The core projects in the loess plateau involve the construction of key dams to retain flood runoffs. While the dams will trap sediment and prevent its accumulation in downstream channel, it also stores the surface runoffs from their respective watersheds. Some government planners are fearful that an extensive implementation of such projects will reduce the annual flow of Yellow River, and thus deprive the downstream sites their precious water resources. Such a notion is totally misguided (see below), though unfortunately it seems to play a role in many decisions. [Indeed, the reservoirs created by the core projects are substitutes for small lakes and ponds, which normally belong to an integral part of any topography, but are totally absent in the loess plateau.]
In the loess plateau nearly half of the percipitation comes during the flood season. The remaining half of the precipitation comes down in minor rainfalls and does not constitute significant runoffs for downstream use. The flood runoff contains a heavy sediment load, and by itself it not enough to transport the fine(-grained) portion of the sediment out to sea. In reality, by retaining the sediment locally, a substantial part of the river flow normally wasted in flushing sediment is salvaged. It is in excess of the surface runoff retained by the reservoirs, and thus creating a new source of useful water.
The changing pattern of the water budget for the Yellow River basin can be seen in the following set of data. On average the river receives an input of surface water from all tributaries in the basin by an estimated amount of 58 billion cu m annually. Out of this, in 1980, 27.1 billion cu m was diverted for irrigation and other uses, but with water consumption increasing at the rate of 4.5% each year, by 1987, water diverted from the river shot up to 37.6 billion cu m. Underground-water extraction also increased from 6.4 billion cu m to 15.9 billion cu m during the same time periods. With an estimated underground-water reserve of 19 billion cu m, it is being rapidly depleted.
All this time about 32 billion cu m of water flowed out to sea bringing with it the river sediment. The average anuual sediment load is estimated to be 1.35 billion tons (which is somewhat less than the long-term average of 1.6 billion tons, largely because precipitation in the past 15-20 years had been below average, and this value is consistent with the surface water volume given above). It is estimated that a minimum of 20-24 billion cu m of the annual flow is needed to accomplish the sediment-flushing process. About 1/4 of the sediment, mostly coarse-grained, is left behind accumulated in the downstream channel, while 3/4 of it (mostly fine-grained) is flushed.
Currently, the river is also being asked to supply in addition 4.1 billion cu m of water to cities like Tienjin and Qingdao, and to energy-resource bases in Shanxi and Inner Mongolia, which lie outside the river basin. Annual water shortage by Year 2000 is placed in the range 8.5 - 16 billion cu m.
Clearly, if the sediment load in the river is reduced by half, 10 billion cu m of water presently wasted on sediment flushing can be put to better use. (Note that the expensive Central Water Diversion Project is designed to serve just two cities, Beijing and Tienjin, providing each with 4 billion cu m of water annually. Its cost is estimated conservatively to be around 25 billion yuan.)
The present basinwide water shortage can be partially relieved by creating new water resources through reducing the river's sedment load. (For some inexplicable reason, this very simple deduction had never been proposed until very recently when a member of the Institute of Geography, Academy of Sciences, Jia Shao-feng, submitted an article in the obscure journal Ke Ji Dao Bao, No. 9, 1994, entitled "A substitute proposal for the North-South Water Diversion - water and soil conservation in the middle Yellow River valley." I
only read an abridged version of the article in the journal People's Yellow River, No. 3, 1995.)
Water is being extracted from the river regardless whether or not new resources are available. If the sediment load is not reduced, there will not be enough flow in the main channel to flush the sediment out to sea. Sediment accumulation in the main channel will increase, further worsening the flood discharge capability of the river, and leading possibly to future disasters.
When I attended the workshop on "China's Water Resource Strategy in the 21st Century," on Oct 17-19, 1995 in Beijing, organized by the China Center of Advanced Science and Technology under the Academy of Sciences, I heard talks on numerous water saving proposals-from introducing non-dripping faucets to drip irrigation, and on water diversions south-to-north and agriculture-to-industry, but I heard no mention of applying sediment reduction to salvage water currently wasted on sediment flushing. I made a request to the participants, many of whom represented the Ministry of Water Resources, to add sediment reduction to the laundry-list of possible means to generate new water resources so that people will look into its feasibility, whether or not they believed in its viability. My plea was politely ignored. I actually came out of that workshop feeling rather depressed.
Picture source: YRCC, Soil & Water Conservation in the Huanghe River Valley, Shanghai Educational Pub House, 1988.
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