Tropical rain forests are drenched with rain whereas much of the US deserts receive less than 25 cm a year. The second reason is that human civilization has pushed well beyond the earth’s carrying capacity, exceeding the readily available supply of this renewable resource, even in the areas with abundant rainfall.
 One of the methods, how to increase water supply is saltwater desalination, the removal of salts from seawater and brackish (slightly salty) water. The two main methods are evaporation and reverse osmosis. In evaporation, or distillation, the salt water is heated and evaporated, leaving behind the salts and minerals. The steam produced is then cooled, and pure water condenses out. In reverse osmosis, water is forced through thin membranes whose pores allow the passage of water molecules but not the salts and minerals.
 Since 94% of the water on earth is in the oceans, desalination might seem like the best answer to water shortages. Unfortunately, water produced by desalination is four to ten times more expensive than water from conventional sources.
 We can also do many things to cut down on water use. For instance: in showers - by installing flow restricters or taking shorter showers, in toilets - by installing low-flush toilet, in faucets - by installing low-flush faucets or flow restricters. While brushing teeth and shaving - do it with faucets off, followed by brief rinse.
 Recycling and conservation can make important contributions to increasing water supplies in the near future. Since agriculture uses a large percentage of our water, devices to reduce irrigation losses that are inexpensive and easy to install are needed. Individuals can assist by reducing water consumption.









The hydrologic cycle (fig.1)

The hydrologic cycle is the chain of the history of the water. The global water cycle is driven by solar energy, and a little bit by the rotation of the earth and tide forces. The cycle involves the total earth system comprising the atmosphere, the hydrosphere, lithosphere and biosphere. The hydrosphere includes all the water at and near the near the surface of the earth. All of the water in the hydrosphere is caught up in the hydrologic cycle. The largest single reservoir in the hydrologic cycle, by far, consists of the world’s oceans, which contain 97,5% of the water in the hydrosphere. Lakes and streams together contain only 0,016% of the water. The main processes of the hydrologic cycle involve evaporation into, and precipitation out of the atmosphere.