Jiuzhaigou's "Blue" Issue

Author: Li Xiaohui, Mao Ping, Sun Geng (Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu Branch, Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

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There is a mysterious wonderland hidden in the majestic Minshan Mountains. It was not until half a century ago that its true appearance was gradually known to outsiders. The gurgling water flows through the mountains and dense forests, forming a series of fascinating blue lakes. The lakes here are like pieces of blue jade, changing into various colors under the sunshine, like a dream; the lakes here are like pieces of bright mirrors, reflecting the mountains, rivers, grass, blue sky and white clouds, presenting the wonders of "birds flying in the water and fish swimming in the sky". It is Jiuzhaigou.

The whole picture of Jiuzhaigou Five Flower Lake (Photo credit: Li Xiaohui)

Wuhua Lake is the most famous scenic spot in Jiuzhaigou, known as the "essence of Jiuzhaigou". The blue of the lake after the snow echoes the white snow, which looks like a fairy tale world.

Jiuzhaigou Five-Colored Pool (Photo credit: Sun Geng, Zhu Dalin)

The Five-Colored Pond is one of the most colorful lakes in Jiuzhaigou, and is famous for its beauty, color, purity and transparency. The Five-Colored Pond is also the most exquisite lake in Jiuzhaigou, and is known as the "Eye of Jiuzhaigou".

The water of Jiuzhaigou is dazzlingly blue, intoxicatingly blue, but what kind of blue is Jiuzhaigou's blue? You say it is azure blue, I say it is turquoise blue, he says it is lake blue, she says it is emerald blue, which really makes it even more difficult to say. Hey, please stop making things difficult for the readers, okay?

Things you don't know about Blue

In addition to Jiuzhaigou, there are many famous blue lakes in the world, such as Bear Lake in Utah, the United States, Plitvice Lakes National Park in Croatia, Blue Lagoon in Iceland, Blue Lake in the Gambier Mountains in southern Australia, and Lake Tekapo on the South Island of New Zealand.

Seeing these beautiful views of the sky and water, I, an uneducated person, can only sigh, wow, the sky is so blue, the lake is so blue! However, there are 41 kinds of blue in the world, including dark blue, cobalt blue, Dodge blue, navy blue, etc. Just like lipstick colors, each blue can be distinguished by color number or color RGB value. You may know the lipstick color number very well, but do you know what color numbers are the blues of the lakes above?

PANTON blue color classification chart (Image source: Baidu Encyclopedia)

Among the lakes mentioned above, the colors of each lake are different due to differences in geographical location, ecological environmental conditions and water chemical properties.

Powder blue

The Blue Lagoon is the largest geothermal hot spring in Iceland. The color of the Blue Lagoon is a powdery blue that is almost like milk. According to research, the blue color of the Blue Lagoon is due to the presence of a special blue-green algae in the hot spring, while the milky color is due to the large amount of silicon in the water.

Tiffany Blue

Lake Tekapo is located on the eastern foot of the Southern Alps in the South Island of New Zealand. It is a glacial dammed lake, and the blue color of the lake water is called "Tiffany Blue". The color of the lake water is actually caused by the refraction of sunlight by rock particles suspended in the lake water.

Turquoise blue

Bear Lake and Plitvice Lakes are bright turquoise blue. The color formation process of Bear Lake and Plitvice Lakes is related to calcium carbonate particles. The geological composition of both regions is mainly limestone, and a large amount of calcium and magnesium ions are dissolved in the lake water. The high concentration of micro-suspended calcium carbonate particles in the water reflect blue-green light, making the lake appear turquoise blue.

Dark Blue

The Blue Lake in the Gambier Range in southern Australia is known as the most difficult (blue) lake in the world. The color changes with the seasons. Due to the absorption of incident light by humus, the lake water appears gray-blue in winter. In summer, calcite crystals generated by degassing of carbon dioxide and rising temperatures have a co-precipitation effect with humus, which has a regulating effect on the humus concentration on the surface of the lake. In addition, calcite crystals themselves help scatter short-wavelength light, so the color of the lake changes from gray-blue to dark blue with the seasons.

Original author: Thom Devereux (Image source: Wikipedia has been modified)

Jiuzhaigou Blue (What Kind of Blue Is It?)

It is generally believed that different objects reflect, absorb and scatter light differently, thus presenting different colors. Just like the various lakes mentioned above, the presence of smaller particles such as rock particles, water-soluble silica particles, colloidal sulfur particles, colloidal aluminosilicate particles, glacial powder, etc., makes the lake water have unique reflection and scattering characteristics for sunlight, so the lake will appear blue.

Most of the lakes in Jiuzhaigou are karst lakes. The formation of their water color is related to the low turbidity of the lake water and the calcium carbonate particles present in the water. The Rayleigh scattering and Mie scattering of the incident light by the suspended calcium carbonate particles can make the lake water appear blue, and the differences in scattering intensity in all directions, the depth of the lake and the water quality give the lake a colorful blue.

Let me tell you what kind of blue is Jiuzhaigou blue?

Although there have been studies on the causes of the color of karst lakes, the special hydrological system and geographical environment of the karst area make the formation mechanism of lake color complicated and difficult to study. There is even less research on the formation mechanism of blue in plateau karst lakes such as Jiuzhaigou, and there is no scientific and reasonable explanation for why the lakes appear "colorful blue".

At present, Li Xiaohui, a master's student from the team of Sun Geng, a researcher at the Chengdu Institute of Biology, Chinese Academy of Sciences, and others have used hyperspectral equipment and combined it with indoor water chemistry analysis methods to quantify the color of Jiuzhaigou's karst lakes for the first time, revealing the color formation mechanism of Jiuzhaigou's world-famous blue karst lakes, establishing a regression equation for lake water quality and color, and reasonably explaining the cause of the colorful blue of Jiuzhaigou's lakes. The equation has also been applied to the prediction of lake color.

Let's talk about how this research was done.

1. Color can also be “quantified”

To explain the color of Jiuzhaigou's lakes, researchers first had to quantify the color.

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

We all know that color is a visual perception of light produced by the eyes and brain. White light can be decomposed into seven monochromatic lights: red, orange, yellow, green, cyan, blue, and purple. Because of the presence of red, green, and blue cone cells in the human eye, we are most sensitive to red, green, and blue light. Most of the colors we perceive can be mixed in different proportions by the three primary colors of red, green, and blue. This is the famous "three-color theory." Objects of different colors have different spectral characteristic distributions formed by reflected light. After entering the human eye, they are selectively absorbed by the three types of cone cells, and after a series of photochemical reactions, the color of the object is formed in the brain.

A simplified diagram of the human eye's color perception principle (Image source: Li Xiaohui)

Seeing this, many people may be confused, so let me give you an example to explain.

When the sky clears after rain or when the weather is clear, the sky is often blue. This is because there are fewer suspended particles in the atmosphere at this time. Under the strong selective scattering effect of atmospheric molecules, blue light is scattered from the sunlight and diffuses throughout the atmosphere. The blue light that enters the human eye accounts for the vast majority, which makes us feel that the sky is blue.

Blue sky and ocean (Photo credit: Mao Ping)

Based on the principle of human eye color perception, the researchers used a hyperspectral device (ASD FieldSpec HandHeld 2 portable ground spectrometer) to measure the off-water radiance data of each sampling point in the lake water body within the visible light range of the human eye (380nm-780nm) (the spectral radiation of light returning to the atmospheric interface after passing through the water body). Drawing on the theory of colorimetry, mathematical integration operations were performed within the visible light range to derive the color tristimulus values ​​XYZ. The color coordinates (x, y) were calculated using the XYZ values, and the color of each sampling point can be determined by corresponding to the CIE1931 colorimetric diagram. The color RGB value at each sampling point can also be calculated by performing matrix operations using Matlab software. In this way, by simulating the process of human eye perception of color, after the researchers determined the spectral data of the lake water body, they can quantify the complex lake color into simple color RGB values. This is conducive to the subsequent study of the color formation mechanism and lays a solid foundation for exploring the relationship between lake color and water quality.

The above principles are the real stuff, let’s go straight to the pictures below!

Schematic diagram of the lake color measurement process (Image source: Li Xiaohui)

The quantitative determination of lake color can be viewed as the process of human eye perceiving color. The sophisticated ASD spectrometer can be roughly viewed as a human eye receiving incident light, and the complex integration and matrix calculations are actually simulating a series of photochemical reactions in the human eye.

After analyzing the lake's off-water radiance data, researchers found that Jiuzhaigou's water body has strong wavelength selectivity in its absorption, reflection, and scattering of visible light, and the proportion of blue-green light in the reflected light is significantly higher than that of red light. This is closely related to the lake's unique water environment conditions and directly determines the color of the lake observed by the human eye. According to the measured data, the average proportion of red, green, and blue light reflected by Wuhuahai's water body is 9.2%, 49.6%, and 41.2%, respectively. This unique composition of reflected light makes the lake appear bright blue-green to the human eye.

2. The color secrets hidden in water

In order to explore the reasons why lake water selectively reflects and scatters visible light, the researchers also collected lake water samples for indoor water chemistry analysis.

After research, it was found that the turbidity of the water in Jiuzhaigou Lake is low. Except for a few sample points, the turbidity of most waters is around 0.69NTU, and the transparency is extremely high. Observed with the naked eye, the lake water is basically the same as canned Yibao. The reason is that the water of Jiuzhaigou Karst Lake mainly comes from atmospheric precipitation, mountain snowmelt and underground karst water, which has low sand content and high cleanliness. In addition, the lakeshore vegetation in Jiuzhaigou Scenic Area is rich, the surface runoff is small, and the water body has low sand content, which makes the lake water turbidity low and the transparency extremely high. Imagine if there is a lot of sediment in the lake water, it will never show a beautiful blue color.

Clear light is not enough. The unique travertine particles in Jiuzhaigou Lake are the key to the color of the lake. The water chemistry of Jiuzhaigou Lake belongs to Ca2+-HCO3- and Mg2+-HCO3-. 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 high concentrations of Ca2+ and HCO3- and alkaline water environment make the calcite saturation index (SIC) of the lake water high, that is, there are a large number of travertine (calcium carbonate) particles in the lake. These travertine particles have strong selective reflection and scattering effects on short-wavelength light (380-570nm) in visible light due to Rayleigh scattering. In addition, the travertine deposition process has a fixing effect on nutrients such as phosphates. The colloidal solution formed by calcium carbonate and water also has a dissolution precipitation effect, which can absorb suspended impurities in the lake water and has a purifying effect on the water body, which further reduces the turbidity of the lake. The interaction between Rayleigh scattering of sunlight by travertine particles and low turbidity causes the lake water to mainly reflect and scatter short-wavelength visible light, but the human eye is not sensitive to purple light in visible light, so the color of the lake we perceive is mainly blue-green.

So the question is, how to explain the colorful blue of the lake? As mentioned above, calcium carbonate particles selectively reflect and scatter the shorter wavelength blue-green light in visible light due to Rayleigh scattering. In fact, in the process of Rayleigh scattering, the scattering intensity of light in each direction is also different, which makes the lake appear blue at different depths when observing the lake surface from different angles. At the same time, due to the high clarity of the lake, when the lake reaches a certain depth, the highly clean water selectively absorbs long-wavelength visible light such as red light and mainly reflects short-wavelength blue-green light. At this time, the deeper the lake, the bluer the lake color observed by the human eye. Therefore, in different waters of the same lake, if there is a difference in depth, the lake water will often show different degrees of blue. In addition, different lakes and different seasons have different water quality characteristics that will directly lead to changes in the deposition rate of travertine, so different lakes have different abilities to reflect short-wavelength blue-green light. The above factors have created different degrees of blue in different lakes in the same ditch, different waters of the same lake, and different seasons of the same lake in the scenic area. This is the reason for the formation of the unique landscape of Jiuzhaigou's colorful lakes.

Schematic diagram of the color formation mechanism of Jiuzhaigou karst lakes

(Photo source: Li Xiaohui)

The colloidal solution formed by calcium carbonate and lake water has a dissolution and precipitation effect, which can absorb 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 and mainly reflects blue-green light; coupled with the selective reflection and scattering effect of calcium carbonate particles in the karst lake water on the incident light, the proportion of short-wavelength blue-green light entering the human eye is higher, so the human eye perceives the lake as blue-green.

The blue of the Wuhua Lake at different depths in Jiuzhaigou (Photo credit: Li Xiaohui)

Pure water has the highest absorption rate for visible light at 750-760nm wavelength. When the lake reaches a certain depth, the lake water absorbs a large amount of long-wavelength red light and mainly reflects and scatters blue-green light. At this time, the deeper the lake, the bluer the color.

3. Predicting Color Using Equations

Through correlation analysis, redundancy analysis and other analytical methods, the researchers found that the color of the lake is closely related to changes in water quality. In order to further explore the possible impact of water quality changes on the color of the lake, the researchers conducted a stepwise linear regression analysis on the water quality data obtained from water chemistry analysis and the lake RGB color value data, and established the water quality-color equation of Jiuzhaigou Lake:

The independent variables in the equation are water quality indicators such as transparency, ion concentration, conductivity, dissolved oxygen, etc., and the dependent variables are color R value and B value. 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 2019 into the equation and predicted the color changes of Jiuzhaigou Wuhua Lake, Mirror Lake and Rhino Lake. 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.

Jiuzhaigou Lake Color Forecast (Photo credit: Li Xiaohui)

Don't underestimate this equation. In the future, we may be able to monitor the changes in lake water quality and predict the color changes of lakes, so as to better protect the beautiful lakes in Jiuzhaigou. Similar methods can also be applied to other lakes.

Through the above content, have you gained a deeper understanding of Jiuzhaigou's blue lake? Jiuzhaigou is a treasure bestowed by nature on the world. The magical Jiuzhaigou still has many secrets worth exploring. Protecting it is to protect the environment on which human beings depend for survival.

Postscript: The most scientific way to show off wealth

——I rowed a boat in Jiuzhaigou

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

You think the sampling is like this:

Photos of tourists in Jiuzhaigou (Photo credit: Deng Xianzhi)

But, the fact is this:

Serious work photos (Photo credit: Li Xiaohui, Zhang Mengqi)

The time for playing is always short but beautiful. When you wake up, it's time to start working.

The people floating on the lake in Jiuzhaigou are not necessarily fairies, but also researchers collecting samples. When I was competing with people for resumes, the person in front of me boasted that he had wiped pandas’ butts and collected panda feces. I could only write a poem to express my pride: "I often remember the sunset in Jiuzhaigou, when I was collecting samples but didn’t know the way home. I returned to the boat late at night, and mistakenly entered the depths of scientific research..."

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