Sustainable Irrigation Systems in Vineyards: A Literature Review on the Contribution of Renewable Energy Generation and Intelligent Resource Management Models

Abstract

Agriculture being an essential activity sector for the survival and prosperity of humanity, it is fundamental to use sustainable technologies in this field. With this in mind, some statistical data are analyzed regarding the food price rise and sustainable development indicators, with a special focus on the Portugal region. It is determined that one of the main factors that influences agriculture’s success is the soil’s characteristics, namely in terms of moisture and nutrients. In this regard, irrigation processes have become indispensable, and their technological management brings countless economic advantages. Like other branches of agriculture, the wine sector needs an adequate concentration of nutrients and moisture in the soil to provide the most efficient results, considering the appropriate and intelligent use of available water and energy resources. Given these facts, the use of renewable energies is a very important aspect of this study, which also synthesizes the main irrigation methods and examines the importance of evaluating the evapotranspiration of crops. Furthermore, the control of irrigation processes and the implementation of optimization and resource management models are of utmost importance to allow maximum efficiency and sustainability in this field.

1. Introduction

Agriculture has always been one of the most important sectors of activity for the survival and prosperity of humanity, becoming one of the main pillars of support for any country. As a field that is constantly evolving, there are various methods and techniques from the primordial times, which end up improving and making this whole activity more efficient [1]. Nowadays, the importance of agricultural resources is becoming increasingly evident, and a considerable impact on this sector triggers a whole negative reaction on a global socio-economic level. However, most of the population mistakenly takes it for granted or does not think about the entire process required to put a product from agriculture on the market [2,3].

In addition to the human losses and destruction caused by Russia’s invasion of Ukraine, also known as the “breadbasket of Europe”, several adversities and challenges have complicated the supply of energy and food. These difficulties have ended up further accentuating the vulnerabilities of economic sectors already weakened due to climate change and the COVID-19 pandemic, and one of the sectors that suffered the most was agriculture [4,5]. This impact is visible in the graph in Figure 1 provided by the Food and Agriculture Organization (FAO), which shows that the FAO Food Price Index (FFIP) has seen a significant increase [6].

Therefore, it is increasingly important to develop systems that make it possible to intensify what is sustainable agriculture to optimize the efficient usage of available resources, in terms of the use of arable land, water, and energy resources.

Agriculture turns out to be one of the most intensive activities in terms of land use, with a large percentage of habitable land being used for the production of crops, a figure that continues to increase exponentially over the years [7]. Unfortunately, the agriculture we practice today is destroying the planet. The use of excessive fertilizers, herbicides, and insecticides, which end up being washed into other environments, such as river basins, is a worrying aspect as it causes change and pollution in other ecosystems [8].

Nowadays, the vast majority of the population recognizes that one of the major factors influencing the success of agricultural activity is the existence of adequate moisture and nutrient conditions in the soil. For this reason, agriculture ends up being an activity that requires a great deal of water, and it is expected that, by 2030, this need will increase by 50% due to the constant increase in the world’s population. As agriculture is one of the main contributors to water scarcity, it is essential to create appropriate resource management and control systems to make the whole process more efficient and save a valuable asset [9,10].

According to a statistic search in FAO Aquastat [11] and Eurostat [12] databases, it is evident that the indicators related to water and energy usage in agriculture are increasing, which explains the importance of developing efficient solutions to better manage these resources. Considering the area of implementation of this study, it was decided to place a special focus on Portugal, comparing it with the average values of the European Union.

In Figure 2a,b, one can see an increase in the total percentage of cultivated land and water withdrawal for agricultural purposes relative to the total water collected.

One other indicator that is also relevant to this study is the Sustainable Development Goal (SDG) 6.4.1, which tracks the cost per volume of water in cubic meters, by a given economic activity over time. This indicator allows us to assess how economic growth depends on water resources, considering all economic activities, focusing on agriculture, industry, and the service sector. According to the graphic in Figure 3, it is clear that the indicator related to the agriculture sector is increasing over the years. The below-average value in Portugal is not surprising, given that there are countries like Denmark where this index is around 267 $\mathrm{EUR}/{\mathrm{m}}^3$ or even Luxembourg where it scales exponentially to 1112 $\mathrm{EUR}/{\mathrm{m}}^3$.

In terms of energy, the total consumption for agriculture purposes was analyzed, and the percentage of this energy was compared to the total energy used. As seen in Figure 4a,b, both of these indicators also increased during the past years.

The analyzed data also extends to one of the largest parts of the agricultural sector, which is the wine industry. Mankind has been fermenting grapes into wine for centuries, and today, wine is a consumable commodity worldwide. Despite its global popularity, wine production is a fairly exclusive industry, and not all land offers the ideal soil and atmospheric conditions for this activity [13]. In Portugal, a large part of the agricultural plantations are vineyards, making it the 10th country with the highest wine production and 2nd in consumption per capita worldwide [14]. To maintain this sector as a major contributor to the growth of the national economy, the efficiency of the entire process leading to wine production is of utmost importance [15].

Like other agricultural sectors, the wine sector requires an adequate concentration of nutrients and moisture in the soil [16]. For this reason, the development of systems that allow for greater efficiency in this activity’s practice is crucial, especially in terms of the irrigation processes that are indispensable for its success [17,18].

A search in Scopus [19] using the query “renewable energy AND irrigation” until the end of 2023 revealed a total of 1464 documents. This is a very low number of publications considering the importance of renewable energy for efficient and sustainable irrigation management. This gap has been identified by the scientific community, and one can see it is being addressed by the significant increase in related publications since around 2010, as shown in Figure 5. Most of these publications—nearly 60%—refer to journal publications, as highlighted in Figure 6, which supports the relevance of the topic. The interest and contribution to the advances in this subject are shared among multiple research and development areas, as shown by Figure 7, which include energy, engineering, environmental sciences, among many other complementary fields. The results displayed intentionally cover a broader context considering the exploratory literature review nature of this work.

Despite all the current systems and solutions which, depending on the availability and need of resources, are successful in increasing the efficiency of irrigation processes, there are still some limitations stemming from the energy and communication dependencies of the equipment used (sensors and actuators).

In certain cases, these limitations can be easily overcome; however, when it comes to land dedicated to wine production characterized by its large scale and adverse characteristics, particularly in terms of terrain accents, it is difficult to guarantee that these dependencies can be easily dealt with. In addition, there is also a growing need to make appropriate use of available water and energy resources, which, more than ever, cannot be considered infinite or guaranteed.

One of the main difficulties with irrigation smart systems sometimes lies in the existence of wiring to the sensors or actuators, which can be difficult to implement or damaged by agricultural machinery. A way of overcoming this problem is by implementing wireless-based systems that can be easily applied in any location, with no need to install cables to communicate the data acquired by the equipment [20]. This makes it possible to respond to the main limitation related to the size and adversity of the terrain, as it is possible to carry out remote communication between the sensors and the system controller. Of course, as the objective is to eliminate the routing of cables across the terrain, the power supply for these sensors and communication modules also has to be provided in another way, namely through batteries (rechargeable or not) and/or photovoltaic cells [21].

With these problems in mind, this article aims to study solutions and alternatives that will make the entire irrigation system more effective, particularly in the automatic management of resources that will optimize production efficiency in the wine sector.

Given the complexity of the tasks arising, there is a need to dismantle this main objective, obtaining more specific tasks, such as the following:

• Analysis of the resources needed for successful agricultural activity;
• Study of the most commonly used irrigation methods and their relevance to the wine sector;
• Analysis of existing concepts that make irrigation processes more efficient;
• Identification of effective sensors, actuators, and controllers for these solutions.

After this introductory section, Section 2 presents the importance of renewable energy sources in agriculture, and Section 3 provides information about the irrigation methods most frequently used. More technically in-depth, Section 4 briefly introduces the importance of evapotranspiration in agriculture, and Section 5 shows a study about some methods to control the irrigation processes. To conclude this study, Section 6 analyzes existing optimization and resource management models, and Section 7 summarizes the entire research of this article.

2. The Importance of Renewable Energies in Agriculture

With the continuous technological evolution in most economic sectors, agriculture has not escaped the constant innovation that allows the practice of the activity more autonomously and efficiently. However, compared to conventional agricultural activity, modern agriculture ends up requiring a greater energy demand, which very often resorts to fossil fuels [22].

Given the continuous rise in greenhouse gas emissions and consequent climate change, the promotion of renewable energies is increasingly important in all sectors of activity, despite all the challenges that may be imposed [23]. As stated by the European Green Deal, one of the main objectives of its program is to ensure zero net emissions of greenhouse gases by 2050, transforming the European Union into a resource-efficient economy [24].

Achieving sustainable agriculture is not an easy task, but with technological advances, renewable energy sources are becoming increasingly accessible. The decreasing costs of photovoltaic modules, wind technologies, and batteries for storing electricity are increasingly contributing to the implementation of green and sustainable agriculture [25].

The energy used throughout the agricultural production chain can be divided into two main areas: primary energy or energy used directly, namely in lighting, cooling, or heating systems and irrigation processes; and energy used indirectly, such as in the production of fertilizers or agricultural chemicals [22]. As irrigation is one of the most important stages in the success of agricultural activity, it is more than essential to ensure that the energy used by irrigation systems is acquired and used as economically and sustainably as possible.

Within the universe of renewable energies in the agricultural sector, it is important to highlight a few examples, such as solar, wind, hydroelectric, and biomass. Since wind and solar are the most common renewable energies used in this sector, they will be addressed with more focus in the article, although all of them have great potential for ensuring sustainable agriculture.

In addition to all the advantages renewable energies offer, particularly in terms of environmental benefits, sustainability, availability, and security, they are also characterized by their portability and ease of implementation. These turn out to be one of the most important factors about renewable energy sources because not every place has direct access to an electricity grid and sometimes securing a connection can result in a huge expense, which can make it unfeasible or even impossible [26].

The hillsides have always been the place of choice for growing quality vines, as can be seen in the Douro vineyards in Portugal [27]. Places characterized by rockier, less fertile soil end up producing smaller vines than regions with more fertile soil. This is not necessarily a disadvantage, as smaller vines result in smaller grapes, which offer a higher ratio of concentration and intensity in their extract, producing more unique and exceptional wines. In addition, vineyards grown on slopes have fewer problems with frost and offer natural water drainage, which prevents the land from flooding and consequently destroying the crops [28].

However, the great disadvantage of these lands is their topographical steepness, which immediately makes it difficult to work with agricultural machinery and access the electricity grid due to their remote and adverse nature. With this in mind, implementing an electrical self-production system based on renewable energies is extremely important as it guarantees access to clean, renewable energy, which reduces production costs, taking an important step towards ensuring sustainable agriculture [29,30].

In addition to all the sensors, actuators, or weather stations that can be placed on agricultural land to measure the many variables that influence irrigation processes, one of the pieces of equipment that can have the greatest energy demand is electric water pumps. Used to direct water from a reservoir to a specific destination, this equipment may be necessary to guarantee the presence of water on agricultural land.

A system made up of electric water pumps powered by photovoltaic panels can become a promising way of watering crops, creating systems often known as Solar Powered Irrigation Systems (SPIS) [31,32]. As an example, the use of solar-powered electric pumps to extract water from lakes, rivers, or canals is considered a very useful option in Bangladesh, where 35% of them are used purely for irrigation purposes [33].

In SPIS, the energy is generated by a set of photovoltaic panels that are used to power the electric pumps responsible for extracting, lifting, or distributing the water across the terrain [34]. Figure 8 shows a schematic of how this type of system is usually implemented.

In addition to the supply of energy obtained from solar power, another type of energy that has also had a major impact is wind power. With the constant increase in investment in this renewable alternative, the goal set by Net Zero is that, by 2050, around 18% of the energy used globally will be supplied by wind. With constant growth every year, wind energy saw a record increase of 273 TWh produced in 2021 compared to the previous year, making it the largest increase among all existing renewable energies [35,36,37].

This same principle can also be applied to powering pumps in the agricultural sectors. Instead of conventional photovoltaic panels, a wind turbine can be used to supply energy when the wind is on site, as shown in Figure 9.

This type of solution could be viable, for example, in windy steep terraced slopes and dry schist soils, such as the Douro valleys, which are characterized by a range of microclimates and often used for wine production. The so-called northerly winds, which come from the north, can be intense in these regions. However, during the growing season (typically from March to October), the winds are more moderate and are particularly beneficial for the growth of the vines. The existing air currents not only cool and refresh the crops but also prevent disease by ensuring good air circulation. Of course, the extra advantage of these climates is precisely the implementation of wind energy sources that can make the most of the winds in the region [38,39].

Once the energy supply is guaranteed, the system (both photovoltaic and wind) can be easily integrated with a controller or autonomous model, which manages the irrigation process according to the requirements of the crops or the availability of resources. In addition, the structure that supplies energy can be connected to a battery that guarantees the supply of energy to other equipment on the plantation, namely sensors and actuators associated with the irrigation systems. However, despite the increasingly promising battery developments, they remain considerably expensive and have limited life cycles [40].

An efficient way of avoiding this problem is by storing electrical energy in the form of hydroelectric potential energy, where water is pumped during sunny hours into a tank or reservoir located higher up. Through this reservoir and using the principle of hydroelectric energy production, it is possible to implement a hydroelectric generator made up of turbines that can supply energy in sunless hours. In addition to supplying energy, this water can also be used to irrigate crops, thus playing a major role in the autonomy and efficiency of irrigation processes [41].

3. Main Irrigation Methods

As previously mentioned, the main purpose of irrigation processes is to supply the amount of water needed by the soil for the proper development of cultivated crops.

Despite all the variables associated with each irrigation method, they all require adequate knowledge of soil characteristics and crop water requirements. One of the main factors to consider when estimating a crop’s water needs is evapotranspiration, a concept that results in the integration of the water transpired by the plant itself and the water evaporated [42]. Although the soil itself does not have a direct influence on evapotranspiration, it does influence the calculation of the volume of water to be applied, as well as the actual conduction of water through the soil, which is why it is essential to determine the best irrigation method for the plantation in question.

Within the universe of existing irrigation methods, they can be divided into three broad categories: surface irrigation, sprinkler irrigation, and localized irrigation [43,44].

Surface irrigation, where the effect of gravity is the main protagonist, is characterized by the way water runs off the surface of the soil and infiltrates along its path. Used in a large part of the world’s irrigated areas because of its simplicity, surface irrigation is distinguished by being a low-tech method that does not need any kind of pumping method, except to place the water on the surface of the land. In this process, water can be applied to the surface of a plot of land in the form of controlled flooding or waterlogging (visible, for example, in rice fields), or it can be applied through the defluence of water through existing channels or strips which guarantee the supply of water to the crops (Figure 10).

On the other hand, a more complex system is sprinkler irrigation, where the water is projected onto the crops in the form of rain. Unlike surface irrigation, this method already requires water to be conveyed by pressure along a network of pipes, which requires the implementation of a pumping system with consequent energy consumption. Essentially, sprinkler irrigation systems can be characterized as stationary, where the sprinklers remain in a fixed position, or mobile, where the sprinklers are mounted on moving platforms, as illustrated in Figure 11, also known as pivot irrigation.

Despite the advantages that the previously mentioned methods can bring to the most varied types of crops, particularly in terms of practicality or ease of installation and execution, the irrigation method most used in the wine sector is localized irrigation, also known as micro-watering or drip irrigation.

In this irrigation technique, water is directly applied only to the areas of the soil where plant roots develop and is dispersed slowly and evenly. For this reason, localized irrigation immediately presents a higher efficiency due to the amount of water used to satisfy all crops on a given plantation. This is particularly efficient due to the reduction in water losses as a result of evaporation, which often occurs in previous methods and ends up causing excessive irrigation.

Within the universe of localized irrigation, some specific processes stand out that correspond to distinct hydraulic processes. The micro-sprinkler method, where water is sprayed onto the surface of the soil, ends up being similar to sprinkler irrigation with the difference that small moist areas are created only in the root zone. In drip irrigation, which is one of the most economical options, water is delivered directly to the root area using drippers. Similarly to the previous process, there is subsurface irrigation, where water is applied under the surface of the soil through the implementation of, for example, a buried perforated piping system that discharges water directly to the roots. In Figure 12, an example of localized irrigation is represented, highlighting the difference between surface and subsurface drip irrigation.

In general, localized irrigation can contribute to the efficient use of water resources. These types of systems, if planned correctly, reduce the percentage of water waste due to runoff and infiltration into the soil and eliminate almost all losses due to evaporation. In addition to the benefit in production costs, drip irrigation also presents a strong advantage in reducing the growth and appearance of fungi that can cause diseases in the crop and consequent loss of quality in the final product [47].

However, the initial cost of implementing a system of this type ends up being higher compared to other, simpler systems. The value of these systems will always vary depending on the characteristics of the terrain, the composition of the soil, the type of crops, and the source of water used to feed the system. Another disadvantage is that, as they are exposed systems, they can be difficult to combine in plantations where the use of agricultural machinery is frequent, as there may be damage caused to the piping network or to the emitters, a situation that can be overcome with the use of subsurface methods [47].

Despite its disadvantages and expensive implementation, drip irrigation continues to be one of the most used and most economical systems in the long term in the wine sector. Characterized by being the least prone to problems related to soil variability and having the lowest level of difficulties due to the topography of the land, localized irrigation systems are the only ones that can guarantee greater control uniformity in water distribution. For these reasons and when combined with data sensing techniques or optimization models using renewable energies, it is possible to guarantee the highest level of efficiency in wine production [48,49].

4. Evapotranspiration Concept

Having determined the best irrigation method for the wine sector, another factor already mentioned and which is extremely important is evapotranspiration (ET), a concept that determines the water needs of a plantation. Generally speaking, vines can grow healthily and efficiently in soils with varying moisture conditions. However, excessive water can result in a wide range of negative impacts on grape production, such as a reduction in the set of flowers and their fruit, abnormal development of the vine trunk, and even dehydration of the grapes [50].

Other studies also revealed that variations in climate and atmospheric conditions largely influence the growth and development of the vine and the consequent yield and quality of the wine produced. In addition to its direct impact, a hotter and drier climate can negatively influence the growth of vines, particularly in terms of increasing soil salinity and, consequently, general soil degradation [51,52].

Although vines show strong resistance to challenging environmental conditions, estimating trends in soil irrigation needs and requirements, including vine evapotranspiration components under varying soil texture conditions, can be an excellent data source to guarantee sustainable production with guaranteed yield and quality [53].

Evapotranspiration, as the name suggests, is a parameter that defines the amount of water removed from the soil through plant transpiration and direct evaporation. The amount of ET from a plant is not a fixed value and varies depending on geographic location and over time. Some main factors determine its value, namely temperature, relative humidity, wind, solar radiation, and the type of plantation itself [54].

To study plant evapotranspiration, it is important to take into account two essential terms: $E{T}_0$ (reference or potential evapotranspiration) and $E{T}_c$ (plantation or crop evapotranspiration). $E{T}_0$ is a parameter that indicates the ET of a normal lawn under ideal conditions, and $E{T}_c$ indicates the evapotranspiration of the type of crop. These values are usually represented in the unit mm/day; however, they can be represented in other temporal forms (for example, mm/m, mm/min).

As described in FAO Irrigation and Drainage Paper 56 (FAO-56) [55], the set of methodologies presented below is among the most used to determine the evapotranspiration of a plantation. This approach is frequently applied given its simplicity and robustness, as it requires less input data and provides acceptable ET estimates when compared to other strongly parameterized models or models that take into account measurements via satellite or sensors placed on the ground.

To determine the value of $E{T}_0$, we can adopt an indirect way that uses the FAO Penman–Moneith method. This method is characterized by having a formula (Equation (1)) that allows calculating the value of $E{T}_0$ using some meteorological data.

$$E{T}_0=\frac{0.408\Delta (R_n-G)+\gamma \frac{900}{T+273}{u}_2(e_s-e_a)}{\Delta +\gamma (1+0.34u_2)}$$

(1)

where,

• $E{T}_0$—reference evapotranspiration [$\mathrm{mm}\hspace{0.17em}{\mathrm{day}}^{-1}$],
• $R_n$—net radiation at the crop surface [$\mathrm{MJ}\hspace{0.17em}{\mathrm{m}}^{-2}\hspace{0.17em}{\mathrm{day}}^{-1}$],
• G—soil heat flux density [$\mathrm{MJ}\hspace{0.17em}{\mathrm{m}}^{-2}\hspace{0.17em}{\mathrm{day}}^{-1}$],
• T—air temperature at 2 m height [°C],
• $u_2$—wind speed at 2 m height [$\mathrm{m}\hspace{0.17em}{\mathrm{s}}^{-1}$],
• $e_s$—saturation vapor pressure [kPa],
• $e_a$—actual vapor pressure [kPa],
• $e_s-e_a$—saturation vapor pressure deficit [kPa],
• $\Delta$—slope vapor pressure curve [kPa °${\mathrm{C}}^{-1}$],
• $\gamma$—psychrometric constant [kPa °${\mathrm{C}}^{-1}$].

The equation, despite presenting complex parameters and data, uses simple meteorological records of solar radiation, air temperature, humidity, and wind speed which, as previously mentioned, will directly affect the value of $E{T}_0$.

Based on the value of $E{T}_0$ at the plantation location, some approaches allow calculating the value of $E{T}_c$ through a parameter $K_c$ (crop coefficient). The value of the $K_c$ coefficient is not always the same for the same crop and varies depending on the stage of development the plant is in. To obtain more accurate results, FAO-56 determines two approximations to determine $E{T}_c$, which are the simple approximation and the Dual Crop Coefficient (DCC) approximation [56,57].

For the single coefficient used in the simple approximation, the effect of plant transpiration and soil evaporation are combined into a single coefficient $K_c$, obtaining Equation (2) for determining $E{T}_c$.

$$E{T}_c=K_c\times E{T}_0$$

(2)

On the other hand, the DCC approach separates the single coefficient $K_c$ into a coefficient related to transpiration ($K_{cb}$) and one related to soil evaporation ($K_e$), as can be seen in Equation (3). This approach requires additional parameters related to climate, soil, and crop to estimate transpiration and, of course, implies higher computational costs. However, this approach is recommended by FAO-56 when improved estimates of the value of $K_c$ are needed to, for example, schedule accurate daily watering.

$$E{T}_c=(K_{cb}+K_e)\times E{T}_0$$

(3)

For the purposes of normal planning and management of irrigation processes, the average plantation coefficients obtained by the simple approximation are relevant and more convenient than the $K_c$ calculated by the DCC approximation. However, it is always necessary to analyze the plantation in question and the available resources to determine the best approach.

5. Control of Irrigation Processes

Taking into account all the previously mentioned concepts, to achieve the status of sustainable agriculture, irrigation processes must be managed in the most intelligent way possible, both in terms of decision-making and in terms of resource use [58]. In this sense, there is a wide range of solutions already implemented that allow intelligent management of irrigation processes and that vary their efficiency depending on their complexity and practicality.

5.1. Hourly Scheduled Watering

Starting with a more traditional solution that is widely used, the programmed irrigation system appears. These simple systems are relatively developed and continue to make a big contribution in this field, since many irrigation projects still use this technique due to its low price and simplicity. The basic principle most frequently implemented in these systems is the concept of hourly irrigation [59].

Through intelligent programmers that offer a simple interface, the user can define the days and times at which they want to start irrigating a certain area of the land, indicating parameters such as the irrigation time or the percentage of water to be delivered, relative to the available flow rate. With the interconnection of electrovalves that allow the opening or closing of a flow of a given fluid through an electrical drive, the intelligent controller can carry out previously defined irrigation processes by the user, automatically.

5.2. Solutions Based on Measuring Soil Moisture

One of the solutions implemented for decision-making in irrigation processes are systems that work by obtaining values related to soil moisture in real time. These values can be acquired through the installation of a network of soil moisture sensors at several strategic points on the land, and through intelligent management by an implemented controller, it is possible to define very rigorous irrigation with great savings and less waste [60,61].

A method used to obtain the value of soil moisture is its gravimetric measurement; however, this process ends up being time-consuming and complex, as it would require the constant removal of several samples from different locations. That being said, soil moisture sensors are often used to obtain these measurements indirectly, using other soil properties, which can be divided into volumetric sensors, tensiometers, and sensors that measure the electrical resistance of the soil between two probes [62,63].

Volumetric sensors directly measure the volumetric water capacity in the soil—Volumetric Water Content (VWC). This type of sensor is capable of obtaining these readings through the use of neutron probes, heat dissipation sensors, or through the dielectric constant of the soil, which is the most common option. The soil dielectric constant is a property dependent on the soil’s moisture content and can be obtained through Time Domain Reflectometry (TDR), Time Domain Transmissiometry (TDT), or electrical capacitance or Frequency Domain Reflectometry (FDR). These volumetric sensors end up being a considerably expensive option (costing more than EUR 100 per unit and around EUR 500 to EUR 1100 for a necessary electronic interpreter); however, they have high precision and allow instantaneous readings, being used mainly in research centers or on plantations with high monetary value, where time and data precision justify the investment [64].

On the other hand, tensiometers are devices capable of measuring soil moisture through the tension with which water molecules are retained by soil particles. When the soil has a high amount of water, the root of a plant does not need to exert much force to absorb it, in the same way that the tensiometer does not need to, so the tension is low. In the opposite case, where there is a low amount of water in the soil, the tension increases. Tensiometers are a cheaper option compared to the previous one, with costs of around 65 EUR, and do not require any data interpreter as the values obtained are direct.

Finally, some sensors measure soil moisture through electrical resistance, which is an even cheaper option compared to the previous ones. With costs of no more than EUR 20 (when already incorporated with comparator modules), these sensors have a relatively simple operating mode. Having a shape similar to a fork, this sensor has two conductive probes that act as a variable resistance depending on the water content of the soil. The value of this resistance appears inversely proportional to the value of soil moisture, which means, that the higher the water content in the soil, the better the electricity conduction, meaning the resistance read between the two ends is lower, and vice versa. Through this variable resistance, it is then possible to determine the soil moisture in relative terms (whether it is wetter or drier). As mentioned, it is common to purchase these sensors with comparator modules, which allow obtaining a digital output (by calibrating a potentiometer) or an analog output, both in the form of an electrical voltage.

Through a network of soil moisture sensors properly placed throughout the land, the user can define the ideal minimum and maximum values (threshold) of soil moisture for each zone, making it possible to automate the irrigation process efficiently. As soon as the controller, through reading the values of a given sensor, detects that the moisture value is below the stipulated minimum, the irrigation of that same area is automatically started. As soon as the maximum value is reached, irrigation is automatically terminated, avoiding excessive watering and maintaining soil moisture values within the stipulated thresholds. With this type of system, as long as it is properly calibrated for the terrain in question (taking into account the type of soil, plantations, or sun exposure), high efficiency is guaranteed, opening up more possibilities for energy and water sustainability [65].

5.3. Solutions Based on Estimation of Evapotranspiration

As previously mentioned, crop evapotranspiration is directly related to the soil’s water needs; therefore, on a day when the plantation’s ET value is higher, it is necessary to carry out longer irrigation to compensate for these losses due to evaporation and transpiration [66].

An ET controller automatically adjusts irrigation times according to the land’s water needs, based on meteorological evapotranspiration information [67]. Unlike programmed irrigation controllers, in which the user has to constantly update irrigation schedules to avoid excessive or insufficient irrigation, these ET controllers automatically carry out this management [68]. These intelligent controllers can be divided into three distinct categories based on the way they obtain data related to evapotranspiration for each type of plantation: historical, signal-based, or terrain sensors [69].

ET controllers that use historical data generally have pre-programmed meteorological information. They are a relatively cheap option and, in most cases, reset the watering time to a monthly frequency, achieving considerable savings in water and energy.

Signal-based controllers obtain information regarding reference evapotranspiration by receiving data from a public meteorological station. These controllers generally have a service fee for data availability, but some public services provide this information free of charge, such as the Portuguese Institute of the Sea and Atmosphere (IPMA) database. The advantage of these systems is that they do not require on-site sensing equipment. However, although they already allow for high savings, they may not have very high precision, as the data acquired can cover a large territory (an entire municipality, for example).

A solution that allows even greater savings is the use of these ET controllers connected to sensors strategically located along the land. The installation of mini meteorological stations is common, and they allow the controller to obtain data acquired on-site to make continuous calculations of evapotranspiration, automatically adjusting the amount of water required. It is the most accurate and effective option in this type of system, as it provides all the necessary information without large margins of error, but it becomes more expensive due to the need for more expensive equipment [70].

6. Optimization and Resource Management Models

In the vast scientific and technological community, several solutions use most of the concepts covered in the previous chapters and that allow the optimization of productive efficiency in the agricultural sector.

In [71], the authors propose an intelligent energy management model that provides an increase in efficiency and autonomy through the use of renewable energy, namely photovoltaic and hydroelectric. The developed model and case studies are intended for agricultural fields close to rivers, where hydroelectric energy is used to power the water pumping system for a reservoir. Furthermore, for efficient energy management, decision trees are used to analyze and compare several variables, namely the cost–benefit of using the hydro generator or using energy from the electricity grid [72].

In the intelligent system developed in [73], an autonomous model uses the concept of mobile sprinkler irrigation (center pivot) that takes into account the water needs of plantations. Using data obtained from the ground, such as temperature, wind, soil humidity, and precipitation forecasts, the system can estimate the actual evapotranspiration of the plantation [74]. With this value, the model plans the irrigation processes taking into account the energy produced on site (through photovoltaic panels) and the cost of energy from the electricity grid.

In the dissertation [75], the authors used a network of humidity sensors based on the electrical resistance of the soil which allows constant monitoring of soil moisture. This relatively simple model defines a threshold that automatically manages the irrigation process [76]. Furthermore, an interesting innovation in this project was the implementation of a fire detection system, which allows an alarm to be issued in the event of a fire.

In [77], the authors carried out an intelligent approach using a database that contains the daily water needs of several plantations. With this database and taking into account data such as soil humidity and the time of day, the system can decide the amount of water needed for a given type of plantation. The results of the experiments carried out were satisfactory, and the authors valued the low cost of implementation, particularly in terms of the sensor network and access to the database.

Focusing more on the wine sector, the authors of publication [78] have developed a photovoltaic network to meet the energy demands of a winery’s wastewater treatment plant and the pumping station of a vineyard’s irrigation system. An interesting aspect of this article is the use of batteries as short-term storage, and with the surplus energy, hydrogen is produced by electrolysis of water, which is used by a modified battery vehicle.

Some authors of the previous article have also published the scientific paper [79], where they characterize the energy demand of a vineyard and create a prototype of a photovoltaic network with a set of panels floating in an irrigation pond to avoid using land.

Additionally, the authors of the article [80] developed and tested an automatic management system, which involves the coordination of wireless sensors placed at strategic points on the land. All information obtained by the sensors is sent and interpreted by a controller that contains an intelligent decision-making model (Figure 13). The proposed system focuses mainly on the management and optimization of water resources and a reduction in human labor so that wine production costs can be minimized.

7. Conclusions

All the research carried out within the scope of the state of the art highlights numerous intelligent solutions and models that significantly increase productive efficiency in sustainable agriculture worldwide. From the most effective irrigation methods to smart sensors, actuators, and decision-making models, many solutions enable optimal efficiency.

In most agricultural activities, water and energy are not dissociated, and there is a close link between these resources, which is why the analysis of the water–energy nexus deserves special attention. The definition of solutions that allow the identification of these resources’ consumption, interconnection, forecasting, and control, particularly through monitoring, management, and decision support systems, allows greater efficiency.

The studies in this area highlight the importance of using renewable energy as a source of clean energy for irrigation processes, which solves problems generated by the adversity of the most common terrains in vineyards, such as the unavailability of energy on-site. By encouraging the use of renewable sources, vineyards can also reduce their carbon footprint and dependence on non-renewable energy.

Smart irrigation techniques, such as precision irrigation and automated systems, ensure efficient water and energy use, minimizing waste, allowing resource optimization, and enhancing crop growth. Furthermore, using intelligent management systems for irrigation processes in conjunction with green energy such as photovoltaic or wind, a very efficient ecological footprint towards more sustainable agriculture can be achieved.

After determining one of the best and most efficient irrigation methods for vineyards, which is drip irrigation, it is clear that using intelligent solutions to manage irrigation processes brings considerable advantages. Solutions based on measuring soil moisture with sensors or solutions based on measuring evapotranspiration end up delivering a huge benefit in terms of efficiency compared to more traditional irrigation methods, namely the ones based on a schedule.

However, the current idea is that all these techniques are dispersed across several isolated systems. In this sense, there is a lack of an automatic resource management model that maximizes productive efficiency in the wine sector based on the largest number of compatible technologies and ideas mentioned previously.

Nevertheless, the scientific community has created numerous solutions that allow the use of the concepts covered throughout this article. From simpler to more complex projects that consider many variables, these solutions allow the achievement of excellent levels of efficiency and sustainability.