Natural circulation process is a multi-link, global water cycle involving evaporation, atmospheric moisture transport, as well as surface and subsurface circulation water reservoir forms of storage. Precipitation, evaporation and runoff are the three most important aspects of the water cycle . The three water circulation pathways determine the global water balance and determine the total water resources in a region. Evaporation is one of the most important aspects of the water cycle . The water vapor generated by evaporation enters the atmosphere and moves with atmospheric activity. The water vapor in the atmosphere comes mainly from the ocean, and some of it comes from the evapotranspiration of the continental surface. The circulation of water vapor in the atmosphere is a recurring process of evaporation-condensation-precipitation-evaporation. The water vapor over the ocean can be transported to the condensate precipitation over the land, called the external water vapor precipitation; the water vapor over the mainland directly condenses the precipitation, which is called the internal water vapor precipitation. The ratio of total precipitation to external water vapor precipitation is called the water circulation coefficient of the area. The global atmospheric moisture exchange cycle is 10 days. Water vapor transport is one of the most active aspects of the water cycle. Runoff is the difference between precipitation and evaporation in a region (basin). The multi-year average ocean water balance equation is: evaporation = precipitation + runoff; the average annual land water balance equation is: precipitation = runoff + evaporation. However, whether it is ocean or land, the geographical distribution of precipitation and evaporation is uneven. The most obvious difference is the difference in different latitudes. China's atmospheric water circulation pathways include five water circulation systems, including the Pacific Ocean, the Indian Ocean, the South China Sea, the Okhotsk Sea, and the Inland. They are sources of water vapor in southeastern China, south, south, northeast and northwest. In the northwestern hinterland, there is also a small amount of Atlantic water vapor that is prevailing from the west wind and cyclone.
The water cycle that occurs on land (or within a basin) is a complex process of precipitation-surface and underground runoff-evaporation. The exchange between atmospheric precipitation, surface runoff and underground runoff on land is also called three-water conversion. Watershed runoff is one of the most important phenomena in the terrestrial water cycle . The movement of groundwater is mainly related to molecular force, heat, gravity and void nature, and its movement is multi-dimensional. Through the evaporation and transpiration of soil and vegetation, the upward movement becomes atmospheric water; the downward movement through infiltration can replenish groundwater; and the horizontal movement can become part of the river and lake water. Although the groundwater reserves are large, they are accumulated over many years or even thousands of years. The water exchange cycle is very long and the cycle is extremely slow. The mutual conversion of groundwater and surface water is one of the main contents of the study of water quantity relationship, and it is also an important issue in the calculation of modern water resources. It is estimated that the total circulating water volume in the world is about 496'1012 cubic meters per year, less than four ten thousandths of the world's total water storage. About 22.4% of these circulating waters have become land-based precipitation, and about two-thirds of them have evaporated from the land. But the final evaporation is less than the precipitation, which forms the surface runoff.
A pipette, also known as a pipette gun, is a device used to quantitatively transfer liquids. When conducting research in analytical testing, a small or trace amount of liquid is generally removed with a pipette. Pipettes can be divided into gas piston pipettes (Air-displacement pipette) and external piston pipettes (Positive-displacement pipette) according to the principle. Gas piston pipettes are mainly used for standard pipetting, and external piston pipettes are mainly used to handle special liquids such as volatile, corrosive and viscous. Many people ignore the correct use of pipettes and some of the details.
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