Water resources

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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 natural resources of water that are potentially useful for humans,[1] for example as a source of drinking water supply or irrigation water. 97% of the water on 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.[2] The remaining unfrozen freshwater is found mainly as groundwater, with only a small fraction present above ground or in the air.[3] 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.

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.[4]

Natural sources of fresh water

Natural sources of

frozen water
.

Surface water

Parinacota
volcano in northern Chile

Surface water is water in a river,

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

pipeline
.

Brazil is estimated to have the largest supply of fresh water in the world, followed by Russia and Canada.[5]

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.[6]

Groundwater

Relative groundwater travel times in the subsurface

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,[8][9][10] and desalinated seawater. However, the economic and environmental side effects of these technologies must also be taken into consideration.[11]

Wastewater reuse

Desalinated water

Water desalination
Methods

Desalination is a process that takes away mineral components from saline water. More generally, desalination is the removal of salts and minerals from a target substance,[16] 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.[17] 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.[18]

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.[19][20] 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.[21][22]: 24 

Research into other options

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[23]
Map of water stress and spatial variability of water yield along the delineated near-offshore region of 200 km across the world[23]

Researchers proposed "significantly increasing freshwater through the capture of humid air over oceans" to address present and, especially, future water scarcity/insecurity.[24][23]

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.[25][26][27]

Water uses

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

Irrigation of agricultural fields in Andalusia, Spain. Irrigation canal on the left.

landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been developed by many cultures around the world. Irrigation helps to grow crops, maintain landscapes, and revegetate disturbed soils in dry areas and during times of below-average rainfall. In addition to these uses, irrigation is also employed to protect crops from frost,[28] suppress weed growth in grain fields, and prevent soil consolidation. It is also used to cool livestock, reduce dust, dispose of sewage, and support mining operations. Drainage
, which involves the removal of surface and sub-surface water from a given location, is often studied in conjunction with irrigation.

There are several methods of irrigation that differ in how water is supplied to plants. Surface irrigation, also known as gravity irrigation, is the oldest form of irrigation and has been in use for thousands of years. In sprinkler irrigation, water is piped to one or more central locations within the field and distributed by overhead high-pressure water devices. Micro-irrigation is a system that distributes water under low pressure through a piped network and applies it as a small discharge to each plant. Micro-irrigation uses less pressure and water flow than sprinkler irrigation. Drip irrigation delivers water directly to the root zone of plants. Subirrigation has been used in field crops in areas with high water tables for many years. It involves artificially raising the water table to moisten the soil below the root zone of plants.

Irrigation water can come from

desalinated water, drainage water, or fog collection. Irrigation can be supplementary to rainfall, which is common in many parts of the world as rainfed agriculture
, or it can be full irrigation, where crops rarely rely on any contribution from rainfall. Full irrigation is less common and only occurs in arid landscapes with very low rainfall or when crops are grown in semi-arid areas outside of rainy seasons.

The environmental effects of irrigation relate to the changes in quantity and quality of
aquifers through overdrafting. Soil can be over-irrigated due to poor distribution uniformity or management wastes water, chemicals, and may lead to water pollution. Over-irrigation can cause deep drainage from rising water tables that can lead to problems of irrigation salinity requiring watertable control by some form of subsurface land drainage
.

Industries