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Water demand is defined as the volume of water requested by natural and human users to satisfy their needs. Water sustains human health, well-being, food production, and economic development. But less than three percent of earth’s water is freshwater, out of which nearly 70 percent is stored in glaciers and icebergs, and is not available for direct human use. The freshwater that is available comes from precipitation, surface, or groundwater sources.
Due to rapid population growth, potential water availability worldwide decreased from 12,900 cubic meters per capita per year in 1970 to less than 7,000 cubic meters in 2000. In densely populated parts of Asia, Africa, and central and southern Europe, current per capita water availability is between 1,200 and 5,000 cubic meters per year. The global availability of freshwater is projected to drop to 5,100 cubic meters per capita per year by 2025. It is estimated that three billion people will be in the water scarcity category of 1,700 cubic meters per capita per year by 2025.
Approximately two-thirds of total water consumption is used by the agricultural sector, 20 percent is consumed by industry, and 10 percent by private households. However, regional differences exist. In Europe, more than half of the water consumption is used by industry. In Asia and Africa, the agricultural sector consumes more than 85 percent of the available water. It has been estimated that the global water demand will rise by 20 percent for agriculture, about 50 percent for industry, and 80 percent for private households until 2025.
Agricultural Water Demand
Plants require adequate water at the right time for establishment and growth. Crops have specific water requirements that vary depending on local conditions. Sources of water for crop production are rainfall, shallow groundwater, and irrigation water, which is water diverted from surface flows or groundwater. For instance, water withdrawals for agriculture account for about 91 percent of all water withdrawals in the Middle East and North Africa and 95 percent in Central Asia. Drylands, where irrigation plays an important role, have the highest level of water withdrawal for agriculture.
In most developing regions, hundreds of millions of people are served by hand-pumps fed by boreholes, hand-dug wells, or communal stand-posts or yard-taps fed by elevated tanks and distribution networks. The daily task of fetching and carrying water remains a major burden for women and girls throughout the developing world; valuable time is lost for schooling and education. The world’s irrigation areas totaled approximately 253 million hectares in 1995. By 2025, they are expected to reach about 330 million hectares. Therefore, irrigation systems urgently have to be modernized to reduce water consumption and wastage.
Industrial Water Demand
In the industrial sector, the biggest share of freshwater is stored in reservoirs and dams for electrical power generation and irrigation. Industrial uses account for about 20 percent of global freshwater withdrawals. Of this, 57-69 percent is used for hydropower and cooling in nuclear power generation, 30-40 percent for industrial processes, and 0.5-3 percent for thermal power generation. Not all withdrawals are consumptive, as water is recharged into the water cycle after use. The volumes of industrial water demand are quite different within individual branches of industry and from country to country, and depending on the technology of the manufacturing process.
Increasingly, industrial enterprises convert to more rational water uses, for example through water reuse and closed water cycles. Water saving reduces overall costs and saves energy. Environmental pressures and water pricing have stimulated an increasing amount of recycling and reuse by industries in the developed world, but so far, there has been less progress made in developing countries.
Domestic Water Demand
Around one-tenth of global water consumption directly meets the needs of private households. This includes, besides drinking water, the use of water for cooking and hygienic purposes. Another important domestic use of water is for productive purposes around the household, including activities such as growing vegetables and fruit trees or giving water to small stock. Enormous regional differences exist: In rural areas of dry land regions in Africa, water consumption amounts to less than 20 liters per day per capita, while the United States on average it reaches 295 liters per day per capita. According to the World Health Organization (WHO), a minimum of 25 liters per capita a day is needed to meet basic needs (for drinking water, cooking, hygiene).
The classic domestic water cycle of better-off urban residents involves house connections to deliver enough high-quality water for all lifestyle needs, and sewer connections to take away the wastewater for centralized treatment and return to watercourses. In many crowded peri-urban settlements, construction of water mains and sewers remains unattractive for government and private investors, and therefore impractical. There, residents depend on communal water points or water vendors, and on a range of often unhygienic ways of disposing of solid and liquid waste. This situation may be changing: One United Nations (UN) Millennium Development Goal is the aim to halve the proportion of people who lack access to hygienic means of sanitation by 2015.
Water Pollution Problems
Water pollution is the contamination of streams, lakes, groundwater, bays, or oceans by substances harmful to nature and humans. The quality of water sources is often negatively affected by pollution from agriculture or industry. Excess fertilizer applications, improper disposal of hazardous materials from industry, municipal dumping, poorly constructed septic tanks, or poorly managed transport systems are major pollutants. Plants and animals require water that is moderately pure, and they cannot survive if their water is loaded with toxic chemicals or harmful microorganisms. If severe, water pollution can kill large numbers of fish, birds, and other animals, in some cases killing an entire species in an affected area. People who ingest polluted water and consume polluted aquatic resources become ill, and, with prolonged exposure, may develop cancers or bear children with birth defects.
Around 80 percent of all diseases in developing countries are caused by the lack of access to clean drinking water and sufficient sanitation. Two of five persons in the world do not have access to an adequate sewage system. Approximately 1.1 billion people do not have access to clean drinking water. About 90 percent of the wastewater in developing countries infiltrates freely into the ground, or runs into rivers, and returns into the water cycle.
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