation, biochemistry (for example, plant water content, altered pigment composition), and canopy structure (such as leaf area index (LAI), leaf size or leaf orientation). At the community level, the distance between individual plants or plant morphology (for example, plant height) could be impacted [39]. There have been three main spatial scales of remote sensing incorporated into agricultural management: • Satellite scale: for this case, the information is obtained through sensors mounted on satellites or shuttles, which over the last decades have had substantial improvements in terms of spatial, temporal, and spectral resolution [40], Today, a large range of satellite sensors provide us regularly with data covering a wide spectral range (from optical through microwave). Data are acquired from various orbits and in different spatial and temporal resolutions [41]. Some of these platforms are LANDSAT, MODIS, and Sentinel. However, a significant disadvantage of these systems is that monitoring is highly dependent on the time series provided (revisiting time) and the spatial resolution of the sensors [1]. For example, the Landsat satellite, orbiting at 705 km above the Earth’s surface, can record images with a 30 m × 30 m pixel size. This can be very useful for the study and modeling of large areas. However, it does not allow the more precise management required in precision agriculture, particularly in water management [42]. • Airborne scale: This scale refers mainly to unmanned aerial vehicles (UAV). The use of this type of tools is steadily increasing because of the main advantages of these types of systems, which are the temporal frequency (depending on the needs of the user) and the ability to incorporate high-resolution sensors, providing high spectral and spatial resolutions (Ali & Imran, 2021; Baluja et al., 2012), providing near-real-time monitoring in key phenological stages of the plant’s [44]–[50]. However, these technologies also present some constraints in their implementation at the field level, such as the cost of instrumentation [51], the difficulties in the flight plan programming [44], limited battery duration [52], restricted cargo capacity [53] and data manipulation, correction and interpretation [1]. • Ground-based systems: considers the use of portable hand-held devices such as thermometers, spectroradiometers, and porometers, which can provide a rich quality of information, given that they perform measurements almost in direct contact with the object of study [40]. This type of sensor can detect plant stresses using the imaging or spectrometry of plant leaves in the visible (red-green-blue or RGB), near-infrared (NIR), infrared (IR), and ultraviolet (UV) wavebands [54]. This is possible because it is known that many physiological and chemical properties of plants influence the way their tissues interact with light. Consequently, different biotic and abiotic factors alter this response [38]. Depending on the instrument selected for the spectral measurement, a trade must be made between spatial, radiometric, and spectral resolution. In this regard, the terms multispectral and hyperspectral arise. They usually define instruments according to the number of wavebands of information that are recorded for each image pixel. The more general adjective ‘multispectral’ describes instruments that record information in only a small number of wavebands, typically 2–10. Hyperspectral instruments record information in many wavebands, typically greater than 10 [55]. This causes the spectral profile of a plant to be more poorly described when using a multispectral instrument. Among plant measurement-based approaches to determining water status, canopy temperature has received much attention over the past two decades, especially with the advent of portable infrared radiometers and thermal imaging cameras [56]–[58]. Through these sensors, it is possible to precisely monitor the temperature of the canopy surface, which is related to the water potential, through the calculation of the Crop Water Stress Index (CWSI) [59]. The CWSI is based on the difference between canopy temperature and air temperature (Tc-Ta), normalized by the vapor pressure deficit (VPD), and is based on the fact that water stress induces stomatal closure, thereby decreasing evaporative cooling and increasing leaf temperature [60]. This index has been widely used over several crops such as grapevines [61]–[64], olive trees [42], [65], maize [66], [67], soybean [56], among others, highlighting the usefulness of this index and other remote sensing tools. III. CONCLUSIONS The accurate management of irrigation water to improve the overall productivity of crops has become an issue of great importance in the current scenario of population growth and climate change. In the case of Chile, agriculture faces constant Authorized licensed use limited to: Amrita School Of Engineering - Kollam. Downloaded on January 11,2023 at 07:13:59 UTC from IEEE Xplore. Restrictions apply. variations in climate derived from different atmospheric phenomena. In this sense, the current tools used to assess plant water status, such as physiological measurements and methods to estimate ET0 have severe disadvantages, limiting their application over large surfaces. Because of the above, it has been demonstrated that incorporating new technologies in the form of remote sensing can overcome the difficulties presented in physiological measurements to assess the plant water status of several crops in a fast and representative way. 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