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Evaporation capacity

The amount of water that evaporates and spreads into the air

Evaporation capacity（ evaporation ）The process of water changing from liquid or solid to gas and escaping into the atmosphere is called evaporation. Evaporation refers to the amount of water dispersed into the air through evaporation in a certain period of time, usually expressed in millimeters of the thickness of the evaporated water layer. The water evaporation amount of water surface or soil is different evaporator determination. Generally, the higher the temperature, the lower the humidity, the greater the wind speed, and the lower the air pressure, the greater the evaporation capacity; On the contrary, the evaporation capacity is smaller. Determination of soil evaporation and water surface evaporation agricultural production And hydrology. Rainfall is scarce Groundwater source And areas with little inflow runoff, such as large evaporation, are prone to drought.

Chinese name: Evaporation capacity
Foreign name: evaporation

Interpretation: The amount of water that evaporates and spreads into the air
Represent: Thickness of evaporated water layer in mm

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Evaporation capacity

Consumed by surface soil evaporation and crop transpiration soil moisture The amount of water from phreatic water is called phreatic evaporation. The phreatic water evaporation is Water cycle Part of Phreatic layer Groundwater to soil water and Atmospheric water A form of transformation.

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Macro significance

Evaporation capacity

Evaporation makes the water on the ground rise to the air, while rainfall and snowfall make the water in the air fall to the ground. They are not only two opposite processes, but also two interdependent processes. If the water on the ground no longer evaporates into the air, there will be no rain or snow on the earth in 10 days.

Evaporation is not only interdependent with precipitation, but also related to rivers on the ground. In extremely arid areas, precipitation is very small. Its actual evaporation is equal to the precipitation. There is no river on the ground, not even a small dry ditch. This is the case in our desert areas. At the source or upstream of the river, the precipitation there is greater than the actual evaporation. This excess water forms a river and slowly flows along the valley into the ocean or lake. In any natural basin, its evaporation, precipitation and river flow are basically balanced. The formula is: any one Closed basin : precipitation falling into the basin=evaporation+river water flowing out of the basin.

Meteorologic significance

All weather stations have evaporation data, which is often quoted by people. People often use the comparative data of precipitation and evaporation to explain how dry a place is. In fact, there are problems with this statement. Provided by many regions Data It shows that the local evaporation is far greater than the precipitation. But if it were true, human beings would have been unable to survive there. The terrain of the earth's surface is complex. In a region or even a county, there are often desert, oasis and mountain areas. In mountainous areas, precipitation is far greater than evaporation; stay desert And in the desert, basically how much water can be lowered evaporation How many? In oasis, although evaporation is greater than precipitation surface runoff Supplement, or suitable for human survival.

In very humid areas, the evaporation measured by the meteorological station is about 60% of the natural evaporation. Therefore, it is still possible to use it to roughly analyze the difference of evaporation. However, there is a serious deviation between the evaporation measured by meteorological stations in arid areas and the actual evaporation.

For example, Xinjiang Turpan Basin Of Toksun The annual evaporation measured by the meteorological station is 3.7 meters. Some people say that the evaporation capacity there is amazing. However, the actual situation is there Annual precipitation Less than 1cm thick. So local natural condition The maximum evaporation capacity that can be provided under is 1 cm. This is 370 times less than 3.7m.

It is obviously distorted to describe the evaporation measured by meteorological stations as the actual evaporation in arid areas. The evaporation is actually Evaporation pan Intermediate survey The data obtained only show the evaporation capacity of this area, not the actual evaporation capacity. The meteorological department should change the evaporation capacity of the meteorological station to evaporation capacity, which will reduce people's misunderstanding. When people quote evaporation data, they first understand its exact meaning, which will also avoid this misunderstanding.

meteorology

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Evaporation capacity

Evaporation is Surface heat balance And water balance water cycle Most directly affected by land use And climate change. At the same time, evaporation is also an important factor in heat exchange. Therefore, evaporation is used to estimate land evaporation, crop water demand and crop water balance It has important application value. The study of evaporation change is of great significance for understanding climate change and exploring the law of water cycle change. In practical terms Hydraulic engineering design Agriculture, forestry and animal husbandry Soil improvement , Soil Moisture regulation 、 Irrigation quota Development and research Water resources , Development Climatic regionalization And other aspects have important guiding significance.

global warming Evaporation in some areas decreases instead of increasing

according to Intergovernmental Panel on Climate Change （ IPCC ）The latest report shows that the global temperature has increased by 0.6 ± 0.2 ℃ on average in the past 100 years. Therefore, it is expected that global warming may dry the atmosphere, leading to an increase in water evaporation on land. On the contrary, pan evaporation in many areas has shown a significant downward trend. The reason is related internationally Expert statement Not one. Peterson, a famous American climatologist, and others attributed the decline in pan evaporation to cloudiness Increase of; Brutsaert et al. believed that the decrease of pan evaporation was due to the increase of ground evaporation; Michael from all over the world Daily temperature range Based on the fact of becoming smaller, it theoretically explains that the decrease of pan evaporation is mainly due to solar radiation Caused by the reduction of quantity; Stanhill and Cohen, on the other hand, believe that the increase of cloud cover and aerosol is the main reason for In recent years, the decrease of global solar radiation. In comparison, 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 adopted Survey station There are few data, so it is difficult to form a systematic and comprehensive theory.

Therefore, a large amount of station data and long time scale It is necessary to study the change trend and causes of pan evaporation in China. stay National Natural Science Foundation of China General items and Natural Science Foundation of Jiangsu Province In the "Research on the Change Characteristics and Causes of Pan Evaporation in China in the Past 45 Years" funded by the project, Shen Shuanghe and others analyzed the measured data of 20 cm diameter pan at 472 meteorological stations in China from 1957 to 2001 Small evaporating dish The change trend of evaporation and its reasons.

Change trend Evaporation decreases by 34.12mm every decade

Through the statistics and calculation of the data, Shen Shuanghe found that although the annual average temperature in China during the 45 years Annual increase The trend of 0.2 ℃ increases, but the pan evaporation generally decreases by 34.12 mm every 10 years. Among them, the rate of decline and the range of decline are the largest in summer, 15.59 mm per 10 years, followed by spring and autumn, with no obvious change in winter. "This change trend is generally consistent with that of the world," Shen Shuanghe explained. "The temperature increase is mainly concentrated in winter. For many regions, the summer temperature, which accounts for a large proportion of the total annual evaporation, does not rise but falls, which leads to a significant decline in the annual evaporation."

From the perspective of regional distribution, the significant increase in pan evaporation is only concentrated in a few areas, such as the northern part of the Great Khingan Mountains and the Beishan Mountains; The areas with the greatest decline are concentrated in the east, northwest north, south and Tibet south.

Key factor wind speed and Sunshine hours decline

Shen Shuanghe passed the Penman formula After analyzing the energy balance term and aerodynamic term, it is believed that the decline of pan evaporation in the east is mainly due to the significant reduction of the energy provided for evaporation, while the decline of pan evaporation in the west is mainly for evaporation Power down Caused by. Just Meteorological factor In his opinion, when the wind is weak, the airflow is slow, Evaporation surface Of water Vapor is not easy to be brought into the atmosphere. When sunlight decreases, the evaporation surface receives less energy, the kinetic energy of water molecules decreases, and the diffusion of water vapor slows down. In this way, the evaporation capacity of the evaporating pan decreases.

Therefore, he proposed that the main reason for the decrease of pan evaporation was the decrease of wind speed and sunshine hours. “ Xiaoxing'an Mountains The increase in pan evaporation is mainly due to the significant increase in temperature in this area, while the sunshine and wind speed are not significantly reduced. " He used one Counterexample To explain.

Later, he passed the Meteorological factor conduct trend analysis and correlation analysis This conclusion is further proved.

May affect the change of water cycle and water resource distribution

The fact that "evaporation in most regions of China is declining" will affect water cycle What impact does it have on agriculture, forestry and other related fields? Shen Shuanghe explained that the decrease of pan evaporation in most regions of China can be ruled out as a result of the decrease of temperature, the increase of ground evaporation and Air humidification It should be attributed to the reduction of sunshine and wind speed. Therefore, the decrease of pan evaporation may have some impact on the component of water cycle, such as the decrease of ground evaporation Water vapor transmission The precipitation will also change accordingly. yes agricultural production For example, it will lead to farmland Evapotranspiration Weakening, crop water use efficiency Increase and alleviate drought to some extent. In addition, China Northeast China The pan evaporation has increased to a certain extent. In addition, the temperature has increased significantly. On the one hand, it can expand the agricultural planting area crop yield And forests Carbon sequestration and Wood stock ； On the other hand, the increase of water consumption may have a certain impact on the local water cycle and water resource distribution.

measuring method

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Common instruments

Evaporation capacity

The commonly used instruments for measuring evaporation are small evaporator , large evaporation drums and Evaporation pan And so on.

Small evaporator It is a round metal basin with a diameter of 20 cm and a height of about 10 cm, Pelvic orifice It is in the shape of a knife edge. In order to prevent birds and animals from drinking water, the upper part of the mouth is covered with a metal wire mesh ring that is stretched outward into a trumpet shape. When measuring, put the instrument on the shelf, with the mouth 70cm above the ground, and put quantitative clean water every day. After 24 hours, measure the remaining water volume with a measuring cup. The reduced water volume is the evaporation capacity.

The large evaporation barrel is a cylindrical barrel with an opening area of 0.3 square meters. A straight pipe is installed at the center of the bottom of the barrel and the upper end of the straight pipe Measuring needle Seat and water surface indicator pin, the barrel body is buried in the ground, and the barrel mouth is slightly higher than the ground. Observe at 20:00 every day and insert the measuring probe Pincushion , read the water surface height, and precipitation Calculate evaporation.

Evaporation capacity

The specifications of evaporating dishes are mostly the same as Rain gauge Similarly, it is also a round vessel with a diameter of 20 cm, and the upper edge of the vessel mouth is 70 cm higher than the ground. The evaporating dish is 10 cm deep. It is because its thickness is less than the diameter that it is called a dish. Add 2cm deep water layer to the evaporating dish every day, and pour the remaining water into the evaporating dish at night Measuring cup , measure the remaining water depth. Subtract the remaining water depth from 20cm to get the evaporation of the day. If there is rain on that day, the remaining water will also be deducted from the precipitation on that day. This is why the diameter and height of the evaporating pan should be consistent with that of the rain gauge. Otherwise, the two cannot be simply subtracted.

Situation handling

When the measurement of evaporation is affected by non-human factors or human factors, people can adopt the following methods:

1. Due to precipitation and other natural reasons, the evaporation capacity is negative (No matter how many negative values), it is recorded as 0.0. There is no need to comment in this case.

2. If the evaporation is negative due to human factors, it shall be treated as lack of measurement.

Evaporation capacity

4. If the station who is not on duty at night finds that the evaporating pan water (snow) is indeed overflowing the next morning, the pan can be Internal water (Snow) Pour it out, add 20mm clean water again, and add parentheses to the evaporation capacity of the day, and indicate it.

5. Considering the phenomenon of fog, dew and frost evaporator On water (ice) surface and Rain gauge The condensation state on the metal surface is different, so the amount of pure fog, dew and frost is not considered in the calculation of evaporation.

6. When the station located on an island or high mountain is really hit by evaporation pan water and strong wind Blow out The records shall be enclosed with brackets and noted.

7. The evaporator is frozen, and the sand frozen in the ice cannot be removed, so it can be weighed and recorded as usual. However, the water shall be changed immediately after observation.

8. No special evaporation Platform scale There is no ordinary platform scale station with the unit of gram. When the evaporator freezes, the following methods can be used to deal with the evaporation capacity to ensure the integrity of ten day and monthly records.

The column of evaporation on each icing day (icing during observation) is marked with "B", and the total amount of this section is measured and calculated on the day when the ice melts, and recorded in the column of evaporation on the observation day. but Icing period To enter the next ten day period, a certain amount of warm water must be added to melt the ice at 20:00 on the last day of this ten day period for observation. The amount of warm water added is deducted from the measured value, and the calculated evaporation amount is recorded in the column of evaporation amount on the observation day. The above situations shall be noted in the remarks column of the book and table.

9. When the E-601 evaporator encounters icing, mark "B" in the column of evaporation on each icing day. After the ice melts on a certain day, measure the total amount since the stop of measurement and record it in the column of evaporation on that day; If the ice turns into the next month, the total amount since the suspension of measurement will be measured when the next month melts, by day Average distribution The accumulated value obtained shall be recorded in the column of evaporation on the last month and this month melting day respectively, so as to obtain a complete monthly calculation value, which shall be noted in the column of remarks in the sheet.

Since evaporation and precipitation are both observed at 20:00 every day, the daily evaporation and precipitation measured are actually the amount from 20:00 yesterday to 20:00 today, rather than the amount from 24:00 yesterday to 24:00 today.

data processing

The evaporation amount indicates the water branching out condition of the area. However, due to the dynamic and thermodynamic conditions of the evaporator itself and its surrounding air Natural water It is different. The evaporation measured by the evaporator is more than lake The evaporation capacity of actual water bodies such as reservoirs is large. Therefore, the evaporator's Observations It must be multiplied by a reduction factor (generally 0.7-0.8) before it can be used as the evaporation capacity of natural water bodies. The spatial variation of evaporation is affected by temperature, land and sea, precipitation and other factors. The lower the latitude, the higher the temperature, the stronger the evaporation capacity, and the greater the evaporation capacity; Under the same temperature, the evaporation on the ocean is greater than that on the mainland, and has a significant decrease trend from coastal to inland; Generally speaking, where there is more precipitation, there is also more evaporation. On the contrary, there is less evaporation. Whether a place is wet or dry depends on the place Wetting coefficient K， The formula is K=P/E, where P is precipitation and E is evaporation. K Greater than or equal to At 1 o'clock, it indicates that the water income is greater than or equal to the expenditure, which belongs to the wet condition; When K is less than 1, it reflects that water income is not enough to spend, and it belongs to semi humid, semi-arid or arid. K size, pair natural landscape The formation of structural characteristics has a profound impact.

Natural measurement

weather station The method of measuring evaporation is simple and easy. If the daily measured evaporation is added up, the annual evaporation can be obtained. Weather stations all over the country do this. We also know the annual evaporation throughout the country. But this method has disadvantages: it can not fully represent the real evaporation capacity of nature, and sometimes it has a very large deviation. The diameter of the evaporating dish is too small, resulting in a large evaporation capacity. Other natural Underlying surface Some are dry and some are wet, and there are swamps, farmland, lakes or oceans. These different underlying surfaces Actual evaporation Obviously, they are different.

In order to study evaporation under different natural conditions, people also choose evaporation dishes with larger diameters or measure evaporation from soil, water, farmland and even leaves. These evaporation measurement methods are complex in technology and high in cost, and only a few test stations can carry out them. In addition, some formulas are also studied to calculate evaporation indirectly.

Change trend

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Evaporation capacity

Evaporation is reduced by 34.12mm every decade. Through the statistics and calculation of data, it is found that although the annual average temperature in China has increased by 0.2 ℃ every 10 years in the past 45 years Evaporation pan On the whole, evaporation decreases by 34.12mm every 10 years. The research is only based on the analysis of the observation data of meteorological stations Representativeness And reliability are paid more attention, so the contribution of urban development (including building height and density, environmental and air pollution, etc.) to the weakening of wind speed, the reduction of sunshine, and the rise of temperature needs further research.

Distribution

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evaporator

On the earth, the amount of evaporation varies with different terrain and climate. The area with the largest evaporation in China is Qinghai Provincial Chaerhan Salt Lake , annual average evaporation（ Evaporation capacity ）3518 mm. The evaporation capacity of each continent is Asia 、 Africa 、 South America 、 North America 、 Oceania Europe.

Evaporation significance

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evaporator

Nature is really a wonderful machine. It has both storms and silence Evaporation process 。 Have you ever thought that without evaporation, there would be no rain in the sky, no rivers, no plants and animals.

The evaporation process connects the rivers on the ground, the vegetation and landscape on the earth, and our agriculture, forests and grasslands. On a large scale Diversion irrigation 、 south-to-north water diversion When planting trees and protecting the environment, a scientific analysis should be made of the changes in evaporation caused by each step. Correct understanding Evaporation process And evaporation law have very important theoretical and practical significance for the protection, utilization and transformation of nature.

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