Water consumption for food production, including crops and livestock, accounts for about 86% of the total societal water consumption, though, locally, household and industrial uses can be predominant, particularly in major urban areas. Thus, securing water resources for agriculture, while reconciling the competing water needs of growing cities and surrounding rural areas, is a major challenge of our time. Climate change is expected to further enhance local water scarcity, especially in the subtropics. In fact, while climate warming is slightly increasing global precipitation, the global patterns of rainfall distribution are expected to become more uneven with an intensification of aridity in the dry subtropics, and an increase in precipitation in the wet tropics and the midlatitude temperate zone . The temporal variability of precipitation will likely increase, thereby enhancing the probability of drought and flood occurrences .Despite recent developments in desalinization technology , 2016, most human activities related to food and energy production rely on the consumptive use of freshwater. Desalinization remains limited to specific uses that require relatively small amounts of water and to societies that can sustain the associated costs . The freshwater available for human activities is stored in continental land masses either in soils or in surface-water bodies and groundwater aquifers. Often referred to as “green water,” soil moisture is retained in the ground by capillary forces and can be extracted only when it is subjected to a suction that overcomes the action of capillarity. Plants exert such suction through root uptake. Although most of terrestrial vegetation in natural ecosystems relies on green water , soil moisture remains for most part unavailable to direct human use because it is difficult to extract. In contrast, water stored in surface water bodies and aquifers, referred to as “blue water,” hydroponic net pots is more mobile and contributes to surface-water and groundwater runoff. Thus, green water leaves land masses in the water vapor phase as evapotranspiration , whereas blue water flows to the ocean in the liquid phase as runoff .
Since antiquity, human societies have engineered systems to withdraw blue water from rivers, lakes, and aquifers and have transported it through channels and pipes to meet the needs of a variety of human activities. Today, the main consumptive use of blue water is for irrigation , which strongly increases green water flows at the expense of blue water flows. Irrigation is a major human disruption of the water cycle ; indeed, many rivers are so strongly depleted that they no longer reach the ocean , while lakes in basins with internal drainage are drying out . Irrigation can modify the local climate, possibly by increasing evapotranspiration and effectively cooling the near-surface atmosphere . Irrigation may also moderately enhance precipitation downwind of irrigated areas and induce mesoscale circulations driven by the contrast between irrigated areas and the surrounding dry lands . It has been estimated that globally, irrigation uses a water volume that is roughly 2.56 × 1012 m3 /year , which accounts for about 2% of the precipitation . Although water is a renewable resource that is conserved in the Earth system, freshwater stocks can be depleted when their use exceeds the rates of natural replenishment. A typical example is groundwater that is often used for agriculture andis being depleted in many regions of the world , including the North American Southwest, Northern Africa, the Arabian Peninsula, and India . In some cases, groundwater use is depleting water stocks that accumulated in epochs with a wetter climate. In these aquifers “over pumping” leads to a permanent extraction of water resources, a phenomenon that is known as “groundwater mining” to better stress its unsustainability and the irreversible loss of resources that will not be available to future generations. However, even when the depletion of water resources is reversible, its environmental impacts may not be. Excessive water withdrawals from rivers and streams destroy the aquatic habitat and lead to extinction of riparian species. Interestingly, freshwater ecosystems are particularly vulnerable because the extinction rate of freshwater aquatic species is much greater than that of terrestrial organisms . Thus, sustainable use of water resources should prevent not only their permanent depletion but also the irreversible damage of downstream ecosystems.
A rich body of literature has discussed criteria to define minimum flow requirements and minimum flow variability required to conserve the aquatic habitat . A reevaluation of those efforts within the context of water sustainability has led to the formulation of the concepts of “planetary boundaries” and “safe operating space” that define a cap for sustainable water use . Such a cap is typically expressed as a fraction of the natural river flow, ranging from 20% to 60% , though recent studies have suggested referring to season-dependent fractions . Although globally, the current use of water for irrigation is smaller than the planetary boundary for blue water and accounts for only 5.4% of the global blue water flows , in many regions of the world those boundaries are locally exceeded, thereby causing habitat loss . Overall, irrigation is critical to sustaining the present rates of agricultural production. Although only 20% of the global agricultural land is irrigated , it sustains about 40% of the global crop production owing to the typically much higher yields in irrigated systems . Collectively, irrigated and rainfed agriculture accounts for about 10% of global precipitation over land, with green water flows from agroecosystems contributing to roughly 16% of the global evapotranspiration from terrestrial ecosystems . These figures give us a sense of the proportion of the water cycle that has been appropriated by agriculture. Moreover, other economic activities, such as mining, manufacturing, and energy production further increase the human demand for freshwater.Since the onset of civilization, agriculture has claimed land from natural ecosystems, such as forests, savannas, and grasslands. By converting these landscapes into agricultural land, humankind has profoundly altered the water and bio-geochemical cycles . Decades of research on deforestation have highlighted the profound hydroclimatic impacts of land use and land cover change . Compared to forests, rainfed farmland sustains lower evapotranspiration rates because of the smaller leaf area index, surface roughness, and root depth, and the greater albedo . The infiltration rates are also smaller because agricultural soils are often more compacted, typically from leaving the land fallow for part of the year and cultivated with heavy machinery. Smaller evapotranspiration and infiltration rates are expected to lead to higher runoff . However, in areas where agriculture is irrigated, water withdrawals for crop production deplete surface-water bodies and aquifers . Land use change also has an impact on the regional climate.
Land use change alters the surface energy balance and land-atmosphere interaction; these changes modify near-surface temperature, boundary layer stability, and the triggering of convection and convective precipitation . Some of these effects can alter the rainfall regime within the same region in which land cover change occurs, though it has been suggested that the impact also can be on adjacent ecosystems . Moreover, land cover change may modify the rate of emission of biological aerosols, thereby affecting cloud microphysics and cloud processes . The reduced evapotranspiration has the effect of reducing precipitation recycling, which is the fraction of regional precipitation contributed by atmospheric moisture from regional evapotransporation , a phenomenon that is relevant to policies and therefore is receiving the attention of social scientists , despite the great uncertainties with which it can be evaluated . Overall, forest or woodland conversion to cropland over large regions is expected to reduce precipitation and increase diurnal temperatures , though these effects depend on the size of the cleared area . The direct and indirect impacts of human activities on freshwater resources may strongly affect their availability to meet the competing needs of food or energy production and the environment,blueberry grow pot raising questions on the type of institutional arrangements that could improve water governance.Water is by its own nature fluid, renewable, and difficult to quantify , and its biophysical characteristics, such as the fact that it is a key input into biological processes and that is relatively plentiful and widely distributed , make the political economy of this resource very different from other similarly important strategic natural resources . From early human history, water use has led to complex dynamics of competition and cooperation . In a world with increasing societal pressure over scarce water resources and aggravating hydro climatic change, water governance is fundamental in the policy and development dimensions of water management. Even though access to safe water and sanitation is recognized as one of the UN-SDGs , about 4 billion people face water scarcity at least 1 month per year . Water availability may be affected by water quality, particularly in the case of drinking water, as the cost of treatment may become prohibitive in some locations, creating physical water scarcity of costly water resources. The reliance on water markets historically has been, and still is, strongly influenced by neoliberal governance approaches based on privatization, liberalization, and extension of property rights. The core principle behind these approaches is that water markets provide the correct economic incentives to promote the reallocation of water to higher valued uses and improve efficiency. These approaches treat water as a commodity and thus require the recognition of property rights that define the use, management, and trade of water resources . Easter et al. describe a strong legal system as the main institutional condition necessary for water markets to function properly. The creation of water markets in the Western United States and in Chile have been used as exemplary policy and governance models that could be exported and promoted in developing countries .
Since the 1980s, the World Bank has been the main promoter of water markets in developing countries, while also supporting the development of lucrative transnational opportunities in the water sector for private investors . In the context of the FEW nexus debate, however, a water market economy may lead to water resources previously used to produce food being transferred to other uses, such as industrial production or household needs in urban areas. In fact, the economic yield of food commodities may typically be orders of magnitude lower than that of the energy and water utility sectors . Food as a basic human need means that market approaches to water governance also can be evaluated in the context of their impacts on food security, particularly for the poor. For example, water markets could be structured with special consideration for certain industries, including agriculture, to avoid losing water allocations for production of food. A counterargument in favor of water markets stresses their positive environmental outcomes such as when water is partly acquired to reestablish environmental flows and improve aquatic habitat, or if the market sets a cap on the amount of water that can be withdrawn for human uses . The contemporary neoliberal trends of water commodification, that is, the multidimensional process through which goods that traditionally are not priced enter the world of money and markets , could be in stark contrast with the principle that access to water is a fundamental human right . Ostrom describes water resources as an iconic example of common-pool resources, which often have been successfully governed through diverse community and communal-property institutional arrangements. The multiple characterizations of freshwater by different cultures and societies make it difficult for freshwater to be reduced to a monetized commodity. Water can be perceived as a sacred commodity, a human right , a political good, an ecosystem medium, and a security asset . Moreover, the water sector has intrinsic characteristics that can be associated with structural market failures, with large externalities, and interconnectedness that make the level of individual and collective interdependence particularly critical . From a social and environmental justice perspective, the idea that water is not treated as a “common good” but as a commodity has generated criticism around the perpetuation of inequality and violation of fundamental human rights . Different narratives and political perceptions about the value, the meaning, and the function of water in society make a clear and uniform definition of “good water governance” difficult. As described by Meinzen-Dick , rather than considering single solutions for water governance, it may be more productive to have multiple institutions work together in an adaptive learning process.