Water conservation is the use and management of water for the good of all consumers. It is used in agriculture, industry, and the home. Human requirements for agricultural production, flood control, fish and wildlife management, navigation, industrial production, and many other uses have amended natural hydrologic processes.

The hydrosphere refers to that portion of Earth that is made of water, including all oceans, lakes, rivers, streams, glaciers, and underground water. Less than 3 percent of the water of Earth is fresh water, an amount that includes polar ice caps, glaciers, groundwater, surface water of rivers and freshwater lakes, and even atmospheric water. However, the amount of fresh water usable by people and other members of the biosphere is less than 0.7 percent of the total (this is water in rivers and lakes, and in the ground). This relatively small amount of available fresh water is recycled and purified by the action of processes within the hydrologic cycle, including evaporation, condensation, precipitation, and percolation through the ground. All life depends on the availability of fresh water.

Of all the fresh water used directly by humans, agricultural irrigation accounts for about 70 percent of the total. The remainder is used for industrial and domestic purposes. However, these proportions vary widely due to the climatic and economic conditions of the particular locality. Within this century, one-third of the countries situated in areas of water scarcity may encounter severe water shortages. By 2025, two thirds of the world's population is likely to live in areas of moderate or severe water shortage. The need for more effective conservation of the limited supplies of water that are available for use by people and required by natural ecosystems will intensify as water stress grows.

Available freshwater resources are either groundwater or surface water (rivers and lakes). Water that flows on the surface of the land is surface runoff. The relationship among surface runoff, precipitation, evaporation, and percolation is summarized in the following equation:

Surface runoff = precipitation − (evaporation + percolation)

When surface runoff resulting from rainfall or snowmelt is confined to a relatively narrow, well-defined channel, it is called a river or stream.

Groundwater is that water that has percolated downward through the soil and is present within porous spaces in soil and bedrock. It has been estimated that the global groundwater resource is equivalent to about 34 times the volume of all surface waters (i.e., rivers and lakes) of the world. This resource is present nearly everywhere and has the additional advantages of typically needing no storage or treatment. Utilization does require the construction of a well, sometimes presenting a problem in the most impoverished locations.

Water utilization efficiency is measured by the ratio of water withdrawal and its subsequent consumption. Water withdrawal is water pumped from rivers, reservoirs, or groundwater wells, which is then transported for use. Water consumption is water that is withdrawn and actually used for some specific purpose. It is then returned to the environment through evaporation, transpiration, discharge to a river or lake, or in some other way.

Water consumption varies greatly among regions due to differences in economic development. The average municipal use in the United States is about 150 gal (568 L) per person per day, though the rate can be higher than 350 gal (1324 L) in some locations. This includes home use for bathing, waste disposal, and gardening, as well as institutional and commercial usage. Per capita (per person) water usage in Asia is only 22 gal (85 L) per day, and just 12 gal (47 L) in Africa. While the US population has nearly doubled since the 1960s, the water consumption has actually tripled. By 2024, the Environmental Protection Agency anticipates water shortages in 40 states.

According to the World Health Organization (WHO) of the United Nations, people have a minimum water requirement of about 5 gal (20 L) per person per day. This is the minimum amount needed for physiological rehydration, cooking, washing, and other subsistence requirements. However, WHO estimates that nearly a billion people regularly consume contaminated water. This carries a significant risk of developing such waterborne diseases as cholera, dysentery, polio, or typhoid, which kill about 25 million people per year. Both conservation and sanitation are obvious necessities in meeting the huge demand for fresh water.

Because irrigation accounts for 70 percent of the water used by humans worldwide, achieving a better efficiency of agricultural use is a logical step in advancing water conservation. This can be accomplished by lining water delivery systems with concrete or other impervious materials to minimize loss by leaking during transport, and by using drip-irrigation systems to minimize losses by evaporation. Drip-irrigation systems have been successfully used on fruit trees, certain row crops, and horticultural plants. Conservation can also be accomplished by improving the efficiency of utilization of water by crops, including the cultivation of plants that are less demanding of moisture.

Subsurface irrigation is an emerging technology with high water-utilization efficiency. Subsurface irrigation uses drip-irrigation tubing buried 6 to 8 in (15 to 20 cm) underground, with a spacing of 12 to 24 in (30 to 60 cm) between parallel lines. The tubing contains drip outlets that deliver water and nutrients within the root zone at a desired rate. In addition to water conservation, subsurface irrigation has other advantages that overhead sprinklers do not: minimal overwatering, fewer disease and aeration problems, less runoff and erosion, fewer weeds, and better protection from vandalism. However, this system is relatively expensive to install. In California, subsurface irrigation has been used on fruit trees, field crops, and lawns, and it has achieved water-use savings of about 50 percent. However, this methodology can, in arid environments, lead to the buildup of soil salinity levels, damaging plants and reducing crop yields. Balancing the water needs of the plant with maintenance of soil quality is an important component of water conservation measures. Technologically advanced irrigation systems now incorporate climate-based controls. These systems utilize meteorological information to determine the need for irrigation and modify the length and duration of irrigation to match the plant's requirements. Though these systems are currently used primarily on large-scale applications, development of economical models for the small-scale user is underway.

Xeriscaping, or the cultivation of plants requiring little water, is an especially suitable horticultural practice for conserving water in regions with a dry, hot climate. For example, over much of the southwestern United States, more than 50 percent of the domestic water consumption may be used to irrigate lawns and other horticultural plants that are intolerant of drought. Xeriscaping uses plants such as cacti, succulents, and shrubs of semidesert habitat (such as trailing rosemary Rosemarinus officinale and rock rose Cistus cobariensis), which are well adapted to a hot, dry climate and need little water.

Water conservation can also be advanced by improving other domestic uses of water. One simple conservation practice is to install ultra-low-flush (ULF) toilets and low-flow showerheads in homes and other buildings. A ULF toilet uses only 1.6 gal (6.1 L) per flush, compared to 5 to 7 gal (19 to 26 L) by a standard toilet. Replacing a standard toilet with a ULF saves about 30 to 40 gal (114 to 151 L) of water per day, equivalent to 10,000 to 16,000 gal (37,850 to 60,560 L) per year. More recently, advanced toilets and urinals requiring no water have been developed and are beginning to be utilized on a limited basis.

Another way to conserve the freshwater supply is to desalinize seawater. Desalinization is the removal of salts and other impurities from seawater by either distillation or reverse osmosis, and this method is being increasingly used to provide high-quality water for drinking, cooking, and other domestic uses. In 2019, the world had more than 20,000 desalination plants, creating more than 3.5 billion gal (13.25 billion L) of potable water daily. A significant portion was produced in Saudi Arabia and other nations of the Gulf of Arabia, where energy costs are relatively low (the cost of desalinated water is highly sensitive to the cost of energy). Among the largest desalination plants in the world is Saudi Arabia's Shuaiba Desalination Plant, which uses reverse osmosis. Saudi Arabia is the largest producer of desalinated water in the world, with desalination providing 50 percent of the country's drinking water. The process is also gaining popularity in Spain, Australia, and China.

Desalinization is also practiced in Arizona, California, and Florida, among other states. The largest seawater desalination plant in the United States in 2020 is the Claude “Bud” Lewis Carlsbad Desalination Plant in San Diego County, California, where the cost to desalinate one gallon costs half a cent. The plant's daily capacity is about 50 million gal (190 million L), serving about 400,000 people in the the region.

Widespread recognition of the importance of reusing water has begun to change traditional water use methods. As the value of water increases, users are willing to employ methods that may increase the initial cost of a project, with the hope of recouping those costs through water savings in the future. One of the first of these reuse applications was the irrigation of golf courses and landscaping. In many areas, treated wastewater is diverted from its normal disposal path to be reused in irrigation. This has gained in popularity and is also utilized in small artificial ponds for decorative purposes.

Graywater systems capture water that drains from sinks, tubs, laundry, and dishwashers for reuse in Page 4738  |  Top of Articleirrigation. Graywater systems do not incorporate toilet wastes because of the potential health threat. Dual plumbing is required for such a system and some treatment is required prior to reuse. Though home construction costs are obviously increased by including a graywater system, many people have become dedicated believers in the benefits of water reuse, while others question the economic benefit of small-scale systems. The widespread application of graywater systems has, however, been hampered by codes and laws that make such systems illegal in many locations.

As the availability of water becomes more restricted, the costs to both the provider and consumer are increased. In a situation unique to the water supply industry, providers are frequently placed in the position of trying to convince consumers to use less of the commodity that they supply. Most large water providers have departments dedicated to education of the public with regard to conservation. In general, these education efforts have been largely ineffective and conservation of freshwater resources has been best achieved through economic incentives.

Water providers frequently offer rebates for those consumers who are willing to change from older technology to newer, such as using low-flush toilets, modern washing machines, water-efficient landscaping, or other systems that demonstrate lower water usage. The greatest effect has been achieved through tiered pricing. In this pricing structure, users are charged higher rates for each successive unit, or block, of water used. The rate structure penalizes heavy users with greatly increased rates. This technique has been shown to be highly effective in reducing overall usage. In Tucson, Arizona, for example, an increasing tiered price structure resulted in decreased usage of 26 percent over a three-year period. Additionally, some communities have implemented the use of water conservation monitors and water waste hotlines to penalize those who continue to waste the resource. Many communities currently limit the type and size of landscaping as well as the time and nature of outdoor water use. In extreme cases, some communities have completely banned outdoor water use during droughts or other crisis periods.

Throughout history, the availability of water has been a vital factor in the rise and fall of human cultures. This is largely because water is a limiting factor for the carrying capacity for human activities in any region. Humans continue to look for ways to live within the limits of available natural resources, including the supply of fresh water, especially because the supply of usable water is finite. Efforts are made to reduce consumption in regions that are using this resource excessively.

David Goings