The evaporation of 3570 West Lake Tibetan Plateau lakes in one year is calculated in this way
Evaporation is a key part of the global surface energy balance and an important part of the water balance. It is understood that more than 60% of the global precipitation will return to the atmosphere in the form of evaporation.
Recently, researchers in my country have developed a new method for estimating lake evaporation. The total amount of lake evaporation in the Qinghai-Tibet Plateau is estimated to be 51.7 billion tons per year, which is equivalent to the water volume of 3570 West Lakes in Hangzhou.
In addition, the study also found that the ice sublimation water in the lake in winter accounts for about 12.3% to 23.5% of the annual evaporation of the lake, which is an important part of the study of the lake water balance. The length of the non-icing period and the evaporation of the lake in the southern Tibetan Plateau are significantly higher than those in the northern lake.
So, why should researchers study lake evaporation on the Tibetan Plateau? How did they calculate the evaporation of the lake? Is this method suitable for evaporation measurement in other lakes in the world? To this end, a reporter from Science and Technology Daily interviewed relevant experts.
Evaporation is closely related to the surrounding ecology and climate
It is understood that the Qinghai-Tibet Plateau has an average altitude of nearly 4,000 meters. It not only has the largest glacial reserves except for the Arctic and Arctic regions, but also has the highest and largest number of inland lakes on the planet. The lake area in the Qinghai-Tibet Plateau is nearly 50,000 square kilometers, accounting for more than half of the total lake area in China.
"It is the birthplace of many large rivers in Asia, including the Yangtze River, the Yellow River, the Lancang River, the Yarlung Zangbo River, the Indus River, the Syr River, etc., all of which are bred from the Qinghai-Tibet Plateau. The water resources of these rivers nurture billions of people in Asia Therefore, the Qinghai-Tibet Plateau is called the'Asian Water Tower'." Ma Yaoming, a researcher at the Qinghai-Tibet Plateau Research Institute of the Chinese Academy of Sciences, told Science and Technology Daily.
Rich water resources continue to flow from here, feeding the Qinghai-Tibet Plateau and its downstream areas of forests, grasslands, farmland, etc., and also provide a suitable habitat for fish, birds, and animals in the downstream areas, and it is also a productive and human life Important Guarantee.
"More importantly, the water exchange process between the Qinghai-Tibet Plateau and the surrounding areas will not only bring a large amount of water vapor from the Indian Ocean and the Western Pacific to the plateau through the monsoon system, but also transfer water and water vapor from the plateau to the The transportation in eastern China affects the rainfall process in eastern China." Ma Yaoming said that in addition to the water exchange between the Qinghai-Tibet Plateau and the surrounding area, the huge dynamic and thermal effects of the Qinghai-Tibet Plateau will also affect the surrounding area through the process of ground-gas interaction. Climate change in the region.
As an important part of the Qinghai-Tibet Plateau, the lakes here are extremely sensitive to climate fluctuations and can be regarded as an important information carrier that reveals global climate change and regional responses. Lake evaporation, as the output of lake water in the Qinghai-Tibet Plateau, is dominated by internal flow lakes, and precipitation are important components in the calculation equation of lake water balance. Accurate measurement of lake evaporation is the key to studying lake water and energy balance. In recent years, many researchers have estimated lake evaporation on the Tibetan Plateau through various methods.
Previously, researchers from the Qinghai-Tibet Plateau Research Institute of the Chinese Academy of Sciences and other units conducted assessments of water resources in glaciers, snow, runoff, lakes, rainfall, land evapotranspiration, etc. of Asian water towers in order to obtain the water resources reserves of Asian water towers. Preliminary Understanding.
Different measurement methods have obvious differences
In fact, in the study of the water cycle of lakes on the Qinghai-Tibet Plateau, there have been few observations on lake-gas interactions in high-altitude lakes. The lake evaporation obtained by different research methods on the same lake has obvious differences, and the spatial distribution of the lake evaporation and the total evaporation have not been obtained so far.
"There are many methods for calculating lake evaporation, such as methods based on instrumental observations, methods based on energy balance, methods based on water balance, and methods for model simulation," said Dr. Binbin Wang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences.
The method based on instrument observation mainly uses equipment such as evaporating dish and vortex correlator. It is necessary to go to the lake area to set up the observation instrument and spend a lot of manpower and material resources. However, "due to the harsh natural environmental conditions and inconvenient transportation on the Tibetan Plateau, it is not realistic to directly observe large-scale lake evaporation in a short period of time." Wang Binbin said that at the same time, due to the size of the water body of the evaporation dish, weather and environmental background conditions and the reality There are significant differences in lakes, resulting in this observation method often having great limitations.
At the same time, methods based on water balance, energy balance, and model simulation require large amounts of observation data. "Taking water balance as an example, we need to accurately know the rainfall in the lake area, the surface inflow and outflow of the lake, the underground inflow and outflow of the lake, etc. These observations are difficult to obtain accurately, and the existing observations are also There will be certain errors." Wang Binbin said.
The model simulation method also needs to accurately know the lake's depth, transparency and other parameters, and requires a large amount of meteorological data as driving data, and the model's lake process parameterization scheme needs a lot of actual observation data to verify. The traditional energy balance-based method requires the observation of lake temperature chains to obtain lake heat storage, but there are currently few lakes with temperature chain observations on the Tibetan Plateau, which is insufficient to support such calculation methods.
"Therefore, based on the reasonable assumption that the heat storage of the lake in the non-icing period is close to zero as a whole, with the help of satellite remote sensing data and meteorological reanalysis data, we estimate the lake evaporation." Wang Binbin said. In contrast, this estimation method is also operable in combination with the regional characteristics of lakes frozen in the Qinghai-Tibet Plateau in winter and mostly inland lakes. The introduction of satellite remote sensing data can enable the research method to obtain regional expansion, and the meteorological reanalysis data has been verified by the Qinghai-Tibet Plateau observation and research platform to have a more reliable accuracy guarantee.
"Because the lake on the Qinghai-Tibet Plateau is about 50,000 square kilometers, according to the average annual evaporation, the average annual evaporation depth is 900-1000 mm." Wang Binbin said that this is a normal amount of evaporation. The evaporated water vapor is still in the air. Rain and snow will form on the ground, which is a natural water cycle process. In addition, in the context of current climate warming, the water cycle of the Qinghai-Tibet Plateau is accelerating.
The new estimation method obtains the lake evaporation, ice phenology and evaporation water resources of 75 large lakes in the Qinghai-Tibet Plateau based on satellite remote sensing data. These data are of great significance for accurately estimating the lake's water storage and its changes in the future.
The new measurement method is not suitable for all lakes
So is this method suitable for other lake evaporation measurements?
In this regard, Wang Binbin said that this new lake evaporation estimation method takes into account the specific characteristics of lakes in the Qinghai-Tibet Plateau, for example, most of them are inland lakes and usually have icing periods of varying lengths. If you want to use this new method to estimate the average annual evaporation of lakes in other parts of the world, you usually need to analyze the characteristics of specific lakes for application.
However, for some lakes, this estimation method may not be applicable. For example, lakes have huge inflows and outflows of water, and these inflows and outflows are usually accompanied by a large amount of energy exchange, making the lake water body heat storage items negligible Important assumptions are difficult to establish.
"It must be clear that because the lake area of the Qinghai-Tibet Plateau is relatively small compared to the land area, the lake evaporation is relatively small relative to the land evapotranspiration. Therefore, from the perspective of the entire Tibetan Plateau area, lake evaporation The quantity will not affect the climate environment too much." Wang Binbin emphasized when talking about the significance of lake evaporation to the climate environment research.
However, in some specific lake basins, the impact of lake evaporation on the local climate and environment is relatively large. For example, Wang Binbin said that due to the existence of lakes in the Namco River Basin, there is a lake effect in the downwind area of the Namco Lake, resulting in higher rainfall and snowfall in the downwind area of the Namco Lake than in the upwind area.
"How much water is there on the Qinghai-Tibet Plateau? What changes will the Qinghai-Tibet Plateau's water resources show in the context of climate warming? We have been focusing on these issues," Wang Binbin said.