Water or rather the lack of it plays an important role in several tropical diseases. In order to have a better idea about this precious commodity, a short note on water is warranted.

There are gross differences in water consumption: a typical US citizen uses about one hundred times as much water as a citizen of Burundi or Uganda. Humans need about fifty litres of clean water a day to stay healthy, for drinking, washing, cooking and sanitation. But in 55 countries the average water use per person falls below this. One billion people live without access to adequate drinking water, and half the world's population lacks basic sanitation. Water-related diseases such as cholera, typhoid and dysenteria are rife in many parts of the world. Prevention in the long run is linked to eradication of poverty. The problems associated with sewage-contaminated water have been recognized ever since the physician John Snow demonstrated its connection to cholera epidemics in London in the 1850s. Other sources and effects of pollution have emerged only more recently, e.g. the problem of arsenic-containing groundwater in Bangladesh.

Since more than 70% of the surface of the Earth is covered by seas and oceans, it is easy to understand that there is a lot of water on our planet. However, humans can use only a small fraction of this seemingly endless supply. Fresh water is a finite resource. Scientists estimated in 1996 that humans are currently using over half of the accessible fresh water. Between 1950 and 1990, global water demand tripled, and despite conservation measures, it is still rising. If current trends persist, the demand for water might exceed the total available supply by around 2030. Many political forecasters predict that, during the 21^st century, shortages of water are going to become a major source of international tension, a cause of social and possibly even military conflict.

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The long-term consequence of severe water shortage is that people die not of thirst but of hunger. Famine is mainly caused by bad politics and war, but also by drought and crop failure. By far the biggest use to which water is put is agriculture. Between 70 and 80% of the water withdrawn globally is used to irrigate fields. But irrigation is typically rather wasteful. It is estimated that only 40% or so of all irrigation water gets to where it is needed. Most farmers use traditional, inefficient methods of irrigation. Modern methods such as drip irrigation deliver water exactly where it is needed, and in exactly the right amount. However, drip irrigation systems are costly to install. Over-irrigation is not only wasteful but damaging for the soil. If soil becomes waterlogged and the water table rises, salts from deep in the ground are carried to the surface, where they can form a crust when the water evaporates. This salty soil is then infertile, a problem called 'salinization'.

Huge amounts of water are bound up in the composition of the different minerals of the Earth's crust and core. The amount is estimated to be enough to fill all the oceans 30 times over. More important is the amount of free water. Water exists in solid, liquid and gas phases that are interchangeable at temperatures found on Earth. This is rather unique for a chemical. The hydrological cycle describes the movement of water as it passes through these phases. It is a closed system because water is neither created nor destroyed on a large scale. The hydrosphere contains all the free water in the atmosphere, the biosphere, on the Earth's surface and in the crust down to a depth of 2000 metres. Current estimates are that the Earth's hydrosphere contains about 1386 million cubic kilometres. About 97,5% of this amount is salt water in the seas and oceans. Only 2,5% of the total amount of free water is fresh water. However, 68.7% of the fresh water is in the form of solid ice and permanent snow cover in the Antarctic, the Arctic and in the mountainous regions. It is estimated that about 29.9% exists as fresh groundwater in aquifers. Only 0.26% of the total amount of fresh water is concentrated in lakes, reservoirs and river systems. These are of course average values. For short time intervals such as a single year, a couple of seasons, or a few months, the volume of water stored in the hydrosphere will vary as water exchanges take place between the oceans, land, biosphere and atmosphere.

Solar heat evaporates water into the air from the Earth's surface. Land, lakes, rivers and oceans send up a steady stream of water vapour. Each year, the top 1 meter of the oceans evaporates. The rate of evaporation increases with increasing temperature and wind speed and decreases with increasing humidity. Atmospheric water exists as water vapour, droplets and ice crystals in clouds. The actual volume of water in the atmosphere is small and varies with changes in temperature, pressure and geographical location. Water vapour can move long distances in the atmosphere in a relatively short period of time because of the high velocity winds in the upper atmosphere. The average water molecule is in the atmosphere for twelve days before it precipitates. Moist undersaturated air can become supersaturated when it cools (warm air can contain more moisture than cold air). Condensation of water droplets in saturated air happens around condensation nuclei, such as airborne mineral dust and sooty carbon particles from fossil fuel burning. Other nuclei consist of flecks of crystaline sea salt which form when droplets from braking water evaporate in the air. Dimethylsulfide is a trace gas produced by certain types of marine phytoplankton. It is considered to be an important source of cloud condensation nuclei.

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Around two thirds of the rain water that falls on land is returned directly to the atmosphere by evaporation and transpiration (evapotranspiration). Transpiration is the loss of water by plants. This loss occurs through stomata which are open during the day to enable the absorption of carbon dioxide needed for photosynthesis. Transpiration rates depend upon the temperature, humidity and wind speed near the leaves of the plant. Since plants draw water from the soil, transpiration rates can greatly effect soil moisture content.

Precipitation falling on land is the main source of the formation of the waters found on land, such as rivers, lakes, groundwater and glaciers. About 87% of all evaporated water derives from the oceans, while 13% comes from land. Seventy-eight percent of all rain falls on the oceans. The remaining 22% falls on land. The atmospheric movement of water vapor from sea to land and from land to sea is unbalanced. The land receives a net moisture donation from the oceans. This exchange is balanced by runoff water that flows from the land to the sea. Rainfall is unevenly distributed across the planet. Rain or snow that evaporates as it falls into a layer of dry air near the Earth’s surface is called virga. While virga can happen at any time during the year, it is most common during winter, when low-level air is very dry. As falling precipitation evaporates it moistens the dry air from the top down. If the precipitation lasts, the air becomes moist enough to block evaporation and the rain or snow reaches the ground.

A small amount of precipitation permeates into the ground. Once infiltrated, water continues to filter through soil or rock through vertical movement called percolation. The water will drain through the spaces between soil grains until it reaches an impermeable layer of rock or clay. The water will flow down the slope of the impermeable layer, winding it's way through the soil's pores, cracks and fissures. This is groundwater, and the permeable layer of rock through which it flows is called an aquifer. The upper limit of this saturated region corresponds with the water table. Groundwater flows from areas with a higher water table to areas where the water table is lower. A well can be created by drilling below the water table to reach an aquifer. Water which penetrates but does not saturate the soil is called vadose water. It is vadose water which sustains most land plants. If the water table rises close to the surface, the soil becomes waterlogged. In soils where the grains are very small and the pore spaces between them are narrow, capillary action can raise the water level by over three meters.

Every year the turnover of water on Earth involves 577,000 km³ of water. This is water that evaporates from the oceanic surface (502,800 km³) and from land (74,200 km³). The same amount of water falls as atmospheric precipitation, 458,000 km³ on the ocean and 119,000 km³ on land. The difference between precipitation and evaporation from the land surface (119,000 - 74,200 = 44,800 km³/year) represents the total runoff of the Earth's rivers (42,700 km³/year) and direct groundwater runoff to the ocean (2100 km³/year). These are the principal sources of fresh water to support life necessities and man's economic activities.

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The different forms of water in the hydrosphere are fully replenished during the hydrological cycle but at very different rates. For instance, the period for complete recharge of oceanic waters takes about 2500 years, for permafrost and ice some 10,000 years and for deep groundwater and mountainous glaciers some 1500 years. Water storage in lakes is fully replenished over about 17 years and in rivers about 16 days. Freshwater with a period of complete renewal taking place over many years or decades is contained in large lakes, groundwater, or glaciers. How the hydrological cycle might change in a world warmed by the greenhouse effect is a question which falls outside the scope of this note.

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Quickly renewable water resources include waters replenished yearly in the process of the water turnover of the Earth. These are mainly runoff from rivers. This kind of water resource also includes the yearly renewable upper aquifer groundwater not drained by river systems. However, on a global scale, this volume is not large compared with the volume of river runoff and is of importance only for individual specific regions. In the process of turnover, river runoff is not only recharged quantitatively, its quality is also restored. If man would suddenly stop contaminating rivers, then with time water could return to its natural purity. Thus, river runoff, representing renewable water resources, is an important component of the hydrological cycle.

Intensive use of aquifers unavoidably results in depleting the storage and has unfavourable consequences. It disturbs the natural equilibrium established over centuries, whose restoration would require tens or hundreds of years. It is easy to understand that when one pumps more water out of an aquifer than goes in, one gets a situation which is not tenable in the long run. From the moment the water in an aquifer is polluted (pesticides, radioactivity, arsenic, …), it will take a long time before the total water volume will have turned over. Polluted aquifers are really very difficult to remedy.