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Chapter 24. The atmospheric moisture balance in the proposed water transfer region
Institute of Geography, Academia Sinica, Beijing
Wu Yonglian, Zhang Changduo and Qin Geng
Department of Geography, Beijing Normal University, Beijing
IN THIS CHAPTER, meteorological data from May end July, 1969-1973, are used to make some rough estimates of atmospheric water vapour content and transport, the moisture cycle and the possible change in precipitation in the region which would be affected by China's proposed south-to-north water transfer project.
CHARACTERISTICS OF THE WATER VAPOUR CONTENT
In May the depth of precipitable water in an air column from the surface to 200 mb is 10 to 60 mm in east China, decreasing from south to north. It is 20 to 60 mm to the south of 35°N and 10 to 20 mm to the north (Wu and Shen, 1980). Figure 1 depicts the situation in May 1969, a wet year in north China, particularly in the Bo Hai bay region east of the bend in the Huang He (Hetao). In the water transfer region, the depth of precipitable water declines from 25 to 45 mm in the Chang Jiang and Huai He basins to 10 to 25 mm in the Huang He and Hai He basins.
Although there is little annual variation in the depth of precipitable water in May, it does tend to be greater in a wet year such as 1969 than in a dry year such as 1971, especially in north China. As shown in Figure 2, the difference can reach 20 to 30 per cent in north China while it is 5 to 20 per cent in the Chang Jiang basin in the south.
With the change in season from spring (May) to summer (July), the atmospheric water vapour content increases rapidly. The depth of precipitable water may increase to about 25 to 65 mm in July in east China. There is still a general decline from south to north, but with a large centre forming in the middle and lower reaches of the Chang Jiang where the value is at a maximum.
Figure 3 shows the situation in July 1969. The depth of precipitable water has increased to 45 to 60 mm in the Chang Jiang and Huai He basins and to 25 to 45 mm in areas to the east of the bend in the Huang He, including the north China plain. To the north of 35°N the value increases most considerably over May 1969, a point which we will need to consider when we estimate the water balance.
There is also a distinct annual variation in July of atmospheric water vapour content. This may be seen from Figure 4 which compares July 1969, a wet year in the Chang Jiang Basin, with July 1971, a dry year. The difference between the values for the depth of precipitable water is negative to the north of34°N and positive to the south. South of the Chang Jiang, the water vapour content was 15 per cent larger in July 1969 than in July 1971, while there was a decrease of 5 to 10 per cent in the Huang He and Hai He basins.
Figure 1. Atmospheric Moisture Content (surface to 200mb), May 1969 (mm)
Figure 2. Difference in Moisture Content between May 1969 and May 1971, Expressed as a Percentage of the Latter
Figure 3. Atmospheric Moisture Content (surface to 200mb), July 1969 (mm)
Figure 4. Difference in Moisture Content between July 1969 and July 1971, Expressed as a Percentage of the Latter
CHARACTERISTICS OF WATER VAPOUR TRANSPORT
The total water vapour transport expresses the direction and magnitude of atmospheric water vapour. It is an indispensable element in calculating a water balance and is one of the main branches in the moisture cycle. If we are to correctly calculate the atmospheric water balance and estimate the changes in the distribution of moisture which may arise from large-scale human activities such as the south-to-north water transfer, we must understand the characteristics of total water vapour transport and the changes in those characteristics.
In May there are two main branches of vapour currents over the eastern Chinese mainland. One branch is from the northwest and carries little water vapour while the other is from the south and bears a relatively rich amount of moisture (Shen and Zheng, 1963). Although there is little change in China's basic vapour currents from year to year, there is considerable variation in their strength.
Figure 5 presents the total water vapour transport in May of a wet year, 1969, showing the two currents. The moisture borne by the southern branch forms a transport zone south of the Chang Jiang with over 1,000g/cm. see, reaching a maximum in excess of 3,000 g/cm. sec along the southeast coast. The northwest current bears more than 1,200 g/cm. see, with a maximum value in the vicinity of the Bo Hail
These distributional forms remain essentially the same from year to year but their magnitudes change greatly. Figure 6 shows difference of over 20 per cent between May 1969 and May 1971 everywhere except for small areas around Changsha, Nanchang and Shanghai south of the Chang Jiang. The increase was over 60 per cent in the vicinity of 35°N.
In July there are three branches of vapour currents in east China (Shen and Zheng, 1963). In the north there is one from the northwest and in the south there are two, from the southwest and from the southeast. As in May, the main year-toyear change is in strength of the current, especially the southern branch.
The southwest current was exceptionally strong in the Chang Jiang basin during the wet July of 1969 forming a total water transport zone extending from south of the Chang Jiang all the way northeast to Japan, with a maximum transport centre south of the Chang Jiang greater than 4,000 g/cm. sec. The subtropical high shrank towards the ocean so that the influence of the southwest current. Total water transport values were 500 to 2,000 g/cm. see, north of 35°N and 2,000 to 4,000 g/cm. sec to the south (Figure 7).
Figure 5. Transport of Atmospheric Moisture (surface to 200mb) in direction of arrows, in g/cm. see, May 1969
Figure 6. Difference in Transport of Atmospheric Moisture between May 1969 and May 1971, Expressed as a Percentage of the Latter
In July 1971, a dry year in the Chang Jiang basin, the subtropical high was very strong and stretched all the way into China's interior (Shen and Zhu, 1980). In this year the southeast current was strong and the southwest current retreated this year the southeast current was strong and the southwest current retreated to the west. Water vapour transport fell considerably in areas south of the Chang Jiang, with a minimum value less than 1,000 g/cm. sec (Figure 8).
Figure 9 compares the depth of precipitable water in July for 1969 and 1971. The variation between the two years is obvious. Roughly speaking, the area to the south of the Chang Jiang shows positive values, indicating that the magnitude of water vapour transport in 1971 was lower than in 1969, and the area to the north of the Chang Jiang shows negative values, reflective of the higher level of water vapour transport in 1971. In both cases the percentage difference reaches over 50 per cent of the 1971 value. In the northern water transfer region annual variation in July values can reach 25 to 50 per cent. It is necessary to take this variation into account in calculating the water balance.
MOISTURE CYCLE IN THE WATER TRANSFER REGION
As everyone knows, the precipitation of a given area has two water vapour components, that which comes from elsewhere and that which is formed from local evaporation. The exogenous precipitation is caused by the exogenous cycle and the endogenous precipitation is caused by the endogenous cycle. What is the contribution of each? There are two opposing points of view about the role of the endogenous moisture cycle in the formation of precipitation. One view feels that its effect is quite large (Shipchinski, 1952; Rutkovski, 1952) and the other feels that its influence is negligible compared with that of the exogenous cycle (Budyko and Drozdov, 1953; Pogoshyan, 1952).
In this chapter we have used the formula of Budyko and Drozdov to explore the effects of exogenous and endogenous cycles on the formation of precipitation in area 1 (33 to 40°N, 115 to 120°E) and area II (33 to 40°N, 115 to 120°E):
In this formula, the ratio of precipitation formed by exogenous water vapour (r) and the precipitation formed by endogenous evaporation (R-r) (where R is total precipitation) is equal to the ratio of vapour transported into a given closed district from outside (Fi) to the water vapour from endogenous evaporation which participates in the endogenous cycle (L).
Figure 7. Transport of Atmospheric Moisture (surface to 200 mb) in direction of arrows, in g/cm. see, July 1969
Figure 8. Transport of Atmospheric Moisture (surface to 200 mb) in direction of arrows, m g/cm. see, July 1971
Figure 9. Difference in Transport of Atmospheric Moisture between July 1969 and July 1971, Expressed as a Percentage of the Latter
In using this formula, the values of R in the closed regions are obtained from an isohyet map drawn up from the monthly precipitation data of 400 stations. Evaporation (Er)is derived by multiplying the empirical formula for the north China plain E0 = 0.19(20+t)2(1-h) (see Cheng Weixin, Chapter 19) by 0.7 (Zhu and Yang, June 1955) (t=mean monthly temperature; h = mean monthly relative humidity).
To obtain endogenous and exogenous cycles from the Budyko-Drozdov formula, we use L = 1/2 Er (Zheng and Shen, 1959). The results are presented in Tables 1 and 2. From these tables we can see that both in area I and in area II the percentage of precipitation formed by endogenously evaported vapour is smaller than that which stems from exogenous vapour, and is larger in area I (the more extensive area) than in area II. According to a five-year mean (1969-1473), the ratio of endogenously formed to total precipitation is 25.1 per cent in area I and 11.4 per cent in area II. In every individual area, these percentages increase with the area covered.
There is also a very large annual change in the endogenous cycle. In a relatively dry year (e.g., rainfall in May 1971 in area I was 28.5 mm) the percentage of total precipitation which is endogenous is greater than in a wet year (e.g., in May 1969, when the precipitation was 49.5 mm in area I). In both areas I and II, this percentage is about twice as high as in a wet year.
In addition, the effect of the endogenous cycle is generally greater when the amount of water vapour transport from elsewhere is smaller. Tables 1 and 2 show this to have been the case in 1972 when the amount of vapour transport was the smallest of the five years but the endogenous cycle had the greatest effect, involving 33.3 per cent of total precipitation in area I and 14.8 per cent in area II. This is also reflected by the cycle coefficient K.
ESTIMATING THE EFFECT OF SOUTH-TO-NORTH WATER TRANSFER ON PRECIPITATION
In this section we estimate the possible effect of water transfer on precipitation during May. According to the project design, it is estimated that when the south-to-north transfer project is fully completed, a maximum of about 2.68 km, would be transferred along the East Route in May and about 2.6 km³ would be carried by the Middle Route. In addition, preliminary estimates indicate that about 1.5 km³ of water stored during the winter could be used for irrigation along the East Route and 1.0 km³ could be applied along the Middle Route. An additional 1.0 to 1.5 km-l could be added by raising the present rate of utilization of mountain runoff and groundwater during May. This yields a total increase of approximately 9.0 km' in water north of 35° N. An average of 14 mm would be added to area 1. If half this enters the water cycle, L would increase by 7mm. In area 11 there would be an increment of 29 mm and L would go up by 14.5 mm.
Table 1 Elements of the Atmospheric Water Cycle During May in Area I of North China (33 to 40°N, 110 to 120°E). Units = mm
|Water vapour entering into Endogenous cycle||Water vapour entering into exogenous cycle||Ratio of endogenous to total precipitation||Ratio of exogenous to total precipitation||Cycle coefficient|
|Date||R - r||r||R||ET||L||Fi||(R- r)/R||r/R||K = R/r|
|5 year mean
Table 2 Elements of the Atmospheric Water Cycle During May in area II of North China (33 to 40°N, 115 to 120°E). Units = mm
|Item||Endogenous precipitation||Exogenous precipitation||Total precipitation||Evapo- transpiration||Water vapour entering into Endogenous cycle||Water vapour entering into exagenous cycle||Ratio of endogenous to total precipitation||Ratio of exogenous to total precipitation||Cycle coefficient|
|Date||R - r||r||R||ET||L||Fi,||(R - r)/R||r/R||K = R/r|
|5 year mean
Equation (1) indicates that little change in precipitation is likely to be induced by the proposed water transfer, with an average increment of 3 per cent in May. This figure does not vary greatly with area but there are year-to-year changes depending on climatic conditions. The range is roughly 2 to 4 per cent (Table 3). Of course, this conclusion does not include changes in precipitation caused by factors such as increases in localized evaporation, increases in relative humidity and decreases in the condensation level.
Table 3. Possible Increase in Precipitation After South-to-North Water Transfer
|Area I||Area II|
|5-year mean (1969-1973)||2.8||2.9|
From the above discussion we have reached the following conclusions:
(1) There are large year-to-year variations in depth of
precipitable water and water vapour transport value. Both figures
are larger in May in a wet year (1969) than in a dry year (1971).
The difference is especially marked in north China. The
year-to-year variation is also quite pronounced in July, when
both figures were larger in the wetter 1969 south of the Chang
Jiang but smaller to the north of that river.
(2) On the average, 25 per cent of precipitation in area I is formed by water vapour from local evaporation. The figure for area II is 11 per cent, as this percentage declines with smaller regions. The remainder of precipitation comes from outside the area.
(3) There is year-to-year variation in the effect of the endogenous cycle which causes a higher percentage of total precipitation in a dry year than in a wet year.
(4) Whether we look at area I or area II, the south-to-north water transfer would not have a large effect on precipitation during May in regions to the north of 35° N. The average increase would be about 3 per cent with a range of 2 to 4 per cent depending on the weather.
Budyko, M.I. and Drozdov, O.A., 1953, "The Laws of the Water Cycle in the Atmosphere", Report of the USSR Academy of Sciences, Vol. 90, No. 2, New Series, pp. 167- 170.
Pogoshyan, K.I.,19S2, "Water Cycle Patterns in the Atmosphere", Proceedings of the USSR Academy of Sciences, Geographical Series, Vol.5, pp.40-57.
Rutkovski, V.I., 1952, "On the Inland Atmospheric Water Cycle", Problems of Geography, Vol. 28, pp. 156- 167.
Shen Jianzhu and Zheng Sizhong, 1963, "The Transport of Atmospheric Water Vapour over Continental China", Collected Papers of the Institute of Geography, No. 6, Science Press, Beijing, pp. 85-110.
Shen Jianzhu and Zhu Zhihui, 1980, "The Relationship Between the Strength of the Southwest Monsoon and Precipitation in the Chang Jiang Basin", Proceedings of Symposium on Tropical Weather, Science Press, Beijing.
Shipchinski, A.V., 1952, "On the Endogenous Water Cycle", Proceedings of the USSR Academy of Sciences, Geographical Series, Vol.6, pp. 60-70.
Wu Yonglian and Shen Jianzhu, 1980, "The Characteristics of (Atmospheric) Moisture during the spring in North China and the relationship with Precipitation", Beijing Normal University Journal of Natural. Science, Nos. 3-4.
Zheng Sizhong and Shen Jianzhu, 1959, "The Atmospheric Water Cycle in the Chang Jiang Basin", Acta Meteorologica Sinica, Vol. 25, No. 5, pp. 346-355.
Zhu Gangkun and Yang Renzhang, June 1955, "The Use of Meteorological Records in Economic Construction (II): A Study of Evaporation Distribution in China", Acta Meteorologica Sinica, Vol. 26, Nos. 1-2, pp.1-24
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