Vanishing Water: Rescuing the Neglected Food Resource

The annual global amount of water consumed to produce food ranges from 600,000 to 2.5 million liters per capita depending on food habits and food waste generation. Humans need approximately 2–3 L of water daily to maintain health, but only 0.01% of the world’s water is drinkable. Food supplies cannot be generated without land, water, and energy use. The current use of water for production of food is most concerning and requires immediate and increased awareness. Minimal attention has been devoted to the increasing water scarcity and loss of drinking water. Food waste also contains water and therefore also adds to water scarcity that is affecting almost 4 billion people. We summarize the human need of water, its significance for life and for the production, processing, and consumption of foods. This review includes an examination of the history of water; the unique properties of water for sustaining life; water for food production including agriculture, horticulture, and mariculture; the properties of water exploited in food processing; water scarcity due to water demands exceeding availability or access; and its consequences for our world. Means to reduce water scarcity, including using water treatment and promoting change of human habits, are discussed. The future of water and the recommendations for action are proposed for decreasing water scarcity and reducing water use during food production, food processing, food preparation, and consumption.

Lebensmittel (means of life) is the most fitting German word comprising food and water.Water (total body water) is the principal chemical constituent of the human body.Water represents approximately 50 to 70% of body weight with a daily turnover of approximately 5 to 10% [1].Normal daily water needs depend on a wide range of factors including human metabolism, diet, climate, physical activity, and clothing.Most normal active people need 2-3 L of water per day to remain healthily hydrated, and this can reach 6 L for more active people in warm climates [1,2].The water which we obtain from what we drink and the foods we consume ranges from 96% (e.g., cucumber) to 30-39% (e.g., cheese, bread) to 15% (e.g., margarine) to less than 3% (e.g., biscuits), and all contribute to our daily water intake.
Humans are dependent on water for survival.Water is the "principal driver of the evolutionary changes that characterized the first humans because it was a constant requirement for their daily lives" [3].Water is one of our most precious resources.It is argued that the progressive desiccation of our planet over the past few million years, especially the three shifts around 2.8 million, 1.8 million, and 0.8 million years ago, triggered the extinction of our closest evolutionary relative cousins, the Neanderthals, about 30,000 years ago.Our planet is called the blue planet because of its vast amount of water.However, only 0.01% of it is available as drinking water [4].Of the potential freshwater resources, twothirds are frozen sea caps and icebergs which are now steadily being "lost" in sea water due to melting [5].Mountain regions supply a large proportion of freshwater, but their hydrological significance depends on the region.Mountain glaciers and snow (symbolically termed "water towers") provide half of the world's freshwater resources [6].However, mountains are warming twice as fast as the global average, and snow cover is declining significantly [7].Major water towers are found in arid and semi-arid regions of the world [8], and mountain rivers in humid tropical climates (e.g., Mekong, Orinoco, and Amazon rivers) are inferior in terms of hydrological significance [9].Freshwater is scarce around the world, with approximately 3.8 to 4.0 billion people living under water scarcity for at least 1 month in the year and 0.5 billion people have to endure water scarcity for 12 months in the year [10,11].
The amount of water consumed annually to produce food ranges from 6 × 10 5 to 2.5 × 10 6 L per capita per year depending on the wealth of people, their food habits, and the percentage of food waste they generate [12].Global water demand for all uses is expected to increase by 20% to 30% up to 5.5 × 10 15 -6.5 × 10 15 L [13].The water scarcity together with the importance of water for life drive the constant need to preserve and recover water for the future of humans on our "blue planet".Humanity may be approaching a point of planetary water overshoot, where water use will exceed sustainable boundaries at the planetary scale [14].This drives the need for urgent attention to water for our sustenance.
Despite the significance of water for life, little has been published on the contribution of moisture in food to total water intake and the variation of water intake on human health, energy intake, weight, human performance, and functioning [15].This review provides a glimpse of the history of water and its importance for survival of mankind and human health, the properties of water and its role in foods as well as in food production, processing, preparation, and consumption.It emphasizes the need for re-balancing water resources and recognizing the needs and uses of water while valuing water as an important food resource, requiring actions to reduce water scarcity (Fig. 1) as a safeguard for improving sustainability of food systems.The future of our water supplies, including the needs for problem solving approaches and water retention and recovery strategies are also covered.

Water and Human Needs
Water was the center of life in many ancient cultures including for the Greeks and Romans, communities around the Mediterranean Sea, Egyptians and the Nile River, ancient Polynesians of the South Pacific, Indians and the Ganges River, people of the Amazon rain forest, and the Vikings and the North, the Baltic Sea, and even the Black Sea [16].Aqueducts, channels of water transport, were used in ancient India, Persia, Assyria, and Egypt as early as 700 BCE.Ancient water technologies, especially those of Roman, Mesopotamian, Egyptian, Greek, Iranian, and indigenous American societies, including water recovery, storage and transportation, and water for energy generation (e.g., mills and "pumps"), as well as for operations which reused water (e.g., Greek lavatories), or channels for water distribution for food production, have been covered extensively and expertly [17,18].Some 10,000 year ago, when humans adopted an agrarian way of life with permanent settlements, they became highly Fig. 1 Balancing water resources dependent on local water resources.The scale and complexity of human societies have increased over the years and have been due in part to increasing agricultural productivity [19] and the consequent need for access to water.The earliest known large settlement ("urban") is Jericho from 800 to 700 BCE, located near springs and other bodies of water [20].The recognition of the importance of water quality, purity, sanitation, and its impact on human health have also resulted in careful selection of settlements near rivers [21,22] and the use of water storage and setting tanks, filters, and the boiling of water [20].Fermented liquids, especially ale and beer, afforded an escape from contaminated water as they were considered as safe beverages [23].
Historically, drinking water quality was examined by the senses.Tasteless, cool, clear, and odorless water was considered best, while stagnant, marshy water was avoided [24].Early water treatment methods as recommended by ancient Sanskrit and Greek texts (dating back 6000 years) included boiling over fire, heating in the sun, dipping a heated iron into the water and then allowing the water to cool, and filtering through sand and gravel [24].Egyptians used chemicals, especially alum as the coagulant [25].Plant seeds, rhizomes of algae, roots of water lily, and different stones were also used.Seeds from Moringa oleifera were known to be highly effective [25], as well as the use of copper pots.It is likely that the effectiveness of plants for water purification was based on their chitinase activity which affect microbial cell walls [26].During the 1700s, sand filters became applied for large-scale applications.Ancient Maya cultures used zeolite for water purification for more than 1000 years [20].During 1847/48, Ignaz P. Semmelweis demonstrated the efficacy of chlorine as a bacteriocide, and chlorine was finally introduced for large scale chlorination of water in the Western world [27].Photocatalytic antibacterial disinfection methods with ZnO-based nanomaterials have attracted attention, and there is exploration of new and effective methods aimed at antiviral contamination [28].Concepts for drinking water supplies for disaster areas and for home water treatment have been previously summarized [29].

Properties of Water for Sustaining Life and the Planet
The remarkable chemical properties of water which account for the presence of life on Earth include the change of the states from frozen to liquid and gaseous and the dipolar nature of liquid water molecules (due to the net charges of the oxygen and hydrogen atoms) and its molecular structure.Water has unusually high melting and boiling points, heat capacity, heat of evaporation, and surface tension, as well as a maximum density at 3.98 °C and expands upon freezing [30]."Water may be commonplace but it is unusually weird" in that unlike most substances it shrinks on melting, has unusually high surface tension, is difficult to compress, and is a good solvent for a wide range of substances [31], and exists simultaneously in all three forms-solid, liquid, and gaseous [5,31].Although like other simpler liquids, water molecules interact with each other through van der Waals forces, they are unlike simpler liquids due to the orientation-dependent hydrogen bonding that results in the open tetrahedral cage-like structuring.This contributes to their unusual volumetric and thermal properties [32].
The chemical and physical properties of water are useful to cells and organisms.Due to its dipolar nature, water interacts with molecules resulting in three important effects which make water the basic ingredient for life on Earth: (1) the tight packaging of water molecules in a liquid means less evaporation from the Earth surface, (2) the capillary action of water which is essential for transport (e.g., water, nutrients) in the soil and in the stems of plants, and (3) the ability of water to dissolve other substances [5].All forms of life depend on water and on the solubility of gases in water.Liquid water constitutes about half the volume of every living biological cell.Food from sea life requires conditions under which oxygen has sufficient solubility in water, and marine plants require carbon dioxide dissolved in water for biosynthesis.Gas solubility in water depends on temperature, pressure, and salinity [32].Surface tension and capillary action of water are also important, allowing water to flow up tubes such as in the stems of plants, where cohesion holds the water columns together to also prevent tension rupture caused by transpiration pull [5,32,33].
Falkenmark has recently discussed the functions of green water (held in soil and available to plants) and blue water (in freshwater lakes, rivers and aquifers) and their interactions with the land system to form a dense interactive network which constitutes the "life support system" of humanity [34].The functions of green water include: (1) regulatory functions (soil moisture, evaporation, and transpiration flows to regulate the Earth´s energy balance and climate system), (2) productive functions (evaporation and transportation to sustain food, biomass, and bioenergy production), and (3) moisture feedback functions (regulating the water cycle over land by evaporation).Blue water serves five functions which are: (1) water for societal supply (available for withdrawal), (2) water as a carrier (nutrients and pollutants) and for transport, (3) water as state (water masses and storage), (4) water with a productive function (for irrigation, for food production, and to sustain aquatic growth), and (5) water as a control function (regulating Earth's energy balance, sea levels, and geological processes) [34].

Water for Food Production
Food resources include the environment (land, water, air) with deer, buffaloes, mushrooms, fishes, and birds as examples; cultivated land, agriculture (plants, horticulture, and animals), greenhouses and hydroponics, and gardens; cultivated waters, aquaculture; foods made by fermentation (traditional, biomass, and precision); and more recently insect rearing.Agriculture is the main user of water worldwide [35][36][37][38].Table 1 provides water requirements for production of various food sources [39][40][41].Water is also a significant resource for production of seafood, with water use efficiency being dependent on the farming/fishing system (e.g., raceways, ponds, or re-circulating aquaculture systems) and type of fish [42].Current annual global consumption of seafood is about 155 million tons with an expected substantial future increase [43,44], with edible food from the sea increasing by 21-44 million tons by 2050 [43].

Water for Agriculture
According to the recent Report to the Club of Rome [45] "agriculture is linked directly to 78% of eutrophication in lakes, rivers, seas, and the ocean."Earlier estimates were that agricultural water made up 70% of water withdrawals from surface water and ground water, with a global irrigation water use of 2.2 × 10 15 -3.8 × 10 15 L per year [46].There is significant ground water depletion, and there needs to be steps taken to reduce groundwater disappearing to prevent water scarcity [47].
Irrigation involves the use of multiple water sources for enhanced agricultural productivity.Irrigation of crops is used for 40% of the world's food supply [48].The amount of water required for crops and irrigation water and its quality are of importance for food production.Water requirements are also affected by climate change and the region of crop production.As an example, about 25 years ago, it was estimated that 7 million liters of irrigation water were needed to irrigate one hectare of irrigated corn, with also significant demands of energy costs for pumping water [49].Pimentel et al. also showed that this additionally requires approximately 8 million kcal of fossil fuel for irrigation energy and suggested that energy costs of pumping the irrigation water from a depth of 30 m were approximately $1000/ha, with few crops being sufficiently valuable to justify such costs [49].Further they calculated that a total of 10 million kcal for machinery, fuel, fertilizers, pesticides, partial irrigation, and other inputs were required to produce 1 ha of corn in the USA.These significant demands on water and energy resources were expected to influence the economics of irrigated crops and selection of specific species worth irrigating [49].A study which examined crop water and irrigation water requirements for rice cultivation in Southern China suggested that both these water requirements will increase in the future [50].A depletion-replenish process has been capturing more water than even the world's largest dams.Bangladesh farmers, taking advantage of this Bengal Water Machine, are irrigating rice paddies during the dry season from shallow water wells, lowering the groundwater table.This creates conditions to replenish groundwater during the monsoon rain season, which made one of the most densely populated country in the world self-sufficient by the 1990s [51].
Water conservation is important to reduce water use in agriculture.Hydroponic systems have revealed increased water use efficiency and improved lycopene and ß-carotene contents in tomatoes compared to irrigated greenhouse systems [52].Wastewater treatment and reuse carried out efficiently is a potential resource for use in agriculture [53].Desalination of surface water using solar desalination or membrane processing [54][55][56], as well as wastewater treatments and reuse processes including ozone applications, ultraviolet irradiation, microelectrolysis, using plants or microbial species, computational approaches in waste water treatment such as bioinformatics and genome sequencing, computational fluid dynamics, or remote sensing and geographical information systems, as well as energy production through wastewater treatment have been summarized in recent publications [53,57].Larger amounts of good quality water will be required in future for irrigation of crops to ensure production of good quality food and feed crops [37,38].The global yield of agricultural crops is dependent primarily on fertilizer use, irrigation, and climate.It is suggested that alteration of nutrient and water management practices can close the yield gaps of major crops [58].Global irrigation water consumption associated with global crop production has been identified as follows: 52% of irrigation is unsustainable, and 15% is virtually exported, with an average 18% increase between the years 2000 and 2015.About 60% of the global virtual transfers of unsustainable irrigation water consumption are driven by exports of cotton, sugar cane, fruits, and vegetables [38].The global economic value of water in agriculture is difficult to evaluate since a global assessment is still missing [59], and in most regions of the world, farmers do not pay for the real value of water (including water infrastructures such as dams, aqueducts, and wells), which has been estimated at $ 0.13 per 1000 L as the median global value.However, other industries competing for water (e.g., mining, oil) pay prices ranging from $0.81 to 1.62 per 1000 L in periods of water scarcity [59].There exists a relationship between the water possession of a country and the capacity for food production.This means that assessing the irrigation needs is indispensable for water resource planning to meet food needs and avoid excessive water consumption [60].Virtual water, which is water used in the production process of agricultural or industrial food products, is often transported from one country to another.This essentially amounts to using food as a means of water export from one country to one other [36,61].
There are also alternative strategies to help reduce the water requirement for production of foods by changing diet.For example, rice growth requires large amounts of water (Table 1) because it was domesticated (as well as other crops) based on human acceptance and safety and not on photosynthetic efficiency.Designing drought-resistant crops using gene-editing technologies (e.g., CRISPR/Cas9) has the potential to produce high yields in water-limited environments [62].Genetic modulation involving the expression of a transcriptional regulated gene in rice resulted in higher yields and shortened growth duration in rice via an improved regulator modulated photosynthesis and nitrogen uptake [63], allowing higher amounts of food being produced per unit area with less nitrogen and water requirements [64].Transformation of food systems is required in the face of climate change by improved crop management and improved livestock management [65].Examples of strategies that could be applied for improved crop management include reducing nitrous oxide emissions from synthetic fertilizer application and reducing methane emissions from paddy rice using agroforestry [65].For improved livestock management, strategies that could be considered include: (1) using better grazing land management, (2) improving manure management, (3) using higher quality feed and reducing enteric fermentation, (4) reducing nitrous oxide through manure management, (5) sequestering carbon in pasture, (6) implementing best animal husbandry and management practices, (7) using non-animal protein sources, and (8) using microbial protein as feedstuff [65].
Domestic wastewater (sewage) is known to be used for irrigation and aquaculture by a number of civilizations worldwide since the Bronze Age (3300 BCE to 1200 BCE), and recycled water is now used for almost any purpose, including potable water [66].Human urine recovery and use, as previously already collected by Aztecs, has been suggested to ease the issues of diminishing phosphate reserves and energy-intensive methods of producing nitrogen fertilizers.Field-scale experiments with barley have been carried out comparing the efficiency of urine versus corresponding amounts of mineral fertilizer, and the results showed that there were no significant differences in protein yield.Also, there were no pathogen indicators, pharmaceuticals, hormones, and heavy metals when urine was used for fertilization of barley, apart from progesterone which was below detectable or Finnish legal limits [67].

Water for Mariculture
Ocean water is fluid and interconnected allowing easier transport (three dimensional) of pollutants or alien species, which can cycle and remain in the ocean for extended periods of time.In addition, there is lack of ownership and responsibility in much of the oceans, making regulation and exclusion of non-authorized activities harder [44].A possible pathway with room for sustainable growth may be achieved by shifting to "blue food" (anything edible from fresh water or the sea), including lesser known marine resources such as sea cucumber, jellyfish, seaweed, and microalgae [44].Additionally, some of these blue foods are seagrass-associated.Seagrass meadows also store and sequester carbon, playing a fundamental role in the filtration of coastal waters (also reducing bacteria and viruses), the trapping of particles (including microparticles), cycling nutrients, absorbing nitrogen from the water, and also protecting coastlines from erosion [68], thus contributing to food security.
As related to the wider issues around sustainability, the human use of marine fish goes back to at least 35,000 to 40,000 years, with the onset of overfishing and whaling early in the seventeenth century [69].Modern fishing vessels also require energy (e.g., fuel, hauling nets, refrigeration of catch), making the average carbon footprint of one kg of wild fish approaching that of 1 kg of beef [44].The main pathways suggested for increasing food supply from the ocean include: (1) improving the management of wild fisheries, (2) implementing policy reforms for mariculture, (3) advancing feed technologies for fed mariculture, and (4) shifting demand, which affects the quantity supplied from all three production scenarios.As for feed input, e.g., current fish meal and fish oil, alternative feed ingredients including terrestrial plants or animal-based proteins, seafood processing waste, microbial ingredients, insects, algae, and genetically modified plants are being developed and used [43].

Properties of Water Exploited in Food Processing
Water has many anomalous chemical-physical properties.The use of the anomalous properties of water lends itself to interesting applications in the fields of engineering, medicine, and physiology [33].Among the thermodynamic anomalies of water are its liquidity at room temperature, surprisingly high melting and boiling points, the temperature dependency of water density (max.at 4 °C), and heat capacity (min.at 35 °C), with the isothermal compressibility being a minimum at 46 °C [33].The pressure-temperature relationship of water that was pioneered in 1912 [70] has been extended, with currently 16 forms of solid crystalline ice phases known, with each of them having different densities driven by different temperatures and pressures [32].

Water in Food Processing
The properties of water are made use of when developing food processing operations to produce safe, nutritious, and quality foods.The existence of liquid and solid metastable phases within the phase diagram of water and watercontaining food products has opened a new range of possibilities in pressure-induced freezing and thawing of biological products [71][72][73] as well as in structure modifications of proteins [74].In relation to water diffusion and viscosity properties, water is a complex glassy material undergoing a glass transition to an amorphous solid in which it is "neither fragile nor strong" [32].The control of the glassy state of water during processing influences the success of spray drying, freeze drying, extrusion processes, and the storage stability of dehydrated foods [75].
Water is a polar solvent at a temperature of 21.85 °C and a pressure of 0.1 MPa.The dielectric properties of water enabled the use of electromagnetic waves for heating processes, especially the development of microwave processes [76,77].Raising the temperature and pressure causes significant changes in the properties of water, especially as a solvent due to variations of its dielectric constant, conductivity, ionic product, and the structure of the hydrogen bond network.Changes in viscosity, heat capacity, diffusion coefficient, and density influence the transport characteristics of aqueous solutions [78].The electrical conductivity of water led to the development of emerging technologies such as electric field process applications in bioscience, engineering, biotechnology, and medicine [79].
Water at normal conditions, at sub-or super critical conditions, and as superheated steam is an effective solvent [32,78].Supercritical water (374.2°C, 22.1 MPa) can be used for detoxification of organic waste, for decomposition of plastic materials (e.g., polyethylene), and for municipal and industrial waste treatment including food waste [78,80].Supercritical water is a highly corrosive medium, and the corrosion stability of metals and alloys is critical.Further work in the development of corrosion stable material development is needed.To date, pure alumina and Al 2 O 3 -ZrO 2 ceramics were found not to be severely corroded in supercritical water [78].
It is important to develop transparent methods for identifying energy-and water-intensive steps for a productspecific life cycle analysis, despite the current uncertainties in estimating unit process water demand [81].As the food processing industry is a major user of water, water re-conditioning and re-use from effluent food processing streams offer a route to water conservation.As potable water is required for each food processing operation, the water treatment must be fit-for-purpose, with recovered streams being well-defined and well-characterized in terms of water quality parameters, including microbial load and chemical composition [82].There are significant challenges for treating water to potable standards.There are various physical, chemical, and biological technologies for water reuse and resource recovery.Among these are filtration, anaerobic-aerobic treatment methods, membrane processes, chemical oxidation and electrochemical methods, and the use of adsorbents [57,83].More efficient use of water in the food industry requires cleaner food processing methods and recycling and reusing water [84].

Water in Foods
Water affects the safety, stability, quality, and physical properties of food [85][86][87][88][89] and has an influence on the processing of foods [90].The major nutrients, carbohydrates and proteins, are hydrophilic and may be plasticized by water.Water is also an active food ingredient used to control reactions, food texture, physical, and the biological behavior of food.Food may lose or absorb moisture during storage.Processing often relies on the physical-chemical properties of water and its state transition properties.When associated with food, water can be placed in three categories: free, absorbed (affected water), and bound water [30].The development of the water activity (a w ) concept (ratio of vapor pressure of water in a solution to vapor pressure of pure water) [91] and subsequent work in the areas of low moisture and intermediate moisture foods [87] led to a better understanding of water sorption behavior and to the glass transition temperature [92] that is related to texture, safety, appearance, quality, packaging requirements, and shelf life of foods [30].Labuza [93] summarized the relevant properties of water with respect to food texture as: (1) water holding capacity (amount of water held in the food under various conditions), (2) plasticity (degree of flowability), (3) water that binds together the various ingredients of food, (4) water for stabilizing foams and emulsions, and (5) effect of dissolved solutes in water on the viscosity and surface tension of water in the food system.However, despite all this progress and research related to water and its influence on food, new aspects and properties keep arising, such as new forms of ice [32], unique protein functionalities [74], hydrogen bond transfer in water [94], and a two-state model based on the dynamic co-existence of two types of local structures [95].

Water Scarcity
The water footprint measures human's appropriation of freshwater sources and includes the consumptive use of rainwater, ground and surface water, and the amount of polluted water [96].It had been estimated that the blue water footprint (consumption of surface and groundwater resources) exceeded the maximum sustainable blue water footprint during 2013 in half of the world's river basins.In two-third of the world's river basins, the pollution assimilation capacity for nitrogen and phosphorus had been fully consumed [97].In addition, freshwater reservoirs are running dry.The Colorado River, supplying 40 million inhabitants in southwestern United States, rarely reaches the Gulf of California [98].There is about 78% global ocean and freshwater eutrophication (pollution of waterways with nutrient-rich pollutants) [99].The World Economic Forum Global Risk Report 2022 listed natural resource crises as an environmental global risk-"Chemical, food, mineral, water or other natural resource crises at a global scale as a result of human overexploitation and/or mismanagement of critical resources" [100].It has been previously stated that "our demand for water has turned us into vampires, draining the worlds of its lifeblood" [41].At a global level and annual basis, enough freshwater is available to meet rising global demands for water, but there is still water scarcity in some regions [11].This is because at a spatial and temporal level, the varieties of water demand and availability are large and lead to water scarcity in several parts of the world and during specific times [11].
The global use of water per capita over the past 100 years has only increased from 209,000 to 230,000 L per year, and irrigation was the largest consumer over the period from 1900 to 2000 [10].Despite these small variations in capita consumption over time, rapidly increasing local populations and increases in total water consumption resulted in a nearly 16-fold increase in the populations under water scarcity within the twentieth century.In the 1900s, approximately 200 million (14% of global population) lived in areas under some degree of water scarcity, and the number increased to over 2 billion by the 1980s (42%) and 3.8 billion (58%) by the 2000s [10].The 2018 edition of the UN World Water Development Report stated that nearly 6 billion people will suffer from clean water scarcity by 2050 [13].Global water demand for all uses, presently at about 4.6 × 10 15 L per year, will increase by 20 to 30% by 2050, up to 5.5 × 10 15 to 6.0 × 10 15 L per year [13].Global urban populations facing water scarcity are projected to increase from 933 million (one third of the global population) in 2016 to 1.69-2.37 billion (one third to nearly half the global urban population) in 2050.The number of large cities exposed to water scarcity during this period is projected to increase from 193 to 284, including 10-20 megacities [101], stressing the urgency for counter measures against these trends.It is expected that by 2030, the world's 8.5 billion people will already consume 6 × 10 15 L of water per year [32].
A FAO report states that "agricultural production has grown 2.5 to 3 times over the last 50 years while the cultivated area has grown only 12% during that time", and more than 40% of that increase has come from irrigated areas [35].While agriculture then used 11% of the world's land surface for crop production, it accounted for 70% of all water withdrawn from aquifers, streams, and lakes [35].Water for irrigation currently accounts for about 70% of global water withdrawals and nearly 90% of consumptive water use.A synthesis of simulations from seven global hydrological models found that irrigation water consumption amounted to 1.26 × 10 15 L per year [102].It is estimated that the number of people living in food production units affected by green-blue water scarcity has gone up from 360 million in 1905 (then 21% of the world population) to 2.2 billion (then 34%) in 2005 [103].While water scarcity research has traditionally focused on scarcity of blue water, it is important to consider green water.A critically limited resource such as green water should be part of assessments of water scarcity, food security, and bioenergy potential.
Human impact on lakes, rivers, streams, wetland, and groundwater has been reducing freshwater biodiversity [104].This degradation occurs on every continent of the planet, in every major river basin on Earth, and more rapidly than in terrestrial ecosystems.Less than one fifth of the world's preindustrial freshwater wetlands remain, and this proportion is projected to reach less than one third by mid-century [104].More than 30% of the global biodiversity has been lost because of the degradation of freshwater ecosystems due to pollution.In 2020, the food system occupied 12.6 million km 2 of cropland, used 1.8 × 10 15 L of freshwater resources from surface and groundwater, and applied 104 tera tons of nitrogen and 18 tera tons of phosphorus as fertilizers [105].
Hundreds of million people around the globe benefit from low-cost proteins from freshwater and ocean fisheries [69,106].However, overfishing and other pervasive human disturbances, including pollution and water quality, had a massive detrimental impact on biodiversity.Overfishing causes disturbances to marine ecosystems.Overfishing can lead to population explosions of microorganisms responsible for increased eutrophication, diseases of marine species, toxic bloom, and diseases that affect human health [69].Human activities have also increasingly interfered with the water cycle and have been putting pressure on water resources.Humans affect water systems directly by modifying water circulation and quality (e.g., water withdrawal, wastewater disposal, river regulation, and modification of downstream structures) and indirectly by land-use activities (e.g., modifying vegetation and soil cover).The human activities create water problems, and hence, efforts need to be made to maintain global water supplies [107].Water quality is also affected by water pollution due to wastewater discharge of aquaculture activities [108].Insights from a study into the nature and causes of water crises suggested that ground water depletion, ecological destruction, drought-driven conflicts, unmet subsistence needs, resource capture by the elite, and water reallocation to nature all contributed to water crises [109].

Management of Water
Global water use in 2019 was at 4.6 × 10 15 L per year with an expected increase to 5.5 × 10 15 to 6.0 × 10 15 L in 2030 [32] or in 2050 [13].A future water scenario of water availability under climate change and population growth for 2050 indicates that 59% of the world population will face blue water shortage and 36% will face green and blue water shortage, stressing the need to integrate green and blue water management [110].This means that there will need to be strategies in place to mitigate against water scarcity.

Water Treatment
Global water scarcity is driven by water quantity and quality issues, and expansion in clean water technologies is necessary to reduce water scarcity [56].van Vliet et al. showed that expanding global desalination from 2.9 × 10 12 to 1.36 × 10 13 L per month can reduce water scarcity [56].Food processing operations generate aquatic, atmospheric, and solid wastes, with water footprints varying with food product types [111].Animal food production is the largest water user (Table 1) and produces the most wastewater [112].Food processing generates water from dewatering processes which could be recovered (e.g., condensates)."Non-thermal" processes (e.g., high hydrostatic pressure, pulsed electric fields, and ultrasound) can contribute to energy conservation during dewatering processes [111].Sound management strategies are needed for the safe disposal of brine generated in processing.A comprehensive review of water management and wastewater treatment revealed that water is mainly used for washing, transporting, and cleaning food raw materials and for cleaning of processing equipment and sites [112].Among the most common wastewater treatment methods, physical methods (e.g., sedimentation, flotation, crystallization, and membrane processes, were highly effective from the perspectives of water, energy, and land requirement to treat gray water (reusable after minimal treatments to remove solids) in food processing facilities.Minimizing organic contaminants requires application of water treatment methods (e.g., membrane processes and bioreactors).Algal treatment methods have also been used in meat, fish, dairy, grains, and edible oils processing industries due to the added value of generating algal biomass [112].Early in the twentieth century, physical, chemical, and biological wastewater treatment systems were developed that were capable of reducing or eliminating water-related diseases such as cholera, dysentery, and typhoid [113].The increasing severity of the water crises in the world has required attention to developing strategies to manage freshwater resources, which includes reuse of treated wastewater due to the increasing difficulties of finding new sources of traditional water [113].

Changing Habits
About one fourth of freshwater consumed in global food production is wasted because the food produced with this water is never consumed, meaning 1.74 × 10 14 L of wasted blue water from uneaten plant-based food.At least a third of the food is wasted globally, and there is a significant potential of reducing water scarcity by reducing food losses and waste along all points of the food supply chain.The water lost because of food loss and waste is dependent on the diet and waste patterns of people.Global food loss water wasted per capita per year is about 2.1 × 10 13 L, with high-income countries reaching 4.3 × 10 13 L per capita per year, and for low-income countries, it is about 4 × 10 12 L per capita per year [114].Marston et al. has stressed the potential of reducing water scarcity by reducing food losses and waste [114].The total European Union food waste averages 123 kg per capita per year of which 80% is avoidable, which amounts to 4.0 × 10 12 L per year of blue water and 5.2 × 10 13 L per year of green water.Marston et al. suggest that food waste policies should have one or more of three potential goals: (1) waste prevention, (2) recovery of wasted food for consumption by humans (e.g., donations to food banks, repurposing manufacturing by-products), and (3) recycling wasted food (e.g., animal feed, generating bioenergy).Further, reducing food loss and waste by 50% would improve water availability for over 720 million people globally and eliminate local water scarcity for 131 million of those people [114].An international analysis on options for reducing the environmental effects of the food system to keep it within environmental limits [105] concluded that halving food loss and waste (and thus also reducing water loss and waste) could reduce environmental pressures by 9-24%.Springmann et al. also stress the need for improvements in water management that increase basin efficiency, storage capacity, and better utilization of rainwater [105].
Diet change can also contribute to reduction in water use.Jalava et al. suggest that water consumption of current diets with that of different dietary guidelines which limit the amount of protein from animal sources to 50%, 25%, 12.5%, and 0% would decrease global green water consumption by 6-21% [115].Springmann et al. suggest that combining changes in some or all measures analyzed (food management, technology, and diets) could reduce environmental pressures by around 25-45% to 30-60% [105].Interestingly, a comparison of diet-related water resources needed in nine major Mediterranean countries was higher compared to that of the EAT-Lancet diet [116,117].Currently, depending on our diet we need 2000 to 5000 L of water to produce the food consumed daily by one person [118].The amount of water needed to produce a variety of foods (Table 1) shows the large varieties in water needs and also points to the issue of virtual water [36].It is expected that global demands for water could be increased by 50% if we do not change our habits now [118].Rising incomes and urbanization are driving a global dietary transition in which traditional diets have been and are being replaced by diets higher in refined sugars and fats, oils, and meat.These trends could contribute to an estimated 80% increase in global agricultural greenhouse gas emissions from food production [119] and thus lead to a global, tightly linked diet-environment-health trilemma.It is estimated that reducing animal products in the human diet offers the potential to save water resources, which is expected to be up to the amount currently required to feed 1.8 billion additional people globally [115].
More than two-thirds of water scarce cities can relieve water scarcity [101].The authors, He et al. suggest four directions to address global water scarcity and to realize the Sustainable Development Goals: (1) promote water conservation and reduce water demand, (2) control population growth and urbanization in water scarce regions by implementing relevant policies and regional planning, (3) mitigate climate change through energy efficiency and emission abatement measures to avoid water resources impact caused by the change in precipitation and the increase in evaporation due to increased temperature, and (4) undertake integrated local sustainability assessment of water scarcity solutions.A meta-analysis, seeking to quantify comparable crop yields for a broad range of crops, revealed that increased yields (cucumber, gherkins, tomatoes) have been found for products produced by urban agriculture (e.g., ground based urban land, indoor facilities, roof tops, gardens/balconies) compared to conventional agriculture yield data [120].New recovery concepts should refocus efforts on the multiple benefits water provides such as improving water use efficiency, integrating new technology for decentralized water sources, modernizing management systems, committing to ecological restoration, and adapting more effective economic approaches [113].

The Future of Water
The UN Sustainable Development Goals (SDGs) has set a target for water.Target No. 6 which is related to water requires ensuring availability and sustainable management of water and sanitation for all.The target was set to protect and restore water-related ecosystems.The relevant targets set to be achieved by 2030 are: (1) safe and affordable drinking water for all, (2) end open defecation and provision of access to sanitation and hygiene, (3) improving water quality, wastewater treatment and safe reuse, (4) increasing water use efficiency and ensuring freshwater supplies, (5) implementing integrated water resource management, and (6) expanding water and sanitation support to developing countries [121,122].Additional complementary indicators for SDG 6 targets that are practicable to support coherent policy making and "ultimately contribute to the Agenda 2030´s aspiration that all countries take action" are required [121].
As for climate change, water has been called the most crucial link in climate adaptation and also the most ignored [123].About 60% of climate change adaptations such as irrigation and harvesting rain can address water-related hazards.Mukherjee suggests that more salt-and droughttolerant crops, better flood and drought information, planned relocation, and support for those who migrate are required to mitigate water-related hazard [123].Closing food yield gaps (difference between yields under optimum conditions and average yields in a particular location) requires research inputs such as water, which might not be locally available and their use could have other possible negative environmental impacts [65].Three water-related tipping points have been identified: (1) global change pressure, glacial melt, sea level rise, drastic rainfall regime change, regional climate processes, (2) water overuse (river depletion, river basin closure, groundwater collapse), and (3) land management (deforestation, salinization, land mismanagement) [14].The Indigenous Peoples Kyoto Water Declaration (UNESCO, 2006) states: "we recognize, honor and respect water as sacred and sustaining of all life", and "we stand united to follow and implement our knowledge and traditional laws and exercise of our right of self-determination to preserve water, and to preserve life".There are 476 million indigenous peoples in 90 countries and seven socio-cultural regions, and they often reside at sites of high biodiversity and occupy over a quarter of the world's land.There are many lessons that can be learnt from how indigenous people approach food sustainability and live in harmony with nature [124].
Irrigated agriculture for food production is currently and will likely remain the key in global water resource management.Water use competition for food and energy production may lead to unintended conflicts between agriculture and power sectors, thus threatening food and energy security [125].New measures will be needed to limit water pollution from the production and consumption of synthetic chemicals.As an example, administered pharmaceuticals enter aquatic environments trough wastewater.Currently 80% of all wastewater flows into ecosystems without any treatment, and virtually all of the 20% that is treated still contains excreted pharmaceutical and pathogens, requiring their removal before they enter the environment via advanced wastewater treatments [126].
Table 2 provides information on the water footprints of foods in liters per kg of food, 100 g of protein and per 1000 kcal [127].These data also point to the need for human food consumption changes towards a more plant food-dominated diet [117].Unconventional water resources ("supplementary water resources that need specialized processes to be used as water supply") have been reviewed and evaluated [128].These include artificial recharge water, agricultural drainage water, cloud seeded water, desalinated water, dew, fog and fossil water, iceberg water towed, and virtual water.
Water pollution is a key driver of environmental degradation and a threat to our food systems [65].Among the mitigation areas suggested by Zurek et al. are the restoration of peatland and reforestation of marginal and unimprovable agricultural land, as well as using agroforestry.Forest, water, and energy interactions provide the foundations for carbon storage, cooling terrestrial surfaces, and for distribution of water resources.Forests are intimately linked to rainfall and water availability, they transport and cool water (locally and globally), and they regulate water supplies (fog and cloud capture, infiltration and groundwater recharge, and flood moderation/integration).In addition, forest biodiversity enhances many ecosystem functions like water uptake, tree growth, and pest resistance [129].Ellison et al. suggest that future research should identify the required species richness, particularly native species, as essential drivers in land management policies and the required richness for optimal water ecosystem services should be identified as well as the effects on biodiversity, ice nucleation, and other rainfall related processes [129].It has been stressed that controlling soil erosion helps conserve water by reducing rapid water runoff, and protecting forests facilitates effective use of water resources [49].In addition, create cultivated fields and pastures, and grassy ecosystems (terrestrial grassland, submarine meadows) to stabilize fertile soil, store carbon, generate oxygen, and provide habitat, building materials, and food [130].Leaves have also been developed as a potential protein source for food [131].

Recommendations for Action
It is important to understand the key roles that water plays during global circulation water to make informed choices about strategies to reduce water scarcity.Falkenmark [34] suggests that these key roles which exist simultaneously and interact dynamically include: (1) the controlling role

Actions
Food processing • Install water condensate reuse systems [111] • Develop and use water replacement technologies (e.g., airstreams) for cleaning and transporting of food raw materials • Apply low water consuming processing technologies such as "electric" (pulsed electric fields, ohmic heating, microwave, ultrasound, cold plasma, ozonization), super critical gases, extrusion, and high pressure, technologies (high hydrostatic pressure pasteurization, sterilization, freezing and modification) • Develop small scale, decentralized water saving food processing facilities • Recover water from water separation processes (e.g., drying, concentration) Food preparation and consumption • Expand desalination and treated wastewater use [56] • Develop food waste policies regarding food waste prevention [115] • Reduce food loss and waste, change diets to healthier, and less water-consuming foods [105] • Shift diets to crops and animal products with lower water requirement, and reduce food waste [40] • Reduce diet's animal caloric fractions [14] • Shift consumer demands towards more sustainable diets such as fish and other aquatic foods (e.g., seaweed, farmed bivalves, small pelagic fish) that have improved food environmental performance [140] • Increase use of leftovers to reduce waste (e.g., soups: liquid supply and nourishment) Fig.
having integrated Earth observing systems for clouds and precipitation.These activities towards a better quantitative understanding of exchanges between Earth's main water reservoirs can aid development of better strategies to combat water scarcity [132].Table 3 and Fig. 2 provide some of the key recommendations for action for the reduction of current water scarcity and contamination issues.

Final Remarks
In considering the future of water, it is worthwhile to reflect on the conclusions of a comprehensive overview and critical analyses of ancient water technologies [18].The editor (L.W. Mays) in the final chapter on "Lessons from the Ancients on Water Resources Sustainability" in the book concluded that ancient people lived in harmony with nature and their environment.Today we do not, and we fail to take adequate measures to prevent pollution and environmental degradation and to avert climate change.By not living in harmony with nature and the environment today, we are creating problems for a sustainable future [141].The concluding comments in the remarkable book on "Water Resilience for Human Prosperity" [14] suggest that there needs to be a new water paradigm which incorporates thinking about green and blue water, and integration of land and water requirements across socio-ecological structures at various scales to build water resilience.We agree with Springmann et al. [105] that only a combination of all possibilities and options (Table 3 and Fig. 2) can deliver reduction of water stresses.We further believe that only big and fundamental changes rather than "cosmetic" attempts will be fruitful, which will require cooperation of all people of our planet.We further want to stress that such fundamental changes are required to reverse the 6th global mass extinction, this time generated by humans due to unsustainable use of land, water, and energy [64].The Intergovernmental Panel on Climate Change Special Report on Extreme Events over the last 64 years identified an increased frequency to flash droughts in more than 74% of region around the globe [142].There has to be more and urgent attention paid to water deficit.Water is a fastvanishing resource.It is important to realize and recognize that water is a highly neglected but utmost valuable resource.While much attention has been paid to developing strategies for conservation of land and energy resources, there has been less of a focus on water security, even though water deficits threaten food security and future sustainability.The time to act is now to mitigate water scarcity so that water will not become a vanishing resource for future generations.

(
key in sustaining all life generating ecosystem services and functions in terrestrial and aquatic systems), (2) the state role (victim of change responding to land-use change and pollution), and (3) the driving role (e.g., social shocks by floods and draughts, driver of conflicts).Water (via energy transformation ice to liquid, liquid to vapor) keeps the Earth

Table 2
Freshwater withdrawals needed (in 1000 L) for production.Comparison per kg of product, 100 g of protein generated or per 1000 kcal production a

2
[132]ecommendations for fundamental changes towards water preservation and protection in food systems temperature 30 °C warmer than it would be without water.Therefore, life on Earth would not be possible without water.Humans need clean and safe water for survival and health, but human activities also contribute to water pollution and water scarcity.Stephens et al. discuss the importance of Earth's water reservoirs and have demonstrated the relationship exists between Earth's water reservoirs, weather, climate and human activities[132].The melting of Earth's ice sheets and glaciers, with subsequent runoff and discharge into the oceans, is a major factor in rising sea levels.Antarctic and Greenland ice sheets together hold 2.5 × 10 19 L of ice, representing enough water to raise global sea levels by 70 m.Stephens et al. suggest the importance of gaining a better understanding by: (1) monitoring Earth's ancient waters (ice sheets, aquifers), (2) tracking the land-ocean exchanges of water vapor, (3) monitoring the greening of Earth (satellite monitoring to record water distribution), and