A sign of something! The “real cause” of the abnormal situation that caused a stir on social networking sites.

For the first time in 29 years!　The cause of the large drop in water level

While the low reservoir levels of dams around the country have been making headlines due to the daily low rainfall, an unbelievable scene is unfolding at Lake Kawaguchi, one of Japan’s most popular tourist destinations. The “Rokkakudo”, which usually looks like a floating island, is now accessible on foot.

The water level has dropped 4.1 meters, the first time in 29 years, and people on social networking sites are asking, “Has the spring water run dry? Fuji running dry?” and “Is this a sign of something? However, the real cause is said to lie elsewhere. We will look at the “unique situation of Lake Kawaguchi” as revealed by an expert and its future prospects.

As of March 6, the water level of Lake Kawaguchi was -4.1 m. It is rare for the water level to drop more than 4 m, and it has been 29 years since 1997. Last year, the rainy season was short and there were not many typhoons. This is probably due to the low rainfall.

says Shinya Yamamoto, chief researcher at the Volcanic Disaster Prevention Research Center at Mt.

However, is not the water in the Fuji Five Lakes, including Lake Kawaguchi, fed by spring water from Mt.

Fuji. “What flows into Lake Kawaguchi is not spring water from Mt. Fuji, but groundwater from the Misaka mountain range to the north, the Tera River, which originates in the Misaka mountain range, and underground water that flows under the rivers that are now dry. When precipitation is low, the amount of groundwater and subsoil water flowing into the area is also low, and the water level drops.

When annual precipitation falls below 1,300 mm, the water level tends to decline significantly, but since the annual precipitation in ’25 was about 1,080 mm, the water level is considered to have dropped.

However, groundwater takes decades and hundreds of years to come up through the mountains and other places. What does last year’s precipitation have to do with this year’s major decrease?

Certainly the rain that fell last year will not run off onto the surface of the earth until decades or centuries later. However, water that has accumulated underground will come up to the surface, pushed out by last year’s rainfall. When precipitation is low, groundwater is less likely to be pushed out, so less of it flows into the lake, resulting in a great reduction in water flow.”

Last year, the rainy season, when precipitation is normally high, was short, and there were few typhoons. As a result, the water level did not rise in the summer and dropped by 4 m in the winter.

Why did the water level drop only in Lake Kawaguchi?

If precipitation is the cause, it would be strange if the water levels of the other lakes in the Fuji Five Lakes have not decreased as well. However, Lake Kawaguchi is the only lake where the water level has dropped by 4 meters. Why is this?

Strictly speaking, precipitation varies from east to west. Fuji, so there is more rain on the east side of the lake. Therefore, the area around Lake Yamanaka receives the most rainfall. The next most frequent areas are Lake Nishiko and Lake Shojin, where clouds come in from the west due to the pressure pattern. The north side, where Lake Kawaguchi is located, receives less rainfall because the clouds that have brought down rain flow over the area.

Serious Impact on Agriculture and Tourism

According to Yamamoto, there are two other causes for the low water level besides low precipitation.

One is leakage. Lake Kawaguchi is made of lava, which has many voids. This makes it easy for water to seep in, creating a natural leakage.

However, the amount of leakage is very small. This does not result in a reduction in water flow.

The other is artificial drainage. Lake Kawaguchi is dammed by lava and has no outlet, so it overflows during heavy rains. During the Meiji period (1868-1912), the water frequently rose, causing flooding damage to the surrounding villages. To prevent this, drainage tunnels were built, and two of them are in use today. In the spring, the tunnels are used to send water from the lake to downstream areas for agricultural use. However, if the water level of the lake drops below the bottom of the tunnels, the water cannot be delivered, which could affect agriculture.

Not only agriculture, but also the tourist industry will suffer as boats will not be able to launch if the shallows continue.

Is it safe for the water level to remain this low?

I think it will be fine once the rains start coming in the spring. In fact, in ’17, the water level was minus 3.7 m. It has been 29 years since the water level dropped more than 4 m, but in ’17 it was only 40 cm lower than it is now.

Although the image of the land dropping to Rokkakudo is shocking, it is a sight that has been seen almost every year from winter to early spring, when precipitation is low, for the past 10 years, he said.

The locals are not worried about it because we see it every winter, too. In the past, however, the temperature often dropped to -10 or -15 degrees Celsius in winter, and Lake Kawaguchi and Lake Yamanaka were sometimes completely frozen over. But these days, the temperature rarely drops below -10 degrees Celsius, and the lakes don’t freeze anymore.

Yamamoto says that warmer temperatures have nothing to do with water levels, but there is a theory that low rainfall is an effect of global warming. Will we see Lake Kawaguchi filled to the brim with water in the summer?

Shinya Yamamoto is a senior researcher at the Fuji Volcano Disaster Prevention Research Center of the Yamanashi Prefectural Fuji Science Institute. His research is to decipher past environmental changes by analyzing organic compounds in sediments and their stable isotope ratios. In particular, he conducts research on sediments from the Fuji Five Lakes to investigate the history of eruptions and the environmental effects of eruptions at Mt. Fuji. He also investigates and analyzes groundwater in the Fuji Five Lakes and Mt. Fuji using geochemical methods in order to clarify the current state of the natural environment of Mt.