Masayoshi Nakawo
The Abundant Oases That support River Systems
A vast arid region spreads out across the center of the Eurasian continent. Far from the sea, and receiving an exceptionally small amount of precipitation, the region gets approximately 100 mm of rainfall per year, and in some areas, less than 50 mm. This is indeed what one calls a desert. Having little rain could be considered the same as having good weather. Day after day, there is strong sunlight that sears the skin.
Sunlight, carbon dioxide, and water are indispensable to the growth and development of plants. This is because plants use carbon dioxide from the atmosphere and water to perform photosynthesis in order to grow. There is plenty of carbon dioxide in the atmosphere around the world. As such, what determines why plants grow very well in some locations and not so well in others is whether or not there is sufficient sunlight and water.
Arid regions are flooded with strong sunlight. Accordingly, if there were only enough water, such regions would be just right for plants to grow and develop. However, it is these arid regions that have little water. Although there is sufficient sunlight, there is very little plant life in the desert.
Scattered around deserts there are towns and cities called oases. The characters used to write oasis in Chinese mean “green island”. When traveling on the dark brown desert and then approaching an oasis, it is as if an island of green has appeared before your eyes. The green flora grows lushly, and the people of the oasis raise various crops such as wheat, maize, vegetables, and cotton. An oasis may also be abundant in fruits such as watermelon, melons, and grapes.
Because an oasis is also located within an arid region, an oasis is not formed just because there is a large amount of precipitation right where the oasis is. The oasis still only gets about 100 mm of rain fall a year. So why, then, is there so much abundance in an oasis?
The secret is a river. An oasis is located in the vicinity of a large river. By drawing water from the river and spreading it throughout the town, it is possible to grow shade trees, fruits, grains, vegetables, and various other types of plants. There is actually a network of waterways, like the mesh of a net, running through the oasis to carry the water from the river. This type of system for carrying water to where it is needed via man-made waterways is called an irrigation system, and the waterways themselves are called irrigation waterways. So basically, an oasis is a place in which water, the one thing lacking in an arid region for growing plants, is maintained through irrigation, making it possible to achieve ongoing abundant production.
An oasis is surrounded by brown desert, where almost no plants grow, so the greenery of the oasis is strikingly refreshing. However, this greenery is not something resulting from nature. It is something that finally reached a sustainable level thanks to the unceasing efforts of the people of the oasis.
The irrigation water applied to the fields, etc., causes the plants to grow, and is then returned to the atmosphere as water vapor evaporates from the surface of the ground and the leaves of the plants.
The flow of the river is decreased by the amount used for irrigation in the oasis. The river continues to flow to downstream areas.
In arid regions, it is particularly easy for water to evaporate from the river surface, or to be absorbed into the ground, as the river flows downstream, so the flow of the river gradually decreases as it moves downstream. Little by little, the river becomes narrower. Eventually, the amount of water lost to the atmosphere and into the ground is equal to the amount of water flowing from upstream, at which point the river disappears. There, the end of the river is formed. In Japan, all of the rivers eventually flow into the sea, but in arid regions, there are many rivers that disappear along the way, and never reach the sea. This type of river is called an inland river.
Some rivers disappear as they flow into lakes. If the amount of water evaporating from the lake surface is less than the amount of water flowing into the lake, the lake may gradually become smaller and smaller, and eventually disappear.
Glaciers as a Water Source
Although located in an arid region with little precipitation, an oasis gets an abundance of water from a river. So where exactly does this river come from?
In the areas surrounding the arid regions at the center of the Eurasian continent, there are many mountain ranges, for example, the Pamir plain, the Kunlun, Tianshan, Altai, and Qilian Mountains. These are high mountains that include many peaks with elevations above 7,000 m.
Even in arid regions, precipitation increases with altitude, a phenomenon we also see in Japan. The annual precipitation in areas near oases located at elevations of approximately 1000 m is around 100 mm, but one finds increasing precipitation as one moves up nearby mountains. As precipitation increases, we begin to see grasses appearing on the surface of the ground. At 2000 m to near 3000 m, we see vast plains, and trees begin to appear. In these abundant plains, herdsmen make their living by raising sheep, goats, yak, etc. At this elevation, the annual rainfall is approximately 200 to 300 mm. Going further up the mountain, near an elevation of 4000 m, the annual rainfall reaches from 600 mm to 800 mm. At this height, one can see magnificent woodlands. The river flowing to the oasis is formed from the abundant precipitation falling in the mountains and flowing down to the oasis.
As one moves up a high mountain, the temperature decreases. Above an elevation of 5000 m, the temperature can drop below freezing even in the summer. At such places, precipitation is in the form of snowfall rather than rainfall. Snow falls and accumulates each year and as the snow accumulates, it compresses under its own weight to form ice. The mass of ice formed in this way begins to flow slowly due to gravity. This is a glacier. Many beautiful glaciers can be seen in the mountain ranges surrounding an arid region.
As a glacier moves downhill along the valleys, the temperature rises as the elevation decreases, and the glacier gradually melts. This melting progress occurs particularly quickly in the summer, so glaciers are very important as sources of water for the river water used by an oasis. In other words, as an oasis water resource, a glacier acts as a natural dam. However, unlike man-made dams, a glacier cannot be easily controlled.
With the warming of the climate, the rate of glacier melting exceeds the rate of snow accumulation on glaciers. The amount of water melting from glaciers and flowing into rivers increases, so the volume of water in a river is greater than it was prior to the warming of the climate. Meanwhile, the glaciers become proportionally smaller. Conversely, if the climate becomes colder, the glaciers become larger and the volume of water in the river decreases. In this way, the amount of water released from the glaciers as natural dams is controlled by changes in the climate. As a result of changes in the glaciers, the volume of water in a river flowing into an oasis also changes.
Recent Water Shortages in Arid Regions
In oases in arid regions of western China, water shortages have recently become very apparent. The Heihe River flows northward across the “Silk Road”, which extends from the Qilian Mountains along the Gansu Province, eventually disappearing in Inner Mongolia. Here, we will look at the Heihe River basin as an example of a recent water shortage.
There once were two lakes at the end of the Heihe River, the Gashun Nuoer and Suoguo Nuoer lakes. Located in the west, Gashun dried up in 1961, and Suoguo, located in the East, dried up in 1992. This is thought to have been the result of a decrease in the flow of water in the Heihe River. Before the water could reach the lakes, the flow of the Heihe River itself terminated and the lakes disappeared.
At the lower reaches of the Heihe River, a grove of Populus euphratica, a type of poplar tree, used to grow by the river, but the grove died out. There was also a striking decline in the grasses that grew around the grove. When the local elderly people were children, many reeds grew tall enough to hide a camel, but these reeds had also disappeared.
Herdsmen, who made their livelihood by having their livestock graze on the grasses growing around the lake, faced difficult times as the grasses decreased. Water could no longer be drawn from wells that had been dug several meters deep to acquire the water necessary for grazing. The wells had dried up. The ground water level had dropped, so it was no longer possible to draw water from shallow wells. To get water, it had become necessary to use machinery to drill wells to a depth of 20 meters or more. And it costs money to dig deep wells.
Efforts to Restore Water Resources
In order to resolve the water shortage problems in the lower reaches of the Heihe River that we have been discussing, it would be necessary to restore the water resources. Two methods for achieving this can be considered. One is to restrict the amount of water taken from the Heihe River for irrigation (the quantity of water intaken) to allow more water to flow downstream. The other method is to make the woodlands more abundant.
As a restriction of water intake from the Heihe River, maximum limits on water use for irrigation were established, with the limits corresponding to the flow volume of the Heihe River as it flowed from the mountains into the oases. These usage limits resulted in an increase in the volume of water in the river flowing downstream.
However, this also created difficulties for the farmers who were able to take less water from the river. When the quantity of water intake was decreased, there was not enough water to maintain the agricultural lands that had been used to grow crops. So, the farmers came up with the idea of using well water. Wells were dug, and water was drawn from the wells to compensate for the shortage of water from the river.
Improvements were also made to the irrigation channels. Up to that point, an irrigation channel was just a ditch dug in the surface of the ground. As water flowed through the ditch, it was also being absorbed into the ground through the sides and bottom of the ditch, resulting in less water reaching the final destination. With the improvements, the walls and bottom of the irrigation channels were covered with vinyl sheets or concrete, successfully decreasing the amount of water loss. Thus, it became possible to more efficiently deliver water to its destination in the agricultural lands.
As for efforts to make woodlands more abundant, the first targets of these efforts were the woodlands in the mountains that are the source of the river. Herdsmen grazed their livestock at the base of a mountain, where there is sufficient precipitation for trees to grow. Livestock browse on tree sprouts, hindering the development of the woodlands, so the idea was formulated that it would be preferable to remove the livestock from the mountains. This effectively meant relocating the herdsmen from the mountains to another area. Because this relocation was being done for the purpose of protecting and preserving the ecology, the policies that resulted became known as “ecological relocation” policies. The destinations considered for the relocation were areas near the abundant oases.
So, what became of the herdsmen who were relocated to areas near oases? They had left the vast plains, so it was difficult for them to get food to feed their livestock. It became necessary to cultivate animal feed. So, new agricultural lands were developed to grow animal feed such as alfalfa and maize. However, this is in arid region. There is plenty of sunlight but no water. It was necessary to construct irrigation systems for these new agricultural lands in order to provide water for the crops.
In order to use water from the Heihe River, permission had to be obtained from the farmers who had always been using the water. It was these farmers who had long held the right to utilize the river water, a right known as “irrigation right”.
However, the farmers themselves were subjected to restrictions on water intake from the Heihe River, and were having difficulty getting enough water for their own agricultural lands. They could not be expected to then share their water with the herdsmen who had moved into the oasis area. As such, the herdsmen who relocated to the oasis area gave up the idea of using water from the Heihe River. Instead, new wells were dug, and the herdsmen relied on well water to supply the water they needed.
Of course, it costs money to develop new agricultural lands and to construct irrigation systems for well water. Once agricultural lands and irrigation systems had been developed, some people gave up raising animals and began farming. However, these herdsmen had done absolutely no farming up to that point, and apparently very few have succeeded in their farming efforts.
However, regardless of whether these people were working as herdsmen to grow feed for their animals, or as farmers to grow food crops, there was still in new demand for water for these agricultural lands. So, more water than ever was needed in the oasis.
Arduous afforestation efforts were also made at the lower reaches of the Heihe River, and volunteers from Japan participated in these efforts. However, the annual precipitation at the lower reaches of the Heihe River is less than 50 mm, so it was necessary to continuously supply water for the planted trees to grow. This also created a new demand for water use. The trees that were watered did grow, but with a shortage of water, it was difficult to continue the watering efforts, so nearly all of the planted trees died.
There were herdsmen making use of the grasses growing along the river in the lower reaches of the river as well. Because people believed that there was no doubt that the livestock of these herdsmen were hindering the growth of woodlands along the river, ecological relocation policies were also implemented to relocate these herdsmen. These herdsmen were told to relocate to the nearby Ejina Oasis, and were told that they could no longer graze their animals along the river. This was the same situation as the aforementioned ecological relocation of the herdsmen from the mountain bases. The herdsmen who were relocated to the oasis had to develop new agricultural lands to cultivate feed for their animals, which resulted in increased water use through well water irrigation. More groundwater was being used than ever before, so naturally, the groundwater level dropped. Shallow wells quickly dried up. To make up for the shortage of water, more and more deep wells were dug from which water could still be drawn.
Considering the Causes of Water Shortages
Here, let us consider the causes of water shortages. Why is it that such water shortages have become a problem?
It is said that the world has been getting warmer recently. There are definite signs of global warming at the center of the Eurasian continent. A temperature increase of 0.5°C over the past 50 years has been observed in the oases in the Heihe River basin. Although there has been some increase in precipitation at the mountain bases, where oases are located, there has also been a tendency for decreased precipitation in the mountain areas. These mountain areas are the source of river water. So is the decreased precipitation in the mountains contributing to water shortages?
We have already discussed the importance of glaciers as a water source for the rivers flowing into an oasis. With the occurrence of climate warming, glaciers are gradually getting smaller. As the glaciers melt, more water is fed to the rivers, increasing the flow volume of the rivers. Observations of the July 1st Glacier in the Qilian Mountains reveal that the glacier has lost 10% of its volume in the last 28 years. The melting glacier means that more water is flowing into the river than the water resulting from precipitation alone.
Thus, the decreasing precipitation results in a decrease in the flow volume of water flowing from the mountains to the oasis, while the increase in glacial melting results in an increase in the flow volume, so these two factors counterbalance each other. In fact, for the Heihe River flowing from the Qilian Mountains, an examination of the flow volume of the river where it flows into Zhangye, its first oasis, reveals that the flow volume has been virtually unchanged for the past 50 years.
However, looking at the flow volume of the Heihe River as it flows from the Zhangye oasis further downstream, we see that the river has decreased to two-thirds its volume over the past 50 years. During this time, the amount of water taken from the Heihe River for irrigation has more than doubled. In other words, more water than ever before is being used by the oases, and this is known to be causing water shortages in the lower reaches of the river. For this reason, restrictions were placed on the intake of water from the Heihe River by the oases in order to increase the volume of water released downstream.
The reason that more water than ever before is being used by the oases is the rapid development of agricultural lands. Over the past 50 years, the area of land used for irrigated agriculture has approximately tripled. Although the area of agricultural land has tripled, the amount of water intake from the river has only slightly more than doubled. This may be attributable to the improvement and modernization of water channels that can transport the water more effectively, and to the increase in the number of farmers using well water instead of water from the Heihe River.
There has been a rapid increase in the use of well water. In just 20 years, from the 1980s to the 2000s, the volume of water pumped from wells has increased six fold. Those using these wells include farmers who are using well water to compensate for water shortages resulting from intake restrictions, and herdsman who have come to the oasis as a result of ecological relocation, and must rely on well water because they are not able to use water from the river. In addition to these factors, there are also the recent effects of the cash economy.
Long ago, the majority of crops grown in an oasis were food crops such as wheat and maize. These crops were vital to the production of the food needed to sustain human life in this region. Recently, however, there has been greater production of cotton and vegetables. This is because, with the development of distribution channels, these new crops can be sold to obtain money more easily than when growing food crops. When growing these cash crops, greater effort must be put into irrigation than when growing food crops.
In regard to the irrigation systems that draw water from the river, numerous conventions have been defined as to how much water can be supplied, and when and where the water can be supplied. Individual farmers cannot change the times when water is drawn, or how much is drawn, to suit their individual needs. However, by digging one's own well, a farmer can water the fields whenever, and however, he pleases. This is one reason for the increased use of well water.
Yellow Sands and Revival of a Lake
At the beginning of spring, the sky may appear hazy, with several yellowish bands. In Japan, this phenomenon occurs primarily in the western part of the country. This is known as the “yellow sands”. Yellow winds are caused by mineral particles that have been lifted into the sky by winds from the desert regions at the center of the Eurasian continent being carried by atmospheric flows. It has been reported that at times, these yellow winds can cross the Pacific to reach as far as Canada, Alaska, and Greenland.
In the early 2000s, abnormally heavy yellow winds flowed into Beijing and Korea for several years in a row. In Japan, even if yellow sands do reach the country, they do little more than make distant mountains look hazy. In China and Korea, however, the yellow winds can reach the severity of dust storms and can cause immense damage. Traffic accidents and traffic jams are often caused by the reduced visibility resulting from the yellow winds. There is also concern that the polluted air can adversely affect human health.
Because the Sogo Lake that was once located at the end of the Heihe River dried up in 1992, consideration was given to the possibility that the frequent dust storms occurring at the beginning of the 2000s were caused by the winds carrying mineral particles from the bottom of the dry lake bed in the lower reaches of the Heihe River. It was theorized that when the lake still existed, extremely fine mineral particles accumulated on the lake bed, and once these mineral particles were exposed to the atmosphere, they could easily be carried into the air. In order to reduce the occurrence of dust storms, the lakes would have to be revived. Therefore, efforts were made to create a water channel from the Heihe River to carry water to the location where the lake had disappeared. The channel created for lake revival was a modern channel with little water leakage. This project proved to be worthwhile. By 2003, Sogo Lake had been admirably revived, although it was still smaller than it had been in the passed. The beautiful lake scenery had been brought back to life.
But please think for a moment. The lower reaches of the Heihe River are suffering from water shortages. So, where did this water come from?
Here is what farmers from an oasis had to say on the matter.
“In the lower reaches of the river, we are already suffering a water shortage. Water intake is restricted so that more water can flow downstream, but this is a big problem for us. Despite our difficulties, downstream, the water is being fed into the lake. After the water accumulates in the lake, doesn’t it just evaporate into the air, making it all for naught? ”
They are exactly right. Even compared with other arid regions, the lower reaches of the Heihe River basin is an exceptionally dry area. Although the annual rainfall is 50 mm or less, when there is water on the surface of the land, more than 3,000 mm of water can evaporate in one year in this area. The creation of a lake thus means increasing the amount of water that evaporates into the atmosphere.
The same could be said for tree planting. The planted trees extend their roots into the ground, absorbing the water from the ground and then releasing the water from their leaves as water vapor into the atmosphere. In other words, trees can function like a pump, drawing up groundwater and dispersing it into the atmosphere. The fostering of a forest means that there will be increased consumption of groundwater by the forest.
In regions like Japan, where there is abundant rainfall and groundwater, groundwater depletion by forests is normally not a major problem. This is because in most cases, precipitation falling from the sky soon makes up for the decrease in groundwater. In arid regions, however, there is almost no precipitation, so the only thing that can be relied on to replace lost groundwater is the water seeping from the bottom and sides of rivers flowing nearby. As such, once a groundwater resource is depleted, it takes an extremely long time to recover. When planting a forest in this kind of place, it is necessary to thoroughly consider where the water will come from that will be needed to replenish the groundwater drawn up by the roots of the trees and then released into the atmosphere.
Where Do the Problems Lie?
The problem of a lack of water in the Heihe River basin is a serious one. In addition to the decline of vegetation and the depletion of well water in the lower reaches, water shortages have also hit the oases, as described earlier in the example of the Zhangye oasis. Water shortages are troubling the farmers who have long worked in agriculture, as well as the herdsmen who were moved to the oases for ecological relocation.
Although ecological relocation policies were implemented with the goal of resolving water shortages, they have actually led to an increase in the amount of water required to sustain the oases. This is because the herdsman had previously not needed irrigation water, but with relocation, they too have come to need new irrigation water in order to cultivate animal feed. The original farmers are also suffering water shortages because of restrictions on water intake from the Heihe River. Competing for water, both the herdsman and the farmers are digging wells and using the well water to make up for the shortage of water from the river. As a result, the levels of groundwater in oases have dropped rapidly. Shallow wells are no longer producing water. So, much money is being used to dig deep wells from which water can still be drawn, and that well water is what is now being used.
Based on test of the water drawn from deep wells, it is estimated that the water may be water from rain that fell in the Qilian Mountains several hundred years ago. That is, the groundwater from deep depths has been lying in the ground as groundwater for several hundred years. Put another way, we could say that if the groundwater is depleted, it will take several hundred years for this water resource to recover. However, in order to deal with the current water shortage problem, people have already begun to tap into this water resource.
The introduction of modern water channels with little leakage has also played a part in the depletion of groundwater. We have discussed how improvements to water channels have made it possible to efficiently carry water to the fields that are its destination. Here, “efficiently” means that by using new water channels in which the bottoms and sides are covered with vinyl or concrete, less water is lost as it travels through the water channels, so the amount of water does not decrease as it flows. The older water channels lost much of the water into the ground.
However, where did the water lost from the older channels go? It permeated into the ground to become groundwater. When water is no longer leaking from the water channels, the effect of this water adding to the groundwater is also lost. Water leaking from irrigation channels had served to replenish groundwater reserves that had decreased. Thus, when looking only at the visible water on the surface, one might think that the efficiency of water use has increased, but if the groundwater is also taken into consideration, we might not be able to say that efficiency has actually increased.
As discussed at the beginning of this paper, the arid region at the center of the Eurasian continent is an area with extremely strong sunlight. If it only had water, crops could be produced here in abundance. Perhaps at first, livelihood was sustained here by growing crops near the river, or by using spring water.
Eventually, people may have begun to consider that if they just had more water, they could grow more crops. With advancements in science and technology, it became possible to build water channels and create irrigation systems that could deliver water to a broad area. Thinking that even more water would make it possible to grow even more crops, water was drawn from further sources and broader areas were converted to agricultural lands. As this cycle repeated, eventually so much water was being used that river water was no longer reaching the lower reaches of the river.
People began to think that if there was not enough river water, they could dig wells and use the well water. So, many wells were dug. Well water proved to be convenient, and more and more was used. The groundwater gradually decreased, and shallow wells dried up. Digging deeper into the earth, it is possible to find more groundwater. It is this deep groundwater that the people are now starting to use.
If this deep groundwater disappears, the amount of time it will take for this water resource to recover is mind-boggling. In that sense, it may not be possible to consider deep groundwater as a sustainable water resource. As with other resources such as petroleum, it might be better to also regard deep groundwater as a resource that, once depleted, is finished for good. But now, people have begun to tap into this deep groundwater.
With the recent climate warming, glaciers are rapidly becoming smaller. Because of this higher rate of melting, more water is flowing from the mountains into the rivers than comes from the amount of precipitation alone. If this condition continues, however, it is thought that the glaciers will disappear completely in less than one hundred years. If the glaciers disappear, the amount of water flowing in the rivers from the mountains will rapidly decrease. We may indeed have to start to consider what can be done if that happens.
Conversely, the volume of water in the river will also decrease even if climate warming comes to an end and the climate turns colder. With a colder climate, the glaciers will increase in size, and the river water will decrease proportionately. We know that a decrease in river water due to climate cooling occurred from the 13th century to the 14th century.
We are now at a time at which we can surmise that the amount of river water currently flowing from the mountains will decrease in the future, regardless of whether climate warming continues, or the climate turns colder. When water runs out, we bring it from someplace else. When that is no longer sufficient, water is brought from yet another place, and when that runs out, we search for other places that still have water. But can we continue living like this?
（ Quoted from『子どもたちに語るこれからの地球』 講談社（2006））