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.