water resources water resources water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the earth is salt water and only three percent is fresh water; slightly over two thirds of this is frozen in glaciers and polar ice caps. the remaining unfrozen freshwater is found mainly as groundwater, with only a small fraction present above ground or in the air. natural sources of fresh water include surface water, under river flow, groundwater and frozen water. artificial sources of fresh water can include treated wastewater (wastewater reuse) and desalinated seawater. human uses of water resources include agricultural, industrial, household, recreational and environmental activities. global values of water resources and human water use (excluding antarctica). water resources 1961-90, water use around 2000. computed by the global freshwater model watergap. water resources are under threat from water scarcity, water pollution, water conflict and climate change. fresh water is a renewable resource, yet the world's supply of groundwater is steadily decreasing, with depletion occurring most prominently in asia, south america and north america, although it is still unclear how much natural renewal balances this usage, and whether ecosystems are threatened. the framework for allocating water resources to water users (where such a framework exists) is known as water rights. natural sources of fresh water natural sources of fresh water include surface water, under river flow, groundwater and frozen water. surface water lake chungará and parinacota volcano in northern chile surface water is water in a river, lake or fresh water wetland. surface water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation, evapotranspiration and groundwater recharge. the only natural input to any surface water system is precipitation within its watershed. the total quantity of water in that system at any given time is also dependent on many other factors. these factors include storage capacity in lakes, wetlands and artificial reservoirs, the permeability of the soil beneath these storage bodies, the runoff characteristics of the land in the watershed, the timing of the precipitation and local evaporation rates. all of these factors also affect the proportions of water loss. humans often increase storage capacity by constructing reservoirs and decrease it by draining wetlands. humans often increase runoff quantities and velocities by paving areas and channelizing the stream flow. natural surface water can be augmented by importing surface water from another watershed through a canal or pipeline. brazil is estimated to have the largest supply of fresh water in the world, followed by russia and canada. panorama of a natural wetland (sinclair wetlands, new zealand) water from glaciers glacier runoff is considered to be surface water. the himalayas, which are often called "the roof of the world", contain some of the most extensive and rough high altitude areas on earth as well as the greatest area of glaciers and permafrost outside of the poles. ten of asia's largest rivers flow from there, and more than a billion people's livelihoods depend on them. to complicate matters, temperatures there are rising more rapidly than the global average. in nepal, the temperature has risen by 0.6 degrees celsius over the last decade, whereas globally, the earth has warmed approximately 0.7 degrees celsius over the last hundred years. groundwater relative groundwater travel times in the subsurface this section is an excerpt from groundwater. groundwater is the water present beneath earth's surface in rock and soil pore spaces and in the fractures of rock formations. about 30 percent of all readily available freshwater in the world is groundwater. a unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. the depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. groundwater is recharged from the surface; it may discharge from the surface naturally at springs and seeps, and can form oases or wetlands. groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. the study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology. typically, groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can also contain soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. groundwater is hypothesized to provide lubrication that can possibly influence the movement of faults. it is likely that much of earth's subsurface contains some water, which may be mixed with other fluids in some instances. under river flow throughout the course of a river, the total volume of water transported downstream will often be a combination of the visible free water flow together with a substantial contribution flowing through rocks and sediments that underlie the river and its floodplain called the hyporheic zone. for many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow. the hyporheic zone often forms a dynamic interface between surface water and groundwater from aquifers, exchanging flow between rivers and aquifers that may be fully charged or depleted. this is especially significant in karst areas where pot-holes and underground rivers are common. artificial sources of usable water artificial sources of fresh water can include treated wastewater (reclaimed water), atmospheric water generators, and desalinated seawater. however, economic and environmental side effects of these technologies must also be taken into consideration. wastewater reuse this section is an excerpt from reclaimed water. water reclamation (also called wastewater reuse, water reuse or water recycling) is the process of converting municipal wastewater (sewage) or industrial wastewater into water that can be reused for a variety of purposes. types of reuse include: urban reuse, agricultural reuse (irrigation), environmental reuse, industrial reuse, planned potable reuse, de facto wastewater reuse (unplanned potable reuse). for example, reuse may include irrigation of gardens and agricultural fields or replenishing surface water and groundwater (i.e., groundwater recharge). reused water may also be directed toward fulfilling certain needs in residences (e.g. toilet flushing), businesses, and industry, and could even be treated to reach drinking water standards. the injection of reclaimed water into the water supply distribution system is known as direct potable reuse, however, drinking reclaimed water is not a typical practice. treated municipal wastewater reuse for irrigation is a long-established practice, especially in arid countries. reusing wastewater as part of sustainable water management allows water to remain as an alternative water source for human activities. this can reduce scarcity and alleviate pressures on groundwater and other natural water bodies. there are several technologies used to treat wastewater for reuse. a combination of these technologies can meet strict treatment standards and make sure that the processed water is hygienically safe, meaning free from pathogens. the following are some of the typical technologies: ozonation, ultrafiltration, aerobic treatment (membrane bioreactor), forward osmosis, reverse osmosis, advanced oxidation. some water demanding activities do not require high grade water. in this case, wastewater can be reused with little or no treatment. desalinated water this section is an excerpt from desalination. desalination is a process that takes away mineral components from saline water. more generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. saltwater (especially sea water) is desalinated to produce water suitable for human consumption or irrigation. the by-product of the desalination process is brine. desalination is used on many seagoing ships and submarines. most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. along with recycled wastewater, it is one of the few rainfall-independent water resources. due to its energy consumption, desalinating sea water is generally more costly than fresh water from surface water or groundwater, water recycling and water conservation. however, these alternatives are not always available and depletion of reserves is a critical problem worldwide. desalination processes are using either thermal methods (in the case of distillation) or membrane-based methods (e.g. in the case of reverse osmosis) energy types.: 24 air-capture over oceans schematic illustration of a proposed approach for capturing moisture above the ocean surface and transporting it to proximal land for improving water security map of water stress and spatial variability of water yield along the delineated near-offshore region of 200 km across the world researchers proposed "significantly increasing freshwater through the capture of humid air over oceans" to address present and, especially, future water scarcity/insecurity. atmospheric water generators on land a potentials-assessment study proposed hypothetical portable solar-powered atmospheric water harvesting devices which are under development, along with design criteria, finding they could help a billion people to access safe drinking water, albeit such off-the-grid generation may sometimes "undermine efforts to develop permanent piped infrastructure" among other problems. water uses total renewable freshwater resources of the world, in mm/yr ( 1 mm is equivalent to 1 l of water per m²) (long-term average for the years 1961-1990). resolution is 0.5° longitude x 0.5° latitude (equivalent to 55 km x 55 km at the equator). computed by the global freshwater model watergap.the total quantity of water available at any given time is an important consideration. some human water users have an intermittent need for water. for example, many farms require large quantities of water in the spring, and no water at all in the winter. to supply such a farm with water, a surface water system may require a large storage capacity to collect water throughout the year and release it in a short period of time. other users have a continuous need for water, such as a power plant that requires water for cooling. to supply such a power plant with water, a surface water system only needs enough storage capacity to fill in when average stream flow is below the power plant's need. nevertheless, over the long term the average rate of precipitation within a watershed is the upper bound for average consumption of natural surface water from that watershed. agriculture and other irrigation it is estimated that 70% of worldwide water is used for irrigation, with 15–35% of irrigation withdrawals being unsustainable. it takes around 2,000 – 3,000 litres of water to produce enough food to satisfy one person's daily dietary need. this is a considerable amount, when compared to that required for drinking, which is between two and five litres. to produce food for the now over 7 billion people who inhabit the planet today requires the water that would fill a canal ten metres deep, 100 metres wide and 2100 kilometres long. an assessment of water management in agriculture sector was conducted in 2007 by the international water management institute in sri lanka to see if the world had sufficient water to provide food for its growing population. it assessed the current availability of water for agriculture on a global scale and mapped out locations suffering from water scarcity. it found that a fifth of the world's people, more than 1.2 billion, live in areas of physical water scarcity, where there is not enough water to meet all demands. a further 1.6 billion people live in areas experiencing economic water scarcity, where the lack of investment in water or insufficient human capacity make it impossible for authorities to satisfy the demand for water. the report found that it would be possible to produce the food required in future, but that continuation of today's food production and environmental trends would lead to crises in many parts of the world. to avoid a global water crisis, farmers will have to strive to increase productivity to meet growing demands for food, while industry and cities find ways to use water more efficiently. in some areas of the world, irrigation is necessary to grow any crop at all, in other areas it permits more profitable crops to be grown or enhances crop yield. various irrigation methods involve different trade-offs between crop yield, water consumption and capital cost of equipment and structures. irrigation methods such as furrow and overhead sprinkler irrigation are usually less expensive but are also typically less efficient, because much of the water evaporates, runs off or drains below the root zone. other irrigation methods