2 English reference water cycle
Water cycle is a process in which water resources on the earth circulate continuously between land, ocean and atmosphere in gas, liquid and solid state. The water cycle is the medium for the movement of basic elements in the ecosystem. Without water cycle, there would be no biogeochemical cycle of elements. 70% of the earth's surface is covered by water, of which seawater (salt water) accounts for about 97% and fresh water only accounts for 3%. About 3/4 of fresh water exists in polar ice sheets and glaciers, and only 65,438+0/4 is available for human use. The driving force of the water cycle is the attraction of solar energy and water, and the atmosphere is the key to the water cycle. Sunlight shines on waters and land, making part of the water there turn into steam and enter the atmosphere. Most of the water absorbed by plants from soil or water enters the atmosphere through transpiration, and part of the water in animals also enters the atmosphere through evaporation on the body surface. Water vapor in the atmosphere becomes water droplets or ice crystals at high altitude and returns to the ground in the form of precipitation. Assume that the total precipitation of the earth is 100 unit, of which 84 units come from ocean evaporation and 16 units come from land. However, only 77 units of water vapor in the atmosphere reach the ocean through precipitation, while 23 units reach the land. It can be seen that the evaporation of the ocean exceeds the precipitation that reaches there, but on land the situation is just the opposite. This imbalance can be made up by water flowing from land to sea, and rivers are the main channels for water to flow from land to sea. The problem of water is a global problem. Available fresh water is limited and unevenly distributed. In densely populated areas, water shortage and water quality deterioration often occur. Excessive use of groundwater may cause land subsidence, house collapse, road damage and other consequences, so it is urgent to protect water resources and save water.