In some areas of global warming, evaporation does not increase, but decreases.
According to the latest report of the Intergovernmental Panel on Climate Change (IPCC), the global temperature has increased by 0.6℃ on average in the past 100 years. Therefore, it is predicted that global warming may dry the atmosphere and lead to an increase in water evaporation on land. However, the actual results are contrary to this, and the evaporation capacity of evaporating dishes in many areas shows an obvious downward trend. Investigate its reason, the international experts have different opinions. Peterson, a famous American climatologist, and others attributed the decrease of evaporation capacity to the increase of cloud cover. Bulut Sartre and others think that the decrease of evaporation capacity of evaporating dish is the result of the increase of ground evaporation capacity; Based on the fact that the global temperature range is getting smaller, Michael explained theoretically that the decrease of evaporation capacity of evaporating dish is mainly caused by the decrease of solar radiation. On the other hand, Stanhill and Cohen believe that the increase of cloud cover and aerosol is the main reason for the decline of global solar radiation in recent years. Comparatively speaking, although there are many achievements in the calculation of evaporation in China, there are few studies on the changes and causes of evaporation, and many studies are limited to local areas, and the data of stations used are few, so it is difficult to form a systematic and comprehensive theory.
Therefore, it is necessary to use a large number of station data and a long time scale to study the changing trend and reasons of evaporation capacity of evaporating dishes in China. In the general project of the National Natural Science Foundation of China and the "Study on the Characteristics and Causes of Evaporation in China in Recent 45 Years" sponsored by the Natural Science Foundation of Jiangsu Province, Shen Shuanghe and others used the measured data of 472 weather stations 1957~200 1 20 cm diameter evaporating dishes in China to analyze the changing trend and causes of evaporation in small evaporating dishes in China.
Variation trend Evaporation decreases by 34. 12mm every ten years.
Through the statistics and calculation of the data, Shen Shuanghe found that although the annual average temperature in China increased by 0.2℃ every 10 year in these 45 years, the evaporation of the evaporating dish decreased by 34. 12 mm every 10 year. Among them, the rate and amplitude of decline is the largest in summer, which is per 10 year 15.59 mm, followed by spring and autumn, and the change is not obvious in winter. "This trend is generally consistent with the world," Shen Shuanghe explained. "The temperature rise is mainly concentrated in winter. For many areas, the summer temperature, which accounts for a large proportion of the total annual evaporation, does not rise but falls, resulting in a significant decline in annual evaporation."
From the geographical distribution, the significant increase of evaporation capacity of evaporating dish is only concentrated in a few areas, such as the northern Daxing' anling and Beishan areas; The areas with the largest decline are concentrated in the east, northwest and south, and southern Tibet.
Key factors: wind speed and sunshine hours decrease.
After analyzing the energy balance term and aerodynamic term in Penman's formula, Shen Shuanghe thinks that the decrease of evaporation in the eastern region is mainly due to the significant decrease of energy used for evaporation, while the decrease of evaporation in the western region is mainly due to the decrease of power used for evaporation. As far as meteorological factors are concerned, he believes that when the wind is weak, the air flows slowly and the water vapor on the evaporation surface is not easy to be brought into the atmosphere. When the sunshine decreases, the energy received by the evaporation surface decreases, the kinetic energy of water molecules weakens and the diffusion of water vapor slows down. This reduces the evaporation capacity of the evaporating dish.
Therefore, he proposed that the main reason for the decrease of evaporation capacity of evaporating dish was the decrease of wind speed and sunshine hours. "The reason why the evaporation capacity of evaporating dish in Xiaoxing 'anling increased was mainly because the temperature in this area increased obviously, but the sunshine and wind speed did not decrease obviously." He used a counterexample to illustrate.
Later, he further proved this conclusion through the trend analysis and correlation analysis of various meteorological factors.
Changes that may affect the water cycle and water resources distribution.
What impact will the fact that evaporation in most parts of China has decreased have on water cycle, agriculture and forestry and other related fields? Shen Shuanghe explained that the decrease of evaporation in most areas of China can be ruled out as the result of temperature drop, increase of ground evaporation and air humidification, but should be attributed to decrease of sunshine and decrease of wind speed. Therefore, the reduction of evaporation in the pan may have a certain impact on the components of the water cycle, such as the reduction of ground evaporation and the weakening of atmospheric water vapor transport, and the precipitation will also change accordingly. For agricultural production, it will lead to the weakening of farmland evapotranspiration, the improvement of crop water use efficiency and the alleviation of drought to some extent. In addition, the evaporation of evaporating dish in Northeast China has increased to a certain extent, and the temperature has obviously increased. On the one hand, it can expand agricultural planting area, improve crop yield, forest carbon sequestration and wood accumulation; On the other hand, due to the increase of water consumption, it may have a certain impact on the local water cycle and water resources distribution.