Environmental sustainability: a review of the water–energy–food nexus

Water, energy, and food are primary resources on which human life is dependent. This paper presents a review of the water–energy–food (WEF) nexus considering the environmental impacts generated by humans’ reliance on water, energy, and food for their subsistence. Our review assesses the WEF with respect to the agricultural, industrial, and urban sectors and their use of water, energy, and food. The multi-sectorial assessment addresses options for improved management that avoids or mitigates adverse impacts in the agricultural, industrial, and urban sectors. Activities such as the use of fertilizers and pesticides in the agricultural sector, for instance, cause water, air, and soil pollution, which leads to social calamities and environmental degradation. Therefore, examining the effects of mismanagement in one sector on other sectors from the perspective of the WEF nexus is necessary for improved resource management and environmental protection. A literature review revealed that factors or practices of resources use influence sectors differently and with varying degrees of effectiveness in reducing the environmental damage caused by resources use. Improved social awareness on resource consumption, the use of renewable energy, improved energy efficiency, the reduction of food waste, improved animal husbandry, and other factors involved in the WEF nexus are herein examined. This paper’s analysis demonstrates that every action and manner of resource use in one sector also affects other sectors and their resources use, thus calling for a unified analysis of the WE nexus.


INTRODUCTION
Water, energy, and food are essential resources for the successful functioning of society. Freshwater used by humans derives from precipitation, rivers, streams, lakes, reservoirs, and aquifers. Reservoirs are hydraulic structures built to store, deliver, and regulate water for multiple uses and functions (Asgari et al. ). In addition to water supply, many reservoirs provide flood control protection, generate hydropower, provide recreational services, and Interdisciplinarity is a key challenge to achieve successful partnerships between the private and non-private sectors to develop systems modeling that evaluates trade-offs in WEF decision making (Scanlon et al. ). Wiegleb & Bruns () argued that interdisciplinary inquiry is needed to develop a comprehensive understanding of the resource nexus. Kumazawa et al. () cited an example of interdisciplinary concepts for the assessment of groundwater use involving groundwater pumping for geothermal power generation. Interdisciplinary assessment relies on a common language and a common theoretical basis (Defilia et al. ; Kumazawa et al. ). Therefore, nexus concepts are based on a conceptual framework shared by multiple involved analysts and stakeholders.
Siddiqi & Anadon () demonstrated that the energy sector is weakly dependent on water resources; on the contrary, water withdrawal, desalination, and sewage treatment have a strong dependency on energy. Water is used in the extraction, cooling, and processing of fossil fuels. Energy is required for water distribution, storage, conveyance, desalination, and sewage treatment (Lam et al. ). Fossil fuel production is water-intensive; conversely, seawater desalination is an energy-intensive process. Desalinating water for drinking purposes requires 23 times more energy than that required to extract and treat surface water (Borgomeo et al. ).
Water and energy are necessary for food production.
According to report 48 of the UNESCO-IHE (), the production of 1 kg of beef consumes approximately 15,000 l of water (93% green, 4% blue, and 3% gray water footprints). There is a large variation about this global average from region to region. The precise water footprint of beef depends on factors such as the type of production system and the composition and origin of the feed for bovines. The report 49 of the UNESCO-IHE () states the water footprint of a 150-g soy burger produced in the Netherlands is about 160 l. A beef burger in the Netherlands costs consumes about 1,000 l of water. Food is required to generate bodily energy through physiologic processes.
The feedbacks between water, energy, and food are complex and dynamic, with actions in one of the agricultural, industrial, and urban sectors frequently affecting the other two sectors. The conceptualization of resources use as a nexus represents an effort to resolve the complexity of the interactions between water, food, energy, climate, and human activities (Howarth & Monasterolo ). Understanding the interactions and feedbacks between these water, energy, and food use must rely on methods that resort to interdisciplinary, multi-sectoral, and multi-dimensional research (Endo et al. ).
Ackoff () defined a system a set of two or more elements that consists of the three following conditions. First, each element (or subsystem) of a system affects the behavior or features of the whole system. Second, there is an interdependence between the elements of the system that affects the entire system. Third, any subset of the elements affects the entire system, and this effect depends on at least one other subset of the system. In other words, the components of a system are interconnected such that no independent subgroup can be formed. A system so defined implies that the interactions between water, food, and energy can be considered as a system because changes in the amount of consumption and sources of each of these three resources affect the others. For instance, the amount of agricultural production, and the choice of crop type and irrigation method depends on the availability of water resources. Clearly, water, energy, and food form a system that, in turn, is a subsystem of the social and economic system.
Interactions in the WEF nexus are summarized below: • The effect of energy on food production ( • The effect of water in food production (D'Odorico et al.

):
○ Water is used in food production, washing, transportation, food preparation and cooking, food processing, etc.
• The effect of water on energy ( Jalilov et  Providing water to meet growing human demand implies heavier reliance on resources (water, energy, and land) to meet human needs and demand food, which concomitant larger stress on the environment by increasing water diversions, pollution, and changes to the natural environment created by human activities. These environmental impacts must be considered in WEF nexus studies to achieve holistic and effective strategies for resource management.
Understanding the interdependence of water, energy, and food, and the environmental impacts of human reliance on these commodities is a pressing and timely matter. The provision of secure food supplies, water, and clean, renewable, and reliable energy is essential to realize sustainable development. The WEF nexus must be understood and managed properly to maximize the underlying synergies and reduce or avoid adverse impacts from human reliance on water, energy, and food consumption.

Objective
Most previous studies related to the environment and water, energy, or food address only partially, or in a non-integrated manner, the use of water, energy, and food. Yet, there is interdependency in the use of water, energy, and food, which calls for their integrated analysis for the purpose of sustainable resource management. A meaningful understanding of the WEF nexus is only achievable by the comprehensive study of water, energy, and food. This paper's objective is the study of the WEF nexus for assessing the environmental impacts of resource consumption in the agricultural, industrial, and urban sectors. This paper covers a variety of topics pertinent to the WEF nexus and the environment. Figure 1 depicts a conceptualization of the framework of the WEF nexus system proposed in this work. Figure 1 shows three main resources (water, energy, and food) whose use is interrelated in the agricultural, industrial, and urban sectors. The interactions between resources availability and use may cause adverse environmental impacts, social calamities, and economic inefficiency.
The environmental impacts that occur in the WEF nexus and interactions between water, energy, and food are discussed in the following section.

ENVIRONMENTAL IMPACTS IN THE WEF NEXUS
The environment includes conditions in which all living things can live and operate, and makes up our surroundings.
This research considers air, water, and soil as the environment that are constantly affected and harmed by various human activities including emission of GHGs (climate change), soil pollution and erosion, depletion and degradation of water resources, deforestation, reduction of biodiversity, and fisheries depletion.  Howarth & Monasterolo () reported an approach that yields clear and accessible results for better understanding of the interconnectedness between water, food, energy, and climate change, and concluded the water, food, energy, and climate change feedbacks are non-linear, multi-sectoral, and time-sensitive. Wang et al. () showed that exchanges between energy, water, and carbon emissions provide insights for nexus management on how to balance water scarcity issues and develop future energy production in energy and water resources planning. They investigated five scenarios (four low-carbon-development scenarios (S2-S5) and one baseline scenario (S1)) by input-output analysis to reduce climate change impacts. The water-energy-food-environment nexus is a concept that stresses the importance of the interactions between the WEF nexus and the environment. This study considers surface water and groundwater, soil, and climate, which affect water, energy, and food sources through a set of complex feedbacks and interactions. The review of such interactions in this study demonstrates that actions taken in the agricultural, industrial, and urban sectors influence the WEF nexus, and the consequences of such actions must be carefully assessed to avoid adverse and irreversible impacts that may negate short-term gains. Improving water-use and energy efficiency is imperative to avoiding deleterious environmental impacts (Khan et al. ).
The next sub-sections review environmental impacts that arise in the agricultural, industrial, and urban sectors.

The agricultural sector
Food production involves water and energy inputs according to crop type, growing season, and food type (Shannak & Vittorio ). Enhancing food production means enlar- Fertilizer requires energy in its production and is applied to increase crop yields. The use of non-renewable energy to produce fertilizer triggers water and food chain pollution, and exacerbates GHG emissions. The long-term application of fertilizers may cause negative impacts on human health and ecosystems, also (Khan et al. ). They believe that irrigation is a leading energy user in agriculture, which means that achievement of high water efficiency while increasing agricultural productivity is essential to achieve food security and environmental protection. wasted in today's world, which are discharged to the environment thus causing degradation. Treatment and disposal of wasted food must be supervised and regulated. Energy production from such waste must be considered to obtain clean and renewable energies such as bioethanol, biodiesel, biooil, biogas, synthetic gas, therefore reducing waste disposal and raising clean energy generation.

The industrial sector
The importance of using water, food, and energy inputs in industrial production has been addressed in previous works. Less attention has been paid to the relevance that the input of such resources in industrial processes has on the environment. In some countries, Kazakhstan being a case in point, coal power stations account for the majority of the water withdrawals in the energy sector (Karatayev et al. ). The latter authors stated that with the current energy mix, the amount of water use is expected to grow rapidly in the energy sector.
The industry sector is a major consumer of water and energy and is also a source of pollution. This means that addressing the industry sector from a water, food, energy, and environmental perspective is particularly important.
Manufacturing plants input water for process mixing, chemical reactions, extraction, process cooling, steam generation, product washing, and equipment sanitization. Energy is Kumar & Saroj ) and pollutes the air with toxicants harmful to organisms (e.g., particulate matter; sulfur dioxide, SO 2 ; and nitrogen dioxide, NO 2 ; carbon monoxide, CO). GHG emissions contribute to climate change, alter the environment, and may reduce the available water resources in many parts of the world.
All industrial activities follow a lineal sequence, starting with the extraction of raw materials, and the use of technology and labor to convert them into value-added products.
Fossil fuels are often used to generate energy during these stages, which results in the release of large amounts of CO 2 . Also, the transport of raw materials to make them available for processing in industrial processes, and transporting the finished products for delivery to consumers causes emissions of pollutants such as carbon monoxide, nitrogen oxides, hydrocarbons, ozone, and particulate matter. The production of pollution-generating energy can be avoided by resorting to clean (i.e., pollution free) and renewable energy.
Hydropower produces about 16% of the electrical energy worldwide. It is considered renewable in the case of run-of-river hydropower generation, and only partly so when it requires reservoirs for a generation due to sedimen-   Table 1 summarizes the causes of climate change by the emissions of GHGs in the agricultural, industrial, and urban sectors. Table 2

THE ROLE OF GROUNDWATER ON THE WEF NEXUS
Groundwater is an important component of the WEF nexus in the agricultural, industrial, and urban sectors.

The industrial sector
Injection of water to extract oil and gas may threaten aquifers. Excessive groundwater discharge into streams can • Changing livestock feeds, installing solar panels or wind turbines on farms (Horowitz & Gottieb ) • Reducing food waste and use climate-friendly food (Government of the Netherlands) • Increasing the consumption of renewable fuels (Government of the Netherlands) The transportation sector Livestock and dairy operations Groundwater pumping Industry Production of energy to power the industrial sector Energy production and consumption in power plants and industry Industrial process The transportation sector

Urban
The transportation sector Energy consumption for production and water extraction, treatment, and desalination

The urban sector
About 96% of the unfrozen fresh water available globally for human consumption is in the form of groundwater; and 50% of drinking water is groundwater (Smith et al. ).
Groundwater withdrawal may exceed its recharge rate, a condition that causes groundwater overdraft when maintained over long periods of time, say, two or more decades (Loáiciga ), which is commonly associated with adverse effects such as increased cost of groundwater extraction, depletion of groundwater storage, reduction of base flow, land subsidence, groundwater quality deterioration, and contributes to desertification in some instances.
About 70% of groundwater abstraction worldwide is devoted to irrigated agriculture (Rajeevan & Mishra ).
It is possible to reduce groundwater withdrawal by using treated municipal wastewater to irrigate agricultural land However, some toxic metals such as Ni, Cd, and Pb may accumulate in the plant tissue, and NO 3 may pollute wells, which calls for monitoring and proper treatment of sewage before its reuse.
The metropolitan city of Taejon, Korea, is highly dependent on groundwater, which is threatened by pervasive pollution. Jeong () studied the chemical properties and pollution of groundwater in relation to land use in Taejon.
An attempt was made to distinguish anthropological inputs from the effect of natural chemical weathering on the chemical composition of groundwater, leading to the conclusion that most groundwater in the study region is weakly acidic, and that groundwater chemistry is determined mainly by land use and urbanization than the type of aquifer rocks. Jeong () also established that the sources of excess nitrate, chlorine, and CO 2 in groundwater may be leaks in the sewage system, old latrines, and municipal waste at landfills.

FUTURE DIRECTIONS OF THE WEF NEXUS RESEARCH
Most of the analysis and management proposed for the WEF nexus is at a theoretical stage, although current needs call for systematic management more than ever. Solutions emphasizing the importance of having a water, food, and energy-based approach to political and governmental decision-making remain elusive.
The Earth itself is a general dynamic system comprising the WEF nexus. It is timely to develop internationally harmonized WEF nexus methodologies to tackle large-scale comprehensive management of the world's water, energy, and food resources.
Improvement of the performance of WEF nexus management in the environmental field must focus on soil pollution in the future, since soil contamination affects agricultural production and contaminates water in aquifers, which is difficult to treat or remediate. Hatfield et al. () argued that soil is an integral part of water, food, and energy resources, and stated that soil is central to food security and energy supply, and even though scientists have studied it well, its impact on political decisions has not yet been felt, which calls for linking studies of erosion and reduction in soil fertility. In  Comparison between agricultural production with mineral fertilizer and organic fertilizer at the farm level to achieve sustainable agricultural management Reducing groundwater pollution due to the use of organic fertilizers energy, and food, which in turn affect the climate. There have been many studies of climate change; yet, its extent and range of impacts and predictions in the context of the WEF nexus have not been comprehensively addressed.
An overview of the literature on the WEF nexus from an environmental sustainability perspective was herein presented with respect to the agricultural, industrial, and urban sectors. The review shows that studies dealing with the environmental sustainability in the industrial subsystem rarely take into account the complexity of the relations between the WEF resources, and are generally limited to a binary combination of these three components (water and energy or food and energy, for instance).