1. The water are condenses in the atmosphere to clouds form .
2. Every day on the earth falling about 300 cubic kilometers of water.
3. The water through precipitation falls to earth one part goes to the underground.
4. There are about 1.4 billion cubic kilometers of water on the Earth .
5. 97% of the water quantity on the Earth is salt water (seas and oceans) and only 3% of water quantita is fresh water (rivers, lakes, polar ice and groundwater).
6. Every day about 100 cubic kilometers of water flows go into the seas and oceans.
7. Water in seas and oceans comes from rivers, glaciers to melt and from underground.
8. Water from the ocean and soil changes physical state and enters the atmosphere as a gas (water vapor).
9. Evaporation are increases with temperature and wind speed.
10. Water in the atmosphere are condenses again in the form of clouds.
11. This process is continuous and is called the water cycle.
12. During this process, there is no loss of water, or the amount of water on the planet earth and its atmosphere is always the same.

All the above shows that the system of the water cycle works perfectly until the moment when it comes to human interference. Since mostly everything what man do lead to a reduction in the quantity of drinking water, below will be explain  methods for "saving" and "storage" of water.