Humans have always interacted with the hydrosphere, drinking freshwater, and using it for various purposes. However, until a century ago, the number of people on Earth was not high, and human impact on water resources was generally insignificant and local rather than global. Thanks to the renewal process of the water cycle and its self-purification properties, on average, the quantity and quality of fresh waters had not changed much (except for climate-driven natural variability at different timescales). The process of evaporation and surface water systems (rivers, lakes, and, in particular, wetlands) remove a large portion of pollutants from the water, in liquid or gaseous state. There had been an illusion that water resources are infinite, inexhaustible, and perfectly renewable, free goods. The situation has dramatically changed over the last century, when water withdrawals strongly increased due to the dynamic population growth and socioeconomic development driving the increase of human living standards. There has been a dramatic expansion of irrigated agricultural areas, growth of industrial water use (including the power sector), and intensive construction of storage reservoirs worldwide.
The characteristics ofwater resources, in both quantity and quality aspect, which used to be driven by natural conditions (climate, geology, soils, and resultant natural land cover) are now dependent, to an ever-increasing extent, on human economic activities. In many areas of the world, water resources have been adversely affected in quantitative and qualitative terms, by increasing water withdrawal and water pollution, respectively. Problems are particularly acute in arid regions.
Irrigated agriculture consumes, globally, 70% of the world water withdrawals. More and more water is needed to produce food for the ever-increasing population of the globe. Since projections for the future foresee further growth of population, the consequences to food and fiber production are clear and the global demand for water will grow further. Faster growth is expected in less developed countries: in the whole of Africa and much of Asia.
Poor water quality is another severe, and global, water problem. Traditionally, the water quality was mostly related to natural composition of water (salinity). Now, human has changed the quality of the world's water to a large degree. The structure of human-caused water pollution problems has changed in time, with fecal coliform bacteria and organic pollution being the oldest. Later, water pollution included salinization of freshwater (groundwater, rivers, lakes), for example, caused by irrigation or groundwater overexploitation and saltwater intrusion, pollution by metals, radioactive material, organic micropollutants, and acidification. It is estimated that only 5% of the world's wastewater is treated. Important water quality problems are caused by nutrients (nitrogen, phosphorus), whose abundance leads to eutro-phication and toxic algae blooms. Remains of agricultural chemistry products, artificial fertilizers, pesticides, and herbicides, are particularly difficult to eliminate, due to the distributed nature of the source. Some synthetic chemicals, for example, organochlorines (organohalides) have a long half-life time: 8 years in the case of DDT.
In order to improve the quality of water in the countries of the European Community, the Water Framework Directive entered into force in December 2000, setting out a framework for actions in the field of water policy in the European Union (EU). The key objective of the directive, which imposes legal obligations on the authorities in EU member states, is to achieve a 'good water status' for all waters of the EU by 2015.
Even when perennial surface water source is available in a given location, water consumption in untreated state may present a risk to human health because of contamination by pathogens or waste. The number of people dying each year of water-related diseases is of the order of millions. Particularly burning water supply problems occur in informal human settlements, for example, slums around mega-cities, where the poor have no access to public, safe, tap water. They have to buy lower-quality water from vendors and pay much more than the price charged to more wealthy citizens who have access to the public supply of safe water.
Water is not a free goods any more. A future-oriented water resources management should emphasize shaping demands rather than supply extension. It is a must to improve the efficiency of water use, trying to ''do more with less'' (''more crop per drop''). Financial instruments, such as the water pricing not only granting full cost recovery but also accounting the cost of the resource, in the sense of foregone opportunities, can generally improve the efficiency of water use.
Global water consumption has increased nearly sixfold since the beginning of the twentieth century, that is twice stronger than the population growth. Facing the increasing pressures, the business-as-usual approach to water development and management cannot be globally sustainable. The problems of water shortage are likely to be aggravated in the twenty-first century, which was baptized 'the age of water scarcity'. Population growth, economic development, and increasingly consumptive lifestyle impact on the hydrological cycle, boosting water withdrawals and increasing the hazard of water stress and water scarcity.
The need for protection of the aquatic ecosystems is being increasingly recognized. Despite the rising human demand for water, it is necessary to allocate a share of water to maintain the functioning of freshwater-dependent ecosystems, thus meeting conditions of environmental water requirements. This would allow (if flows are regulated) to maintain the water regime within a river or a wetland, that suits aquatic and riparian ecosystems. However, earmarking water for environmental requirements is very difficult in some areas - even large rivers in China and Central Asia run dry, at times. River flow does not reach the sea due to excessive human water withdrawal.
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