There is a mysterious fairyland hidden in the towering Minshan Mountains. It was not until half a century ago that its true meaning was gradually known to outsiders. The gurgling water flows through the mountains and dense forests, forming a charming blue sea. The lakes here are like pieces of blue jade, which change into various colors under the sunlight, like a dream; The lake here is like a mirror, reflecting mountains and rivers, vegetation, blue sky and white clouds, showing the spectacle of "birds flying in the water and fish in the upper reaches of the sky". This is Jiuzhaigou.

Wuhuahai is the most famous scenic spot in Jiuzhaigou, and has the reputation of "the essence of Jiuzhai". After the snow, Haizi's blue echoes the snow from afar, much like a fairy tale world.

Wucaichi is one of the most colorful Haizi in Jiuzhaigou, which is famous for its beauty, purity and transparency. Wucaichi is also the most exquisite Haizi in Jiuzhaigou, and is known as the "Eye of Jiuzhai".

The water in Jiuzhaigou is dazzling and the blue is intoxicating, but what is the blue in Jiuzhaigou? You said it was blue, I said it was emerald blue, he said it was lake blue, and she said it was emerald blue, which really made blue (blue) difficult (south). Hey, don't embarrass the readers, okay?

Besides Jiuzhaigou, there are many famous blue lakes in the world. For example, Bear Lake in Utah, Lake Plitvece National Park in the Republic of Croatia, Blue Lake Hot Springs in Iceland, Blue Lagoon in the gambier Mountains in southern Australia, Lake Tikapo in the South Island of New Zealand, etc.

Seeing the corresponding beautiful scenery of water and sky, I can only sigh that there is no culture. Wow, the sky is so blue and the lake is so blue! However, there are 4 1 kinds of blue in the world, such as deep blue, cobalt blue, dodger blue and navy blue. Like lipstick number, each blue color can be distinguished by color number or RGB value of color. You may know red, but do you know what the blue of those lakes is?

Among the lakes mentioned above, each lake has a different color due to the differences in geographical location, ecological environment conditions and hydrochemical characteristics.

Blue Lake Hot Spring is the largest geothermal hot spring in Iceland, and the color of blue lagoon is pink blue similar to milk. According to research, the blue color of the blue lagoon is due to the existence of a special blue-green algae in the hot springs, while the milky white color is due to the large amount of silicon in the water.

Lake Tikapo is located at the eastern foot of southern alps, South Island, and belongs to glacial lake. The blue of the lake belongs to Tiffany Blue. The color of the lake is actually caused by the refraction of solar light by suspended rock particles in the lake.

Bear Lake and Plitvece Lake Group are bright turquoise blue. The color formation process of Xionghu Lake and Plitvece Lake Group is related to calcium carbonate particles. The geological composition of the two areas is mainly limestone, and a large number of calcium and magnesium ions are dissolved in the lake. The reflection of high concentration calcium carbonate micro-suspended particles in water on blue-green light makes the lake appear turquoise blue.

Blue lagoon, located in the gambier Mountains in southern Australia, is called the most difficult (blue) lake in the world. Colors change with the seasons. Due to the absorption of incident light by humus, the lake appears gray-blue in winter. In summer, calcite crystals produced by carbon dioxide degassing and temperature rise have coprecipitation with humus, which can adjust the concentration of humus on the surface of lakes. In addition, calcite crystal itself contributes to the scattering of short-wavelength light, so the color of the lake will change from gray-blue to dark blue with the seasons.

It is generally believed that different objects reflect, absorb and scatter light differently, so they show different colors. Just like all kinds of lakes mentioned above, the existence of smaller particles such as rock particles, water-soluble silica particles, colloidal sulfur particles, colloidal aluminosilicate particles, glacier powder, etc. makes the lake have unique reflection and scattering characteristics for sunlight, which makes the lake appear blue.

Most lakes in Jiuzhaigou belong to karst lakes, and the formation of water color is related to the low turbidity of lakes and calcium carbonate particles in water. Rayleigh scattering and Michaelis scattering of incident light by suspended calcium carbonate particles can make the lake appear blue, while the difference of scattering intensity in all directions and the difference of lake depth and water quality give the lake colorful blue.

Although the causes of color formation of karst lakes have been studied, the special hydrological system and geographical environment in karst areas lead to complex mechanism of color formation of lakes, which is difficult to study. There is even a lack of research on the blue formation mechanism of plateau karst lakes such as Jiuzhaigou, and there is no scientific and reasonable explanation for the reason why lakes are "colored blue".

At present, Li Xiaohui, a graduate student of Sun Geng's team, researcher of Chengdu Institute of Biology, Chinese Academy of Sciences and others have used hyperspectral equipment and indoor hydrochemical analysis methods to quantify the color of Jiuzhaigou karst lake for the first time, revealed the color formation mechanism of Jiuzhaigou's world-famous blue karst lake, established the regression equation of lake water quality and color, reasonably explained the cause of Jiuzhaigou lake's color blue, and applied the equation to the prediction of lake color.

Let's talk about how this research is done ~

1. color can also be "quantized"

In order to explain the reasons for the color formation of Jiuzhaigou Lake, researchers should first quantify the color.

Before understanding this color quantization process, it is necessary to talk about how the human eye perceives color.

As we all know, color is the visual perception of light produced by eyes and brain. White light can be decomposed into seven monochromatic lights: red, orange, yellow, green, cyan, blue and purple. Because the human eye has red, green and blue pyramidal cells, we are most sensitive to red, green and blue light. Most of the colors we perceive can be mixed by red, green and blue in different proportions, which is the famous "three-color theory". Objects with different colors, due to the different spectral characteristics of reflected light, are selectively absorbed by three kinds of pyramidal cells after entering the human eye, and after a series of photochemical reactions, the colors of the objects are formed in the brain.

Seeing this, many people may be confused. Here is an example.

When the rain stops or the weather is fine, the sky is often blue. This is because there are fewer suspended particles in the atmosphere at this time. Under the strong selective scattering of atmospheric molecules, blue light is scattered by the sun and diffused into the whole atmosphere. The blue light that enters people's eyes accounts for the vast majority, which makes us feel that the sky is blue.

Based on the color perception theory of human eyes, the researchers measured the radiance data from water (the spectral radiation of light returning to the atmospheric interface after passing through the water body) at various sampling points within the visible range of human eyes (380nm-780nm) with the help of hyperspectral equipment (ASD FieldSpec handheld 2 portable ground object spectrometer). Based on the theory of colorimetry, the mathematical integration operation is carried out in the visible light range, and the color tristimulus value XYZ is deduced. The optimal coordinate (x, y) is calculated by XYZ value, and the color of each sampling point can be determined according to CIE 193 1 chromaticity diagram. You can also calculate the color RGB value of each sampling point after matrix operation by Matlab software. In this way, by simulating the process of color perception by human eyes, researchers can quantify complex lake colors into simple RGB values after measuring the spectral data of lake water. This is conducive to the subsequent study of color formation mechanism, and also lays a solid foundation for exploring the relationship between lake color and water quality.

The above principle is dry goods, and the following is directly on the picture!

The process of quantitative determination of lake color can be regarded as the process of color perception by human eyes. The tall ASD ground object spectrometer can be approximately regarded as the incident light received by human eyes, while the complex integration and matrix calculation are actually simulating a series of photochemical reactions in human eyes.

After analyzing the radiance data of the lake, the researchers found that Jiuzhaigou water body has strong wavelength selectivity for absorption, reflection and scattering of visible light, and the proportion of blue-green light in reflected light is obviously higher than that of red light, which is closely related to the unique water environment conditions of the lake and directly determines the color of the lake observed by human eyes. According to the measured data, the average proportion of red, green and blue light reflected by Wuhai water body is 9.2%, 49.6% and 4 1.2%. This unique composition of reflected light makes the color of lake water observed by human eyes bright blue-green.

2. The color secret hidden in the water

In order to explore the causes of selective reflection and scattering of visible light by lake water, the researchers also collected lake water samples for indoor hydrochemical analysis.

Through research, it is found that the turbidity of Jiuzhaigou Lake is low, except for a few other points, the turbidity of most waters is around 0.69NTU, and the transparency is extremely high. Visually, the lake water is similar to Ipoh cans. The reason is that the water body of Jiuzhaigou karst lake is mainly replenished by atmospheric precipitation, mountain snowmelt water and underground karst water, with low sediment concentration and high cleanliness. In addition, Jiuzhaigou Scenic Area is rich in vegetation, with little surface runoff and low sediment concentration, which makes the lake turbidity low and transparency extremely high. Imagine that if there is a lot of sediment in the lake, it can't show a beautiful blue color anyway.

It is not enough to be clear. The unique travertine particles in Jiuzhaigou are the key to the color formation of the lake. The chemical types of Jiuzhaigou Lake are Ca2+-HCO3- and Mg2+-HCO3-, and the water body is rich in Ca2+ and HCO3-, and the concentration of these ions is several times higher than that of ordinary plateau lakes. Such a high concentration of Ca2+, HCO3- and alkaline water environment makes the calcite saturation index (SIC) of the lake high, that is, there are a large number of travertine (calcium carbonate) particles in the lake. Due to Rayleigh scattering, these travertine particles have strong selective reflection and scattering effects on short wavelength light (380-570nm) in visible light. Moreover, the deposition process of travertine can fix nutrients such as phosphate, and the colloidal solution formed by calcium carbonate and water can also dissolve and precipitate, which can adsorb suspended impurities in lake water, purify water quality and further reduce the turbidity of lake. The interaction between travertine particles and Rayleigh scattering of sunlight and low turbidity makes the lake mainly reflect and scatter short-wavelength visible light, while human eyes are not sensitive to violet light in visible light, so the color of the lake we perceive is mainly blue-green.

Then the question comes, how to explain the colorful blue of the lake? Due to Rayleigh scattering, the calcium carbonate particles selectively reflect and scatter blue-green light with shorter wavelength in visible light. In fact, in the process of Rayleigh scattering, the scattering intensity of light in all directions is also different, which makes the lake appear blue at different depths from different angles. At the same time, due to the high clarity of the lake, when the lake reaches a certain depth, the water with high cleanliness selectively absorbs visible light with longer wavelength such as red light, and mainly reflects blue-green light with shorter wavelength. At this time, the deeper the lake, the bluer the color of the lake observed by human eyes. Therefore, in different waters of the same lake, if the depth is different, the lake often appears blue in different degrees. In addition, different lakes and seasons have different reflection abilities to short-wavelength blue-green light, because the difference of water quality characteristics will directly lead to the change of travertine deposition rate. The above factors make the same ditch blue in different lakes, different waters of the same lake and different seasons of the same lake. This is the reason why Jiuzhaigou colorful lake is a strange landscape.

The colloidal solution formed by calcium carbonate and lake water can dissolve and precipitate, and can adsorb suspended impurities in the lake water, making the lake water clear and transparent. When the lake reaches a certain depth, the transparent water body has a strong absorption effect on long-wavelength light such as red light in the incident light, mainly reflecting blue-green light; Coupled with the selective reflection and scattering of incident light by calcium carbonate particles in karst lake water, the proportion of short-wave blue-green light entering human eyes is high, so human eyes perceive the lake water as blue-green.

The absorption rate of pure water to 750-760nm visible light reaches the maximum. When the lake reaches a certain depth, it absorbs a lot of long-wavelength red light, mainly reflecting and scattering blue-green light. At this time, the deeper the lake, the bluer the color.

3. Use equations to predict colors.

Through correlation analysis and redundancy analysis, the researchers found that the color of the lake is closely related to the change of water quality. In order to further explore the possible influence of water quality change on the color of the lake, the researchers made a stepwise linear regression analysis between the water quality data obtained by hydrochemical analysis and the RGB color value data of the lake, and established the water quality-color equation of Jiuzhaigou Lake:

The independent variables in the equation are transparency, ion concentration, conductivity, dissolved oxygen and other water quality indicators, and the dependent variables are color R and B values. With the help of this regression equation, the color change of the lake can be predicted by measuring the water quality of the lake. The researchers substituted the water quality data measured in July, August and September of 2065438+2009 into the equation, and predicted the color changes of Wuhai Lake, Jinghai Lake and Rhino Lake in Jiuzhaigou. It can be seen intuitively from the prediction results that the color of the lake gradually changes from blue to green with the change of seasons.

Don't underestimate this equation. In the future, it may be possible to predict the color change of the lake by monitoring the water quality change of the lake, so as to better protect the beautiful lake in Jiuzhaigou. Similar methods are also applicable to other lakes.

Through the above, do you have a deeper understanding of Jiuzhaigou blue lagoon? Jiuzhaigou is a treasure given by nature, and there are many secrets worth exploring in the magical Jiuzhaigou. Protecting it is to protect the environment on which human beings depend.

During the experiment, in order to determine the colors of different sampling points in each lake, researchers often need to row to the center of the lake to collect water samples and collect spectral data.

Do you think the sampling is like this:

However, the fact is this:

Playing time is always short and beautiful ~ it's time to move bricks when you wake up.

It is not necessarily fairies that float on the lake in Jiuzhaigou, but also researchers who take samples and measure samples. When I was spelling a resume with someone, the person in front praised me for "wiping the panda's ass and pulling the panda shit"; I can only write a poem to express my pride: "I will always remember the sunset in Jiuzhai, and I don't know the way back after sampling." It's too late to go back to the ship and go deep into scientific research ... "