Hydrology for the Water Management of Large Riva- Basins (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 201, 1991.
THE WATER BALANCE OF CHINA AND ITS LARGE RIVER BASINS
LIU GUOWEI AND GUI YUENG Nanjing Institute of Hydrology and Water Resources China
ABSTRACT The Yangtze River, Yellow River and other five large river basins are the largest ones in China, with a total area amount ing to about 4 333 687 km2 and covering both humid and arid/semi- arid regions. Based on the computation of atmospheric vapour transport, precipitation, évapotranspiration and runoff, water bal ance models for the whole country and its seven large river basins have already been developed. Through analyses with the models, some characteristics of hydrologie cycles in the river basins, includ ing the origins and routes of atmospheric moisture flux, the water circulation coefficients, etc., have been determined. The results provide a hydrologie basis for water resources assessment and management in China.
INTRODUCTION
China is located in the East Asian monsoon region, where the hydrologie cycle presents a monsoon climate regime. Every year in May, with the monsoon onset, the rainy season begins in the region south of 25 °N in China. During June to July, the rain band advances to the south of 35°N, and in the whole country the rainy season has developed by August. From November to March of the next year, it is a dry season, and there is a transient season from April to September. The whole country can be divided into three hydrologic-climatic zones: humid, semi-arid and arid zone. There are many rivers in China, including more than 1500 rivers with a catchment area larger than 1000 km2, among which the Yangtze River, the Yellow River, the Zhujiang River, the Hui River, the Hai River, the Liao river and the Songhua River are the largest. There are also many inland rivers in the arid region of west China (Department of Hydrology, Ministry of Water Resources P.R.C, 1987). For the study of the water balance, the river basins are divided into six groups as shown in Table 1, according to their locations and hydrologic-climatic characteristics. For example, the Yellow River, Hai River and Hui River are in North China. For convenience, the boundaries of the six areas and mainland China are approximated by meridians and latitudes (Fig. 1).
153 Lui Guowei & Cui Yifeng 154
Table 1. The Main River Basins of China,
Area Hydro-climatic Area Main River ltfhn2 characteristics
Northeast China Songhua River (I) Liao River Yalu River 124.6 semi-arid region South China Zhujing River (II) Qfantang River Min River 78.8 humid region Yangtze River Yangtze River 180.2 humid region
Southwestw> China Lanchang River (IV) Lujing River Yaluzangbu River 84.1 humid region North China Yellow River (V) Hai River Huai River 166.2 semi-arid region Northwest China Talimu River (VI) Eergisi River Ili River 342.3 arid region
Atmospheric vapour transport and water balance components for each area including atmospheric vapour budget, precipitation, évapotranspiration and runoff, are calculated. Based on the results of the computations, a water balance model for each area can be developed and some features of the hydrologie cycle for the river basins can be determined.
Fig. 1. The simplified boundaries of China and its large river basin. 155 Water balance and large river basins of China
ATMOSPHERIC VAPOUR TRANSPORT
The atmospheric vapour passing through a vertical section of unit length and height associated with change in atmospheric pressure from p„ to pz can be calculated by the following formula (Liu, 1985).
Q== g fpï J^ Uqdtdp (1) where, U is the wind velocity vector (m/s); q is the specific humidity (g/kg); g is 2 gravitational acceleration (m/s ); p„ and pz is the atmospheric pressure on ground surface and at an arbitrary height (hPJ; tj and tj are the beginning and end of a computational time interval. Introducing two averaging operations [ ] and { } ™=tr^ ^udt M=^ri^
{[U]}=P7^ *P!' [U]dp {[q]}=P^P7 tf M*? equation (1) can be written as
Q = | (p. - vu (h -1.) {[qu]} (2)
The physical quantities U and q or [U] and [q] can be broken down into:
U = [U] +U1, [U] = {[U]} + [UP (3)
q = [q] + q1, M = {[q]} + [ql* (4) where U1, q1, [U]*, and [q]* are the deviations from [U], [q], {[U]} and {[q]} respectively. Introducing equalities (3) and (4) into equation (2), we get:
£ {[Uq]} = £ {[U]}{[q]} + 1 [{U]*[q]*} + | {[U'q1]} (5)
(I) (II) (III) (IV)
With (I) denoting the total transport flux of atmospheric moisture, (II) the mean transport flux, and (III) and (IV) the eddy transport, the total transport flux of atmospheric moisture passing through an arbitrary vertical section equals the sum of mean transport and eddy transport (Liu, 1985). Applying equation (5), atmospheric sounding data collected at 150 meteorological stations where observations of wind, moisture and other météorologie elements are made at 0800 h and 2000 h (Beijing time) every day at Lui Guowei & Cui Yifeng 156
1000, 850, 700, 500, 200 and 100 hpa levels, the total transport, mean transport and eddy transport over 1983 (also in every month) were calculated. The spatial distributions of the atmospheric moisture fluxes are shown in Fig. 2 and Fig. 3. Fig. 2 shows that the routes of total transport are in the latitude direction, and that the atmospheric moisture enters China from the south and west and leaves China eastwards. In Fig. 3, the routes of eddy transport shows meridio- nality. The direction of eddy transport is similar to that of the moisture gradient. Atmospheric moisture is transported to the arid region in the northwest of the country from its southeast. This phenomenon is very important to maintain the humidity of the atmosphere over the inland region in northwest China.
CALCULATION OF THE COMPONENTS OF THE WATER BALANCE
The components of the water balance for an area, including the inflow of atmospheric moisture from its outer boundary I, the outflow of atmospheric moisture O, precipitation P, évapotranspiration E and runoff R, are shown in Fig. (4).
Inflow, outflow and netflow
According to equation (1), by integrating along the boundaries of the area, and by using the above mentioned data in 1983 from 150 météorologie stations, the amounts of atmospheric moisture of inflow and outflow, including the total transport, the mean transport and the eddy transport, have been calculated and listed in Table 2 where the sign of inflow is taken as positive and that of outflow negative, the netflow is taken as the difference between inflow and outflow.
Precipitation
Based on the data of ]precipitation observed at 13,000 stations, isohyetal maps for the country have been drawn, and by the isohyetal method the amounts of precipitation for each area have been calculated.
Evapotranspiration
Based on the data of evaporation observed at 1500 stations, isolines of evapor ation from water surfaces can be drawn. These evaporation values from open water surfaces were transformed into évapotranspiration values by using empiri cal formulas (Tan, 1984) for each area. Finally, évapotranspiration for each area was calculated similar to the isohyetal method. 157 Water balance and large river basins of China
Fig. 2. The spatial distribution of moisture fluxes by total transport (aver age over 1983).
Fig. 3. The spatial distribution of moisture fluxes by eddy transport (aver age over 1983). Lui Guowei & Cut Yifeng 158
/CTSW " t _ R Fig. 4. The scheme of water balance components.
Table 2. Atmospheric moisture budget (1983).
Term Area whole country (I) (II) (III) (TV) (V) (VI)
Total transport: Inflow (km'} 2806.1 10272.5 6955.3 4393.73654.4 3647.5 15308.3 Outflow (km3) 2651.0 9430.8 5816.4 3856.2 3496.7 3540.7 12362.7 Netflow (km'} 155.1 841.7 1138.9 537.5 157.7 106.8 2945.6 Eddy transport: Inflow (km') 472.7 983.6 879.5 265.8 980.8 716.8 1527.0 Outflow (km3) 330.5 1060.2 1394.8 229.6 783.9 528.8 1555.6 Netflow (km3) 142.2 -76.6 -515.3 36.2 196.9 188.0 -28.6 Mean transport: Inflow (km3) 2333.4 9288.9 6075.8 4127.9 2673.6 2930.7 13781.3 Outflow (km3) 2320.5 8370.6 4421.6 3626.62712.8 3011.9 10807.1 Netflow (km3) 12.9 918.3 1654.2 501.3 -39.2 -81.2 2974.2
Runoff
Based on the data of runoff observed at 47 basic hydrologie stations which cover 77.2% of the total area except northwest China, the runoff of the control area can be calculated directly. Runoff not controlled by those stations can be calculated according to the relationship between precipitation and runoff. The amounts of precipitation, évapotranspiration and runoff in 1983 are listed in Table 3.
WATER BALANCE MODEL AND ANALYSIS OF THE HYDROLOGIC CYCLE
The water balance equation for a defined area can be written as:
P - E - R + AS = 0 (6) 159 Water balance and large river basins of China while the atmospheric vapour balance equation can be written as:
N - P + E + AW = 0 (7)
Table 3. Amount of precipitation, évapotranspiration, and runoff (1983).
Term Area whole country (I) (II) (III) (IV) (V) (VI)
P (mm) 558.2 1851.6 1157.7 1023.8 538.0 159.6 671.5 E (mm) 417.2 836.8 587.6 546.9 541.7 152.8 405.0 R (mm) 124.3 1060.8 643.4 661.4 110.7 30.2 308.0
where, AS is the change in water storage in the area, AW is the variation of atmospheric moisture storage over the area. Over many years in the average, AS = AW = 0. The state in 1983 is near the average of many years, so from (6) and (7), a equation can be obtained:
N = R (8) where, N is taken as the difference between inflow and outflow of atmospheric moisture transport, i.e. N = I - O. According to Table (2) and (3), the water balance models for each area and the whole country can be developed as shown in Fig. 5. From the water balance as mentioned above, some characteristic coeffi cients of the water cycle can be calculated and are listed in Table 4.
Table 4. Characteristics of the water cycle (1983).
Term Area whole country (I) (II) (III) (TV) (V) (VI)
P (mm) 556.2 1871.2 1181.5 1114.5 543.7 164.7 679.2 E (mm) 431.6 803.0 540.9 475.4 433.6 133.6 369.8 I (mm) 2253.9 13036.2 3911.9 5224.4 2550.2 1062.9 1608.0 W (mm) 12.9 38.1 24.1 14.2 17.2 8.6 16.3 Pg (mm) 507.5 1814.9 1105.2 1066.5 501.1 154.9 609.1 Pe (mm) 48.6 56.3 76.3 48.0 42.6 9.8 70.1 Ke (l 279.4 I I 314.4 2028,9 1954.0 1 — „ .. — 0 2303.1 P E 2851. 155. 692.51 (537.4 155,1 4,4 47,3 1 R -• 417,51 0(413,2 /&Ç7? ( i ) Nortteast China •0 4531,4^110 LiiiL, 4953.1 10272.5 9430.! P 841.7 0_J 1474,5 632.1 841,7 150.4 5373.1 f I Oi 249,4 274,0 TTTTO? ==R (II) South Oiira 1071,910 1)312.2 1881.1 4575.5 — 0 5816.4 1138,9 0—| H-o 2193.71 1331.. 1133,9 8.2 147.6 ^=R 4833.9 H 01160.8 (1) Yangtze River Basin 612.410 1)82.3 ,ma — 0 2703.0 93.7 — 0 P E 3356,2 537.5 0_ 932,31 (399,8 537,5 1487.9 i I 0 171.4 117.5 *W77 = R as (BOSouttoest China — 0 1843.9^111 2897.2 3854.4 3483.7 ~0 157.7 0—I 779.11 (621.4 157.7 108.2 117.6 R 1223.0 n 01266,5 (V) North China 195.2)0 1)793.1 I — — 0 2565.5 2851.3 5347.5 3540.7 f— 0 P 103.1 — I 555.3 1453.5 106.1 26.5 35.0 253.9 II 0 f 467.7 AVAVA " ' t:R ( VI ) tërtfirest China 9,910 1)1205,9 — 0 10974,8 15308,3 12332,7 1—0 2945.6 3520.6 0 — 2945.6 463.1 7333,0(1 ÔT72872 7*TO^r = R (\|) tôle country Fig. 5. Estimated water balance of China and its large river basins. 161 Water balance and large river basins of China In this table W is the atmospheric moisture content (Liu, 1984); Pg is the precipitation formed by atmospheric vapour inflow from outside the area, Pg=P/(l+E/2I); Pe is the precipitation formed by local évapotranspiration, Pe=P/(l+2I/E); Ke=Pe/P is called local hydrologie circulation coefficient; Kg=P/Pg is the general hydrologie circulation coefficient; T is the period of hydrologie circulation. Table 4 shows that: the coefficient Ke <0.1 and Kg > 1.0 in all the areas, implying that the amount of precipitation Pe is less than 10 per cent of the total precipitation P, and 90 per cent of precipitation is formed by general hydrologie circulation. At the same time we find that (i) the proportion of Pe/P in humid regions, for example, in southwest and south China, is 2-3 times larger than that in arid/semi-arid regions, e.g, in north-west and north China, and that (ii) the duration of atmospheric moisture staying over the humid region is about 5 to 7 days, and over the arid/semi-arid region about 11-19 days. These results indicate that the hydrologie cycle in the humid region is active in comparison with that in the arid/semi-arid region. Table 2 shows different contributions of the mean and eddy transport atmospheric moisture in different area. In northeast and southwest China, they both contribute with a positive sign; in south China and in the Yangtze River Basin, the mean transport has positive contribution; in north and northwest China, the mean transport has negative contribution while the eddy transport has negative contribution. Of course, there are obvious seasonal variations both in atmospheric moisture transport and in the components of the water balance (Liu, 1989). It shows the effects of the monsoon climate on the hydrologie cycle in East Asia (Liu, 1990). CONCLUDING STATEMENTS China is a country with water resources shortage. Especially, the distribution of water resources over the country is seriously uneven in space and time. For example, water shortage often occurs in the Yellow River Basin and north China. These regions account for almost 50% of the total area of China, but the water resources are just 14% of the total amount. However, the Yangtze River and south China are rich in water resources, being 81% of the total amount of the country. For tackling the problem of the water shortage in north China, one of the most ambitious water resources projects, the South-North Water Transfer Project (SNWTP), has been under consideration in China for about 10 years. The project necessitates the transport of water from the Yangtze River to the Yellow River and North China, forming the largest water resources system in China. For evaluating the feasibility of the SNWTP, a lot of scientific problems have been formulated. For example, how will the water balance components, such as precipitation, évapotranspiration, runoff and ground water level, be changed? What are the impacts of the changes on the environment? All these scientific problems call for answers by hydrologists, as the water Lui Guowei & Cui Yifeng 162 balance models and hydrological cycle analysis constitute the important bases for them. REFERENCES Department of Hydrology, Ministry of Water Resources of P.R.C. (1978) Water Resources Assessment of China, Water Resources Electric Power Press, Beijing, 8-10 (in Chinese). Liu, (1985) Water vapour transport over mainland China, Journal of Hydraulic Engineering, No. 11, 1-13 (in Chinese). Liu, (1984) The time - space distribution of atmospheric moisture content over mainland China, Journal of Hydraulic Engineering, 5, 1-9 (in Chinese). UNESCO, (1978) World Water Balance and Water Resources, Unesco press, Paris, 71-73. Liu, (1990) Atmospheric moisture transport and water balance over China, Nanjing Institute of Hydrology and Water Resources Press, Nanjing, China, 13-15 (in Chinese). Tan (1984) An inspection of the formulae calculating the evaporation from land surface, ACTA Meteo- rologica SINICA. Vol. 42 No. 2.