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Chapter 15. Water balance in the water transfer region

Liu Changming and Liu Enbao
Institute of Geography, Academia Sinica, Beijing, China

THE FOLLOWING is an analysis of the water balance in the proposed water transfer region of eastern China and the four major river basins associated with it (Chang Jiang, Huai He, Huang He and Hai-Luan He).

WATER BALANCE IN THE FOUR MAJOR RIVER BASINS

The total volume of water is abundant in the four basins. Precipitation is 2,697 km³, or 44 per cent of the national total of 6,170 km³. Total runoff is 1,110 km³, 43 per cent of the national total of 2,600 km³ (see Table 1).

Only in the Chang Jiang basin is annual runoff greater than evaporation. In the three other basins, annual evaporation far exceeds runoff and the major mode of water removal is evaporation. Runoff is less than 22 per cent of precipitation in all three.

The quantity and distribution of water varies widely between the four basins. For example, the annual runoff of the Chang Jiang alone accounts for 88 per cent of the total. As shown in Table 2, the annual runoff in the Huang, Huai and Hai basins is much smaller than that of the Chang Jiang not only in the aggregate but also per unit area.

From the standpoint of unified planning of water resources use in the four river basins, it would be natural to solve the problem of water deficiency in the Huang, Huai and Hai basins by supplying water from the rich Chang Jiang. It would seem that no doubt could be cast on the idea of rationally redistributing water regionally through interbasin transfer, but the level of water deficiency and the appropriate scale and rationality of diversion are key topics which need further study.

SPATIAL AND TEMPORAL DISTRIBUTION OF WATER-BALANCE ELEMENTS

Areal Distribution of Water-balance Elements

The previous section merely reflected the general state of the balance elements delineated according to basins. Here we present in map form a more detailed distribution of these elements in the east China water transfer region, using data from up to 1970 (Hydrogeological Editing Group, 1981). The principal features may be summarized as follows:

Table 1. Water Balance in the Four Basins

Table 2 Runoff as a Fraction of Chang Jiang Runoff

Annual precipitation. Average annual precipitation in the region is 743 mm. Mountainous areas average 724 mm and the plains, 760 mm. In general, precipitation decreases gradually from 1200 to 1400 mm along the Chang Jiang in the south to roughly 600 mm in areas north of the Huang He.

Annual runoff. Average annual runoff in the region is 169 mm. Runoff is markedly higher in the mountainous areas (averaging 220 mm) than in the plains (123 mm). The distribution of runoff coincides roughly with that of precipitation (see Figure 2 in Wei and Zhao, Chapter 7 of this volume).

Figure 1. Three Hydrological Elements

Annual evaporation. Annual evaporation averages 574 mm over the region, 637 mm in the plains and 504 mm in the mountains. There is little variation in the region's evaporation, which lies between 700 mm in the south and 500 mm in the north (see Figure 1).

Annual runoff coefficient. The annual runoff coefficient is higher in the mountains than in the plains and greater in the south than in the north. In the plains it is generally between 0.1 and 0.2 (see Figure 2).

Annual renewal of soil moisture. The areal distribution of this factor is more uniform than that of precipitation or runoff. The Huaibei region north of the Huai He and most areas to the north of it are encompassed by the 600 mm isoline (see Figure 3).

Figure 2. Annual Runoff Coefficient

It is obvious that the water balance elements are affected by the terrain. Figure 4 (a) and (b), synthesizing some research data, presents curves correlating different water balance elements. These clearly reveal differences in the relationship between precipitation and runoff in three kinds of terrain: mountains, hills and plains.

Figure 3. Annual Renewal of Soil Moisture Storage

Figure 4. (a) Regionalization of Water Balance

Figure 4. (b) Correlation Curves of Water Balance Elements for Water Transfer Regions

Characteristics of Water-balance Elements in the Main Provinces and Autonomous Region

Administrative regions are rarely drawn up on the basis of drainage basins. This makes it difficult to calculate provincial water balances. We have used our isograms to make some estimates of the water balance elements in a number of provinces, including Hebei (incorporating Beijing and Tianjin municipalities), Henan, Shandong, Anhui and Jiangsu. The results are listed in Table 3.

It should be pointed out that regional water use is generally planned along provincial boundaries. A provincial water balance therefore makes the calculation of water supply and requirement balances more convenient.

Temporal Distribution of Water Balance Elements

The North China Plain has one of the largest year-to-year variations in precipitation in the country. For example, precipitation reached 1,040.4 mm in 1954 in Gucheng County, Hebei Province, but the next year it was only about onequarter that, or 282.5 mm. Annual runoff variation in the Hai-Luan and Huai river basins is the highest in China, with a coefficient of variation ranging between 0.6 and 0.8.

Three observations should be made here regarding the coefficient of variation for runoff, CV_R:

1. CV_R is correlated with the coefficients of variation for precipitation, CV_P, and evaporation, CV_E; the runoff coefficient, C_r; and the correlation coefficient between evaporation and precipitation, rep.
2. CV_P declines with area.
3. Therefore, CV_R also tends to decline with area. This is evident in Table 4.

The rate of guaranteed water supply could therefore be increased by combining the utilization of river water over a larger area. This is particularly the case in the North China Plain, where the CVR for small- and medium-sized rivers is quite large, generally 1.00 to 1.45, with a very low annual rate of guarantee.

In addition, we have made a synchronous analysis of year-to-year changes in runoff in four major rivers. Our data are presented in Table 5, which shows that abundant, average or deficient water years have occurred simultaneously in all four rivers only five times in forty-seven years. Of these, four were average years and one was abundant. In no years was there a deficiency in all four basins. Annual runoff is thus basically nonsynchronous in the region's major rivers.

We have also drawn up some simulated annual runoff differential mass curves which are presented in Figure 5. These also make it readily apparent that there is a lack of synchronization which would be beneficial to interbasin water transfer.

Table 3 Hydrological Balance in the Main South-to-North Water Transfer Provinces and Autonomous Region

Table 4 Characteristic Values of Annual Average Runoff at Representative Stations, 1951-1976

Table 5 Runoff in Four Main Rivers

Key: +: water abundant (P<25%);
-: water deficient (P>75%);
0: average (25%<P<75%)

Figure 5. Differential Mass Curves of the Four Basins

Annual variations in evaporation are seldom studied. Since the percentage of precipitation which evaporates is over 80 per cent in the Huang, Huai and Hai basins, we may safely assume that CV_E is actually very close to CV_P.

There is quite a large seasonal variation in water balance elements on the North China Plain. Precipitation in March, April and May only accounts for 10 per cent of the annual total while evaporation is intense and evaporativity may exceed 40 per cent of the year's total. At this time precipitation cannot form surface runoff to recharge the streams. Some plains rivers even cease flowing during the spring. In the summer (June, July and August) waves of thunderstorms provide 60 to 70 per cent of the year's precipitation and runoff is nearly half the annual total. Since the summer runoff often appears in the form of flood waters, it is difficult to utilize it in agriculture.

SOME PROBLEMS IN BALANCING WATER SUPPLY AND REQUIREMENTS

Water Supply Analysis

The supply of water depends upon its sources, including surface and groundwater and, for agriculture, soil moisture (atmospheric precipitation may of course be considered the ultimate water source). Because the various types of water resources, both surface and subsurface, are continuously moving in the water cycle, the migrations and transformations of water in different spaces are extremely complex. In the North China Plain, in addition to the difficulty of determining the interchange of surface and groundwater, the recharge of plains surface and groundwater by water from the mountains make the calculation of water sources even more complex.

For the exploitation of shallow groundwater, the use of regionalized precipitation seepage coefficients permits a relatively good estimation of groundwater recharge. According to investigations in various parts of the North China Plain by the Ministry of Geology, the average precipitation seepage coefficient is about 0.22. Using this and the annual precipitation for the regions of 270.705 km³, we estimate shallow groundwater at 59.555 km³.

The amount of recharge from the groundwater into the rivers is relatively small in the North China Plain, amounting to 10 per cent or less of river runoff. Synthesizing data from a wide area, we have chosen 8 per cent to obtain surface runoff into the rivers:
Rs = R-Rg = (1-.08) R= (.92) 43.961 = 40.444 km³,
a depth of 114 mm. Most of the lakes in the plain lie south of the Huang He and are principally fed by surface runoff, so they are not recounted in our calculations of surface water. The above statistics indicate that total water resources on the 360,000 km² North China Plain amount to 100 km³, of which 60 per cent is shallow groundwater.

Water Requirements Analysis

In general, water requirements are determined by the level of socioeconomic development. In the North China Plain, about 65 per cent of the area, or 22 x 10⁶ ha, is under cultivation. This is one-fifth of China's total cultivated land. The plain has abundant heat resources, level land and a high population density. Its industry is well developed and it is one of China's most important grain and cotton areas. Further development of its agriculture is of profound significance to China's agricultural modernization. Nevertheless, the plain suffers frequently from the natural disasters of drought, flooding and salinization. In particular, the spring drought is extremely serious. According to local statistics, spring drought occurs in Henan Province nearly every year (P > 0.97). Drought conditions in Hebei Province are even more severe.

Inasmuch as about 80 per cent of water use is in agriculture, the main goal of water transfer from south to north is to supply agriculture. Agricultural water use is related to crop water requirements. According to Cheng Weixin (see Chapter 19), these requirements vary from 850 to 1,000 mm/annum in the North China Plain. This is about 90 to 240 mm greater than precipitation.

Water Balance and Ecological Equilibrium

Human utilization of water resources must affect the water cycle, which in turn leads to changes in the environmental ecosystem. If we only pay attention to the contradictions between supply and requirements, our use of water resources will harm the environment. Figure 6 presents a simplified view of the relationships between the water balances and the ecosystem.

Figure 6. Relationships Between the Balance of Supply and Requirements for Water Resources and the Environmental Ecosystem

The evaluation of water supply and requirements for agriculture should be combined with geographical research, i.e. the principles of the water/heat balance in geographical zones must be considered. Concretely, a region's moisture conditions (including soil moisture) must correspond with its heat conditions.

Figure 7 is an isogram of D=P - E₀, where E₀ is potential evaporation. D is distributed as follows: the D=0 isoline stretches from the northern part of the Huai He basin through the vicinity of Xuzhou to the Shandong peninsula. From here to the Huang He, the value of D lies roughly between 0 and -300 mm/annum. In the plains in the lower reaches of the Hai and Luan rivers north of the Huang He, D is always less than -300 mm/annum and in the most water deficient Heilonggang district in Hebei Province, D is less than -400 mm/annum. Based on a multiyear average, the water deficiency on the North China Plain may be calculated at about 20 km³/annum.

Figure 7. Water Balance

As mentioned previously, however, the annual variation in precipitation is quite great on the North China Plain. The absolute value of D is commonly more than three times the average during a dry year. From the viewpoint of drought prevention, the scale of a water transfer project cannot be designed on the basis of the average values of D.

The seasonal distribution of water deficiency is also extremely uneven. The largest monthly values of D are in the months of April, May and June. For example, during the dry year of 1972, D reached -328 mm for these three months alone at Shijiazhuang and was -650 mm for the entire year. The annual average for D is nearly zero at Xuzhou, but in the dry year of 1966, the value of D was about 200 mm for April-June and -350 mm for the entire year. The water deficiencies for these two cities were 6,450 m³/ha and 3,450 m³/ha respectively. Obviously, this calculation only applies to drought years. From the viewpoint of average year conditions, these values are excessive and would make water diversion uneconomical. A rational approach would be to increase regulation storage to reduce the amount which needs to be transferred. With full regulation storage, an average of 3,450 m³/ha would have to be diverted according to the annual D value.

CONCLUSIONS

The distribution of water balance elements in the four basins is extremely uneven. The annual runoff in the Chang Jiang is 8.5 times the total runoff in the Huang, Huai and Hai basins.

Because of the uneven temporal distribution of the water balance elements, the water requirements of the North China Plain should be analyzed in terms of the deficiency in dry years and dry seasons. The evaluation and calculation of water supply and requirements should be grounded on an analysis of the water balance and the environmental ecosystem. In principle any increase in soil moisture should correspond to potential evaporation so as to be coordinated with the comprehensive control of drought, excess surface water and salinization.

Although the North China Plain is an important agricultural area, the structure of its water balance is not conducive to farming. Over 80 per cent of the region's precipitation is consumed in evaporation. Surface water resources are meagre, limiting the development of the region's agriculture. Even industrial and municipal water supply cannot be guaranteed. Therefore, redistributing a certain amount of supplementary water to the North China Plain is desirable and worthy of study.

Reference

Hydrogeological Editing Group, Editorial Committee for China's Physical Geography, Chinese Academy of Sciences, 1981, Surface Wafers of China. Science Press, Beijing.

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