Insights on the Water–Energy–Food Nexus

: This Special Issue addresses topics on the water–energy–food (WEF) nexus along with other water-related topics, such as water resources, irrigation and drinking water supply systems, hydraulics and pollution. Several threats jeopardize freshwater availability and quality, energy and food availability. Integrated management approaches are absolutely necessary for pursuing sustainability. This Special Issue addresses various subjects and includes 29 peer-reviewed papers that have been grouped into the following categories: the WEF nexus, water resources and irrigation systems, drinking water supply systems, hydraulics and pollution. Some of them were selected from the Third E ﬃ cient Water Systems (EWaS) International Conference, entitled “Insights on the Water–Energy–Food Nexus,” after a thorough content update. Summaries of the papers are brieﬂy presented in this Editorial.


Introduction
The water-energy-food (WEF) nexus concept sheds light on important interactions between the components of this system, and on the fact that their integrated management is key to achieving the UN's Sustainable Development Goals. Under the pressures of population growth and urbanization, economic development, international trade and climate change, projections indicate that the demands for fresh water, energy and food will increase significantly over the coming decades. The WEF nexus has been established as a useful way to integrate the complex and interrelated nature of our global resource systems. The WEF nexus approach allows one to better understand and analyze the interlinkages between the natural environment and socio-economic activities, and to work towards a more coordinated management and use of natural resources across sectors and scales. Identifying and managing trade-offs and building synergies through this analysis allows for more integrated and sustainable planning, decision-making, policy-making and implementation. In this context, this Special Issue aims to provide insights on the WEF nexus, presenting international case studies to bring together theory and practice.
Along with WEF nexus, this Special Issue addresses water-related topics, such as water resource management, irrigation and drinking water supply systems, hydraulics and pollution. The Special Issue also includes selected papers from the Third Efficient Water Systems (EWaS) International Conference, entitled "Insights on the Water-Energy-Food Nexus".

Water-Energy-Food Nexus
The water-energy-food (WEF) nexus is usually considered a broad concept including land use and climate too. In this context, [1] presents a practical heuristic approach to show the components' interlinkages based on a three-point typology of interactions. This approach was used by taking Greece as the case study. The study resulted in a cross-interlinkage matrix identifying food as the most influential resource and water as the resource mostly influenced by other nexus elements. WEF interactions in countries in the Middle East were studied in [2], wherein WEF resource insecurity and weak planning/management strategies were identified. Specifically, the study showed the interlinkages of the WEF nexus in this area, stressing that the lack of consistent management combined with unsustainable use of resources in the Middle East region will probably pose critical WEF challenges in the near future. The synergies of the WEF nexus in Shenzhen City, China using the synergetic model were studied in [3]. The synergetic model was developed and the synergetic networks were constructed. The study's results showed that the synergies of the nexus system in Shenzhen can be maximized by stabilizing water supply, coordinating the energy imports and exports and reducing the crops sown areas.
An experimental study on the water-energy-food nexus is included in [4]. It is generally known that soil salinization is related to increased energy demand for high-quality irrigation water, so [4] studied the electrical conductivity (EC) of the soil saturated paste extract (ECe) with the EC in the 1:1 and 1:5 soil:water extracts in Greece. The experimental results strongly suggest that EC1:1 and EC1:5 values can be safely used for the estimation of ECe and showed that EC values of 1:1 extracts appear to be better estimators for ECe.
The interlinkage of water-energy was studied by Jager et al. [5], who examined the trade-offs between potential algal biomass production and preservation of adequate surplus water and instream flows to protect aquatic habitats. Sustainability indicators related to water consumption and stress to freshwater habitats were defined and evaluated, showing that basin-specific sustainability targets to avoid conflicts with competing water users at this energy-water nexus should be set. Where conflicts emerge, alternative water sources or production systems minimizing evaporation may be needed for algae cultivation.
Food-energy interlinkages are studied in [6]. More specifically, the viability of the implementation of low-cost innovative solutions, through the clean and eco-efficient production and wastewater pretreatment for fish canneries, is studied. The results provide a framework to allow the canneries to dispose of their pretreated effluents into the urban sanitation system, avoiding the high costs of an industrial wastewater treatment plant to discharge into the river, while constantly evaluating the environmental impacts.

Water Resources and Irrigation Systems
The Water Framework Directive (WFD) 2000/60/EC aims at protecting all water systems in the European Union's territory. Based on the WFD guidelines set regarding water systems, a study to identify environmental policy gaps in Mediterranean national parks (hydro-ecosystems) was done [7]. Using gap analysis, the results confirmed that human pressures altered national parks, and in particular, human pressures and eutrophication are the main factors. The study also revealed that the compliance in monitoring practices with the WFD is poor.
An important factor affecting water resources is climate change. Climate change's effects on groundwater temperature and on spring water quality were studied in [8], by using a dataset of the Campania Environmental Agency (in Italy) to carry out spatial, temporal and statistical analyses to assess the effects of climate variability on 118 springs. The study confirmed that increased yearly average minimum atmospheric temperature will increase yearly average water temperatures for springs. However, temperature increases do not affect a spring's water quality directly, as it is affected by other factors, such as precipitation trends, spring altitude and the presence of geothermal heat fluxes. The importance of the study is that possible long-term effects can be studied, affecting water quality in the long run. Other climate change impacts such as floods are studied in [9]. In particular, a methodology to detect flooded areas and explore the impacts of a flood event by using optical and radar data from earth observation satellites in combination with a high-resolution digital elevation model (DEM) was prepared and applied to the West Thessaly region in Greece. The methodology uses satellite images, water indices and elevation data to estimate flood water depth and to record the land use/cover of the flooded area. This methodology can be a useful tool for the complementary use of multisource spatial data to assess the impacts of floods.
There are papers published in this special issue related to groundwater systems. Specifically, in the general case of an aquifer recharging from a lake, the problem of a sudden rise and subsequent stabilization of the lake's water level is examined in [10]. The authors studied the solution to the one-dimensional second order unsteady partial differential equation (it is known as Boussinesq equation after Boussinesq, who presented the solution in 1904). They concluded that the appropriate water volume quantity with an uncertainty level can be estimated. In another study [11] the possibilities of improving the precision of, and obtaining better, drainage density input data for the Erosion Potential Method, were examined. Four different methodologies using different assumptions were used to derive the drainage density map in the case study of a catchment in Croatia. Another important issue in groundwater systems' management is water withdrawals. Reference [12] presents a metabolic model combined with hydraulic simulations to compare alternative solutions for the reduction of water withdrawals from groundwater bodies. Metabolic modelling provides support to decision-making in the medium-long term, based on sustainability criteria. Key performance indicators are used for evaluation in order to identify which kinds of interventions may be applied to increase the sustainability of the system. This approach was applied in Reggio Emilia Province (Italy). Water allocation to different users is another problem in water resource management. To achieve the optimum allocation of water and to locate one or more new reservoirs in a river-with-reservoirs system, a mixed integer linear programming (MILP) model was used [13]. The model was applied in Machángara river basin in the Ecuadorian Andes. The MILP showed that the water-energy-food nexus can be mitigated by adding one or more reservoirs at a specific basin.
Soil plays an important role in water resource management. Thus, in order to characterize soil's hydraulic properties, disc infiltrometers are used. In the study of Kargas et al. [14], one and three-dimensional infiltration tests were done on three repacked soils (loam, sandy loam and silty clay loam) for two negative pressure heads. The study results showed that the shape parameter seems not to be seriously affected by the soil type. From the study's results of the shape parameter, the criteria proposed for calculating hydraulic conductivity using three-dimensional infiltration data may be fulfilled in most soils.
Three papers in this issue are related to irrigation water. The problem of determining priorities in irrigation plans is tackled in the first paper [15], for which multi-criteria analysis methods were used in a case study area in Croatia. Different irrigation areas were analyzed based on selected criteria related to the environment, water, socio-economic issues and time using six multi-criteria analysis methods. The study's results revealed that such methods can be applied for prioritizing irrigation project development. The second paper presents an experiment wherein bench-scale apparatus was used for continuous irrigation with treated wastewater (TWW) to obtain a high rice yield and quality for animal feed without synthetic fertilizers [16]. Different treated wastewater irrigation directions ("bottom-to-top" and "top-to-top" irrigation) and fertilization practices were used. "Bottom-to-top irrigation is achieved through underground pipes installed at the bottom of the containers with the plants to be irrigated. "Top-to-top" irrigation is achieved when irrigation takes place on the soil surface.
The study results showed that bottom-to-top irrigation with TWW can be considered a potential practice to meet both water and nutrient demands for rice cultivation in order to achieve a very high yield and nutritional quality of cultivated rice without necessitating the application of synthetic fertilizers. To achieve high pearl millet yields, adaptive management of water and nitrogen as applied in [17]. The experiments took place in South Africa, which aimed to combine water and nitrogen applications towards optimum crop requirements. The study revealed that integrated adaptive water and nitrogen management should be considered to reduce high N losses and the cost of crop production, without a meaningful yield penalty, relative to high production input management.

Drinking Water Supply Systems
Papers included in this SI provide insights into drinking water supply systems. Specifically, water demand management and the use of alternative options for drinking use (e.g., water reuse) are discussed in [18][19][20]. The first study [18] focuses on the analysis of drinking water demand in three towns in southern Italy. The study revealed that there are water demand patterns where daily periodicities can be identified in hourly water demands. The same happens for weekly periodicities in daily water demands. Moreover, a physically based regional relationship is able to provide a robust evaluation for the design value of the instantaneous water demand peak factor depending on population. As water availability needs to be improved, Bonoli et al. [19] performed a life cycle analysis of the best technological solutions to enhance water reuse strategies, and in particular to improve the implementation of water consumption monitoring systems, the definition of solutions for greywater reuse and the tool development for environmental sustainability evaluations applied to water systems. Six different scenarios were compared and evaluation of the return times of environmental investments as carried out. Although alternative water sources must be used to safeguard water availability, the question of the perceived acceptance of their users has arisen, threatening the success of their implementation. An integrated water system is presented in [20] comprised of a rainwater harvesting system, a water treatment system and an eco-toilet system. An evaluation model was used to determine the significant factors that can influence the social acceptance of Integrated Water System in the Municipality of Mulanay, Quezon Province, Philippines.
Applications related to leakage management and life assessment of water tanks are presented in [21,22]. Leakage in water distribution systems is responsible for economic and environmental impacts. To tackle leakage at the user level, ref. [21] proposed a real-time household water consumption monitoring and processing system, wherein the consumption data are stored and processed through an empirical algorithm able to automatically identify leakages by looking for non-consumption during certain periods of the day. This system will be an efficient tool for reducing water losses at the user level. Water supply systems' assets suffer from longstanding use. Viccione et al. [22] presented a simplified model for the assessment of the residual service lives of decommissioned water tanks and applied it to a case study. Once the service life is assessed, a plugin is used in Q Geographic Information System (QGIS) to speed up the design of the new pipeline routes in the georeferenced space, thereby overcoming the limits offered by the classic solver of the hydraulic simulation model software tool (EPANET).
Key performance indicators to be used for the improvement of water distribution system management with detailed spatio-temporal supervision are introduced in [23]. The results showed that losses depict intense seasonal variability, while spatial variability, which is linked to the elevation and the different urban land uses, is proven to play a significant role in the neighborhoods' water balances.
Various key performance indicators are suggested and applied for the pressure control scheme, such as leakage reduction, pressure-driven demand reduction and energy and economic savings. The application of the pressure management revealed a significant leakage reduction, annual energy savings and economic savings.
Simulation and optimization tools used in drinking water supply systems are presented in [24,25]. A hybrid, two-stage approach to provide optimal separation of a water distribution network into district metered areas (DMAs), improving both water age and pressure, is presented in [24]. Three algorithms were tested in order to compare their performances, namely, the Gaussian mixture model algorithm, the Student's t-mixture model (SMM-EM) algorithm and the genetic algorithm. The optimization criteria used were: (a) Efficient pressure management in terms of minimizing the pressure times (x) demand product (respecting the down limit of pressure). (b) The water age in terms of the time the water remains inside the water pipes before reaching the customers' taps. The SMM-EM algorithm is very promising, especially for large water distribution networks due to the decreased running time and noteworthy reduction of pressure and water age. The same problem of selecting the optimal equilibrium for water pressure and water age when the distribution network is divided, was studied in [25] using simulation and optimization tools. The methodology was applied in Kos township's water distribution network in Greece, which experiences high water demand peaks and lows during summer and winter time. The study's results showed that combining the optimum formation of DMAs based on the water age criterion and installing pressure regulation afterwards can have similar effects on pressure regulation without the negative impacts on water quality.
Finally, drinking water quality issues related to trihalomethanes' (THMs) formation are discussed in [26]. The development of predictive models for total THMs formation in two water supply systems in Greece using statistical analysis is presented. The models are based on residual chlorine concentration and water pH and total organic carbon concentration. Although they cannot be applied universally due to the complexity of the formation of disinfection by-products, they can be used at local level or in water supply networks with the same operating characteristics, being important decision-making tools for water utility managers.

Hydraulics and Pumps operated as Turbines
Two papers included in this SI relate to hydraulics and the use of pumps operated as turbines (PaTs) in the water supply network. Specifically, the slip phenomenon in PaTs is investigated in [27], using a commercial double suction centrifugal pump. A slip phenomenon can occur in turbines with a finite number of vanes, at the outlet section of the runner. Specifically, the relative velocity vectors at the outlet of a centripetal turbine are subject to a deflection with respect to the direction defined by the blade congruent angle (slip). Thus, the slip phenomenon reduces the capability of the PaT to correctly guide the flow. The inclusion of the slip phenomenon as a parameter in prediction models shows better results (reduced prediction performance errors).
The calculation of multiple critical depths in compound and natural channels is presented in [28], using an adaptive cubic polynomials algorithm (ACPA). The proposed algorithm approximates the specific energy curve of natural or compound sections with multiple cubic polynomials. The algorithm provided accurate results in all cases, and when comparing this algorithm with the Hydrologic Engineering Center-River Analysis System, it was concluded that in most cases their results agreed. ACPA's advantage is that it can be used in more complex sections and it is able to calculate all critical paths.

Pollution
Landfill leachates can cause groundwater pollution. Grosser et al. [29] investigate the effects of ultrasound pretreatment on biological treatment of landfill leachates in the form of processes carried out in two sequential batch reactors. Ultrasound treatment offers significant advantages such as increased leachate biodegradability and reduced toxicity and economic efficiency etc. The existence of a pretreatment step results in higher pollutant removal efficiency.

Conclusions
This SI presents insights on the water-energy-food nexus. In particular, 6 papers on the WEF nexus, 11 papers on water resources and irrigation systems, 9 papers on drinking water supply systems, 2 papers on hydraulics and PATs and 1 paper on pollution are included in this Special Issue.
Interlinkages among water-energy-food-land use-climate are examined in this SI. It is important to study these interlinkages in various spatial scales, as there are regions (such as the Middle Eastern countries) where such studies show that planning and management strategies should be established [2]. Models to study these interlinkages have been developed, such as the heuristic approach [1] and the synergetic model [3]. The quality of the analysis performed is a critical factor for the efficient use of these models. The use of alternative energy sources, such as potential algal biomass, should ensure that water resources and the aquatic habitat are not stressed [4]. Reduced energy demand can be achieved when wastewater from fisheries is pretreated using low-cost innovative solutions to allow the pretreated effluents to be disposed in urban water treatment plants [5]. Another insight in the WEF nexus is the use of EC values of soil over the water mass ratio, which provided very good results in order to identify the suitability of soil for agricultural activity, reducing the energy demand for high-quality irrigation water [4].
Insights are also given in this SI about individual elements of the water-energy-food nexus, such as water resource management. Climate change is a significant factor which is expected to negatively impact water resources by deteriorating water resources quality, causing flood events, etc. In this context, higher temperatures will result in higher water temperature, impacting the water quality of springs in the long run [8]. To address floods' impacts, satellite and radar data can be used to detect flooded areas and explore the impacts of flood events [9]. A problem met in groundwater systems is a sudden rise and subsequent stabilization of the lake's water level in a case of an aquifer recharging from a lake [10]. To ensure the reliable estimation of drainage density, the possibilities to achieve improved precision of drainage density data are studied [11]. The problem of water withdrawals from groundwater bodies is tackled using a metabolic model combined with hydraulic simulations [12]. To solve the problem of optimum allocation of water and to locate one or more new reservoirs in a river-with-reservoirs system, a MILP model is used [13]. Additionally, management measures are assessed in Mediterranean national parks showing that there are inconsistencies with the guidelines set by the WFD (such as in monitoring water systems) [7]. The causes are mainly human interventions and eutrophication causing pollution. Finally, to characterize soil hydraulic properties, disc infiltrometers are used [14]. Water is considered as a scarce natural resource requiring efficient and effective management. As irrigation is the largest water user, irrigation priorities should be set, based on different criteria (e.g., environmental, socio-economic, etc.), using multi-criteria analysis methods [15]. Treated wastewater is used as an alternative resource for irrigation. It is proven that compared to water saving measures, the use of treated wastewater obtains higher yields and reduced use of synthetic fertilizers [16,17]. In this way water and other resources are saved.
The water-energy nexus is a crucial topic in drinking water supply systems' management. To avoid excess water supply and at the same time satisfy the consumers' water needs, a deep understanding of drinking water demand management is necessary [18]. Alternative water sources and combinations of alternative water supply systems are also solutions implemented to tackle the problem of water availability. For example, the best technological solutions used for water reuse or grey water reuse, water consumption monitoring systems, etc., can be assessed using a life cycle analysis [19]. However, the use of alternative water sources systems (such as rainwater harvesting, water treatment and eco-toilet systems) raise social acceptance issues [20]. Water losses and in particular leakages are serious causes of unsustainable drinking water supply systems at the network level or at the user level [21]. To solve the later, a real-time household water consumption monitoring and processing system can be used [21]. Water supply network assets are also of interest regarding water losses. Specifically, the residual service lives of decommissioned water tanks should be assessed [22] to better manage water supply systems. As drinking water supply systems are vulnerable both spatially and temporally, it is important to use key performance indicators (such as leakage reduction, pressure-driven demand reduction and energy and economic savings) [23] for the improvement of water distribution system management. High operating pressure is responsible for high leakage rates, while low operating pressure increases water age. To tackle this problem, simulation and optimization tools are used [24,25] to obtain the optimum equilibrium between pressure and water age. However, drinking water quality is important for the public health. Models for the formation of chlorination by-products such as THMs were developed based on residual chlorine concentration and water pH and total organic carbon concentration [26]. Such models can serve as valuable decision-making tools. Water distribution systems are energy intensive. Energy can be recovered in water supply systems using PaTs, where the slip phenomenon inclusion as a parameter in prediction models shows good results compared to its exclusion [27].
This SI contains some more general papers too. For example, in compound and natural channels the use of an ACPA algorithm provides more accurate results when calculating multiple critical depths and can be used in more complex systems, compared to other methods [28]. Additionally, as pollution is a significant factor for the sustainability of water resources, it was found that ultrasound treatment methods for landfill leachates result in increased biodegradability, reduced toxicity and improved economic efficiency [29].
Author Contributions: The authors led the development of the Special Issue and contributed equally to the preparation of this manuscript. Conceptualization: V.K.; writing-original draft preparation: S.T. and V.K.; writing-review and editing: S.T. and V.K. All authors have read and agreed to the published version of the manuscript.