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Chapter 18. Shallow groundwater resources of the Huang-Huai-Hai plain
Zhu Yanhua and Zheng Xianglin
Hydrogeological and Engineering Geological Institute, Ministry of Geology
THE MINISTRY OF GEOLOGY has carried out hydrogeological surveys of the Huang-Huai-Hai Plain at various scales and compiled the 1:1,000,000 "Hydrogeological Map Series" and the 1:500,000 and 1:200,000 regional "Comprehensive Geological Maps". Since 1979, extensive research has been carried out to reestimate shallow groundwater resources and explore the hydrogeological conditions for underground storage in conjunction with the planning of the south-to-north water transfer project. Here we summarize this research and discuss the current status of shallow groundwater extraction and the implications for interbasin water transfer.
SHALLOW GROUNDWATER RESOURCES IN THE HUANG-HUAI-HAI PLAIN
Principles and Methods for the Calculation of Shallow Groundwater Resources
Water is counted if it is in the phreatic-semiconfined aquifer within 50 m of the surface. Water in this aquifer is not deep, is easy to extract and is relatively abundant. At present it serves as the main source of water supply for industry and agriculture. No specific estimate has been made of the deep confined water resource, but it is quite limited.
The present calculations are not those of the recharge at the present water table depth but of the natural recharge when the water table has fallen to 4 m.
The estimation of annual average integrated amount of recharge conforms to a greater degree to the law of natural formation of shallow groundwater resources.
Shallow groundwater is subdivided into three quality grades: fresh (with a mineralization <2 g/1), brackish (2 to 5 g/1) and saline (>5 g/1).
Analysis is focused on the process of shallow groundwater formation and the principal factors involved therein, based on the hydrogeological conditions of the Huang-Huai-Hai Plain. The magnitude and distribution of precipitation are held to be the basic variables involved in shallow groundwater formation, although the amount of seepage recharge is directly and strictly controlled by the soil characteristics of the unsaturated zone, the strata structure and physicochemical properties.
Therefore we began by compiling a precipitation isogram for the region based on the 20 to 40 year precipitation statistical series of the meteorological departments and combined this with composite soil maps of the unsaturated zone (0-4 m deep) based on a large amount of borehole stratigraphic data (Figure 1). For calculating purposes the region was divided into 277 districts according to the distribution of precipitation and soil characteristics and in conjunction with regional geological, geomorphic and hydrogeological conditions. For each of these subdistricts, balance equations were set up based on the recharge and consumption terms involved in the process of shallow groundwater formation. These equations were used to calculate the combined long-term annual recharge of the shallow groundwater in each subdistrict. The results were then converted into moduli of recharge and the region was subdivided accordingly. The results are shown in Figure 2.
Determination of estimated parameters
(a) The gravity specific yield (m). In extensive regional areas, the gravity specific yield ? is commonly calculated by methods such as field Iysimeter measurements with changing water heads, unsteady flow pumping test estimation and converse reckoning of groundwater level dynamic data. In areas with no shallow groundwater extraction or no runoff effect the decline in the groundwater level is caused directly by phreatic evaporation. Hence specific yields are found by using groundwater level dynamic curves and the empirical formula
h = groundwater buried depth (m),
e0= surface water evaporation intensity (mm),
D0= extreme critical depth of groundwater evaporation (m),
n = empirical coefficient, usually set equal to 1.
Formula (1) indicates that when the phreatic water level is near the surface, the intensity of phreatic evaporation is equivalent toe0; but as groundwater buried depth increases, evaporation intensity decreases, reaching zero at a certain depth D0. Within a time interval t, phreatic evaporation should be equivalent to the amount of water released due to the fall in the water table:
Figure 1.Sketch-map of the Lithology (0-4 m depth) and Precipitation of the Huang-Huai-Hai Plain
Figure 2. Sketch-map of the Shallow Aquifer Recharge Sources in the Huang-Huai-Hai Plain
If two time intervals t1 and t2 are taken from the same water level dynamic curve, then
The gravity specific yield ? can be obtained by solving these simultaneous equations.
(b) The precipitation percolation coefficient (a). In our calculations, we only consider the effects of precipitation, the Ethology and structure of the unsaturated zone and the groundwater buried depth. Most parts of the region use water level dynamic data for estimation. Some areas have used field Iysimeters to make actual observations. One or two localities have already done artificial simulations of rainfall infiltration. Under the same lithological conditions, the value of a is higher in areas of stronger groundwater extraction intensity. Therefore the resulting a tends to be too small in areas with shallow groundwater level buried depth and the water table is held to 4 m in calculations. We have made adjustments in a for the whole region according to the relationships discussed above between the magnitude of a, extraction intensity and water level buried depth.
Irrigation quotas and Regression Coefficients
The irrigation quota varies from place to place because of differences in precipitation, the water requirements of dry land and paddy and the level of irrigation technology. The irrigation quotas of the different provinces and municipalities are set at the following:
Beijing, Hebei and Tianjin: 5,250 m³/ha/annum Henan and
Shandong: 4,500 m³/ha/annum
Jiangsu and Anhui:
dry land: 2,550-3,600 m³/ha/annum
paddy fields: 7,200-9,000 m³/ha/annum
The irrigation regression coefficients are as follows:
Shandong, Henan, Jiangsu and Anhui: 0.1
The Regional Distribution of Shallow Groundwater Resources
Our calculations indicate that there are 56.7 km³/annum of shallow groundwater in the Huang-Huai-Hai Plain, of which 47.6 km³/annum is fresh, 5.4 km³/annum is brackish and 3.7 km³/annum is saline.
Figure 2 shows clearly the uneven regional distribution of shallow groundwater. The most abundant region is the Yan Shan piedmont alluvial fan (especially the Beijing alluvial fan), where the groundwater recharge modulus is 300,000 to 400,000 m³/annum per km². Next are the Taihang Shan piedmont alluvial fan, the Taiyi piedmont alluvial fan, the Huang He alluvial fan and the ancient channels in the Huang He flood plain, where the recharge modulus is 200,000 to 300,000 m³/annum/km². Most deficient are the plain north of the Huai He, the lower reaches of the Huang He and the interfluvial zone, where the recharge modulus is generally 100,000 to 200,000 m³/annum/ km², falling in some districts to 50,000 to 100,000 m³/annum/km².
One prominent feature of this distribution pattern is that when the surface stratum is composed of coarse particles with high permeability, the underlying aquifers have a large storage capacity and there is also a large combined recharge of the shallow groundwater, as the distribution patterns of precipitation and the water abundance of the aquifers coincide. When the surface stratum is composed of fine particles with poor permeability and the underlying aquifers have a small storage capacity, the magnitude of shallow groundwater recharge is controlled chiefly by the soil characteristics and structure of the composite stratum, and the distribution pattern of recharge coincides with that of the soil characteristics of the unsaturated zone.
This feature fully explains why the aquifers in some areas have good water abundance but have a small natural recharge while others have poor water abundance but a relatively large natural recharge. Hence the way to correctly evaluate the water abundance of a rock formation is to study the natural groundwater recharge, for this is a leading factor in calculating the yield of a liftpumping project, estimating a drawback value for extracting groundwater and evaluating the water abundance of a rock formation.
THE STATUS OF SHALLOW GROUNDWATER EXTRACTION IN THE HUANG-HUAI-HAI PLAIN
In recent years, relatively more shallow groundwater has been extracted in the central and upper areas of the piedmont alluvial fans. As a result, the water table in these areas and regions has slowly but progressively dropped year after year. Small and medium-sized cones of depression have been formed in the shallow aquifer in extraction areas of some cities where industry and mining are concentrated or in parts of the low-lying areas between fans. The scope of these cones is quite small, constituting only about 5 per cent of the area where shallow fresh water is distributed. In the vast areas of the central and eastern plains, groundwater depth in general may have reached a long-run equilibrium at 2 to 4 m or even less (see Figure 3). In some irrigation districts which use diverted water there has even been some elevation in the water table.
According to the statistics of the relevant water conservancy departments, total shallow groundwater extraction in the Huang-Huai-Hai Plain was 27 km³ in 1978, only 52 per cent of our estimate of the freshwater resource. Data for 19741978 from the observation station in Hebei Province show an average annual shallow (fresh) groundwater extraction for the province's plains area of about 7.8 km³. Present calculations indicate that there is still a potential of 2.0 km³ of shallow groundwater which can be extracted. Henan Province extracted 9.3 km³ from its shallow aquifers in 1979 and the water table remained relatively stable in over 60 per cent of the areas compared with the 1978 low-flow season. The Beijing area, which has the heaviest rate of extraction, takes about 2.5 km³ each year, but according to our estimate of 2.8 km³ there is still some remaining.
Shallow groundwater resources are far from abundant in Cangzhou and Hengshui prefectures in Hebei Province, where the recharge modulus is generally 150,000 to 200,000 m³/annum, ranging up to 200,000 to 250,000 m³/annum in the relatively bountiful ancient river beds. Statistics from the two prefectures for 1974 (an average year) indicate extraction moduli of 67,000 and 70,000 m³/annum/km² respectively; in 1975, these figures were 94,000 and 105,000 m³/annum/km²,only 1/2 to 1/3 of the recharge. Thus there is still a huge potential for development.
At present, because of the inadequate knowledge of the parties concerned, problems have cropped up in some areas. Instead of conducting thoroughgoing, detailed research on the matter, these areas have blindly pursued the exploitation of the confined water in the deep aquifers. This is expensive and not very productive, and has formed numerous cones of water head depression in these aquifers. In addition, problems of engineering geology such as surface subsidence have appeared. This situation urgently needs to be changed by controlling the extraction of the deep confined water and by advocating the full utilization of shallow groundwater.
The waste of water resources is quite serious. Some places irrigate as much as twelve times a year and over 7,500 m³/ha may be applied annually to dryland crops. Some places still irrigate by flooding the fields with large and wasteful amounts of water. There is enormous potential for scientific and economical water usage in the Huang-Huai-Hai Plain. For example, Shandong and Henan presently divert approximately 9.4 km³/annum from the Huang He, but 4 km³ of this could be saved if flood irrigation were replaced by small border check irrigation. Water use could be reduced by about 50 per cent in places with the appropriate conditions by developing sprinkler and drip irrigation and adopting advanced irrigating techniques. Experiments show that if wheat is watered four to six times during its growing period, the total annual water use is 3,750 to 4,500 m³/ha and grain yield may reach 3.75 to7.5 t/ha. It is clear that grain output can be increased using less water if irrigation norms are worked out and advanced irrigating techniques are applied.
Figure 3. Sketch-map of Depth (in metres) of the first group of Aquifers in the Huang-Huai-Hai Plain
At present there is no unified organization to direct the management of water resources. The general level of management is low and regions and departments are not coordinated with each other. For example, because irrigation expenses are quite low, sometimes without water fees being collected at all, in some places with canal irrigation it is rarely possible to make thorough use of the groundwater by combining well and canal irrigation. Consequently, the water table rises and secondary salinization of the soil occurs.
The utilization and transformation of the extensive saline water resources of the Huang-Huai-Hai Plain have not yet received sufficient attention by the departments concerned, and these resources are rarely exploited. Only a very limited number of organizational units irrigate with brackish water (2 to 3 g/1 or 3 to 5 g/1) during certain growing periods. In many places, industry uses high-quality freshwater and, basically, saline water has yet to be used in China for industrial cooling or circulation. By comparison, 25 per cent of industrial water in Japan and virtually all of the cooling water used in its electric power industry comes from the ocean, as does 31.5 per cent of the cooling water in its chemical industry. Numerous oil fields in the United States use saline water injection to extract petroleum effectively. A large amount of freshwater could be saved in China if we can increase the use of saline water by the industrial sectors.
HYDROGEOLOGICAL PROBLEMS ALONG THE EAST ROUTE
There are 4.3 x 106 ha of cultivated land in the irrigated area to be serviced in the proposed south-to-north water transfer project plans. Of this, 2.0 x 106 ha are north of the Huang He. Irrigation is concentrated in the period from March to June while rainfall and runoff are concentrated in the three-month flood season from July to September. Therefore there must be adequate storage sites in order to regulate the seasonal dynamic water balance and to guarantee the full effectiveness of irrigation during the water-use period. At the same time, the continuous increase in industrial and agricultural water requirements means that the imbalance in supply and requirements of water resources is increasingly felt. These factors make it necessary to develop underground storage.
In 1979 the "Hydrogeological Map of Groundwater Storage" was compiled based on long-term hydrogeological data. The scope of underground water storage areas was determined, water types delineated and the amount of storage estimated, producing a relatively systematic study of the hydrogeological conditions of the groundwater stored in the plains. Seven types of stored groundwater were differentiated based on the soil composition, buried conditions and size of the spatial capacity of the storing strata, and 74 sections were delineated where 9.84 km³/annum may be stored. Table 1 summarizes this information.
Forty-five of these sections are north of the Huang He, with a storage of 6.7 km³/annum but most of these are located in the sloping plains of the Taihang and Yan piedmonts, the alluvial fans of the Huang He and the adjacent ancient river channel zones. Within the irrigated area which would be serviced by the East Route scheme, there are only 17 sections north of the Huang He where water can be stored, and these are quite small in area, with a total storage capacity very roughly estimated at about 0.50 km³/annum (see Table 2). The number of sites suitable for storage is even less if we take into account specific problems of groundwater storage sources, recharge facilities and economic benefits. The proposal for northward diversion via the East Route should treat this matter with the utmost caution.
The shallow groundwater in the central portion of the Huang-Huai-Hai Plain is not very deep, usually 2 to 4 m or less during the low flow period. A large amount of groundwater is consumed in evaporation, leading to large areas of saline soil. According to project plans, 15 km³/annum would be delivered by the East Route to areas north of the Huang He. Of this, 2.0 km³ would be delivered to Tianjin to handle the water supply problem in that municipality and another portion would be consumed in evaporation from water surfaces and absorption into the unsaturated zone. The remaining water would recharge the groundwater and could cause the water table in the vast majority of irrigated districts to rise by 1 to 2 m. If shallow groundwater extraction is maintained at the current level, the water table would approach the surface and subsurface evaporation would increase in order to maintain a condition of equilibrium. This would intensify the pace of salt accumulation in the topsoil layer (the layer of root activity) and produce secondary salinization of the soil over broad areas.
The general direction of shallow groundwater runoff in the Huang-Huai-Hai Plain is from west to east. If the water table were to be raised in the irrigated areas serviced by the East Route, both the runoff conditions and the direction of flow of the groundwater would be obstructed and altered by static pressure conduction. This would aggravate the development of soil salinization in portions of the unbenefited areas to the west because the groundwater recharge would gradually raise the shallow water table.
Table 1. Groundwater Storage in the Huang-Huai-Hai Plain
|North of the Huang He||South of the Huang He|
Table 2. Groundwater Storage Sections in the Huang-Huai Hai Plain North of the Huang He
ENHANCE SHALLOW GROUNDWATER EXTRACTION TO PROVIDE OVERALL CONTROL OF THE HUANG-HUAI-HAI PLAIN
The following are prominent features of the Huang-Huai-Hai Plain:
(1) In the aggregate, precipitation and stream runoff cannot
be considered small for a semihumid region but their strong
seasonality leads to frequent spring droughts and summer
(2) Secondary salinization of the soil occurs over extensive areas of the eastern portion of the plain, notably in the downstream region of the Heilonggang where the land is level with numerous flat, saucer-shaped depressions and a shallow water table (2 m or less), high soil salt content and impeded surface and subsurface runoff. These factors combine to cause the vertical discharge of the shallow groundwater to exceed the recharge into it.
(3) Because of the impact of continental salinization and seawater intrusion, extensive saline water bodies are scattered throughout the middle and eastern plain with a total surface area of about 50,000 km² (24,000 km² north of the Huang He). Of this area, 24,000 km² is brackish and 26,000 km² is saline water. Very little use has been made of the latter. At the same time, locations with saline water bodies tend to have high water tables, aggravating the dangers of soil salinization, waterlogging and flooding.
Thus the problem in the Huang-Huai-Hai Plain is not simply one of "drought" but of coexistent and interrelated drought, flooding, alkalization and salinization. Yet the shallow groundwater is not yet exploited fully, there is enormous waste in water resource utilization and management is poor. Therefore the problems of drought, flooding, alkalization and salinization should be tackled in a comprehensive fashion on the basis of enhancing the extraction of shallow groundwater, developing local water resources and using water economically and scientifically. If the water table is lowered through extraction, subsurface evaporation will be reduced and recharge will be increased, relieving the dangers of salinization and flooding as well as drought.
Research on shallow groundwater resources is an integral component of hydrogeological work. A prerequisite to investigating groundwater formation is to correctly establish concepts of groundwater resources and methods for classifying them. The estimate of groundwater resources included herein can only provide a rough numerical base for industrial location, regional planning in agriculture and the comparison of south-to-north water transfer schemes.
Although the shallow groundwater resources of the Huang-Huai-Hai Plain are far from bountiful, a tremendous potential remains for their exploitation which can satisfy the water requirements of agriculture and industry in the near future.
The only way for an interbasin water transfer (such as the massive south-tonorth project) to proceed safely is on the basis of first acquiring knowledge of natural conditions and their transformation as well as observing the objective laws of economic development.
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