|Long Distance Water Transfer: A Chinese Case Study and International Experiences (UNU, 1983)|
Xu Yuexian and Zhang Xingquan
The Experimental Research Centre of Yucheng
BASIC CONDITIONS OF THE EXPERIMENTAL AREA
THE YUCHENG experimental area, with a total area of 130 km², including 9,270 ha of farmland, was set up in 1966. The main purpose of establishing this area was to explore ways of controlling natural disasters such as drought, waterlogging and salinization of the soil through hydraulic, agronomic and forestry measures in order to provide a scientific basis for the comprehensive control of the Huang-Huai-Hai Plain. Inasmuch as 7,300 ha, 80 per cent of the farmland, was subject to varying levels of salinity, improvement of saline soil was the chief experimental research item. Initially, 330 experimental pump wells were dug and after two or three years of preliminary control the salinity was alleviated. Later, experimental research was discontinued for two or three years. Beginning in 1972, water was diverted from the Huang He for irrigation, inducing secondary salinization. The area of the saline land rose again to about 7,000 ha by 1974. In 1975 the experimental area again determined to rely on well irrigation and adopted comprehensive measures for control, combining well irrigation with drainage by open channels, land levelling, increasing soil fertility, and afforestation. This was quite effective. The area of saline soil had dropped to 2,100 ha by 1979, and the mean annual yield of grain increased from 1.65 t/ha in 1975 to 3.20 t/ha in 1979.
The Yucheng experimental area is situated in the alluvial plain of the lower reach of the Huang He, with an altitude of 21.5 m above sea level. The soil is mostly silt, light loam and medium loam. Climatically, it is in the semi-humid zone. Because of the influence of the East Asian monsoon, the winters and springs are dry while the summers are humid. The annual mean rainfall is 622 mm (from 1951 to 1979),364 mm (59 per cent) of which occurs in July and August. Wheat and maize are the chief crops and there are generally two harvests a year or three in two years. The mean annual depth of groundwater is 2.25 m, falling to 2.51 m from March to June. The mean mineralization of groundwater is 1.34 g/litre. All of these natural conditions are representative to a certain extent of those in the HuangHuai-Hai fluvial plain.
The Yucheng experimental area is surrounded on all sides by rivers. To the north is the Tuhai He, on which the Nanying storage gate has been built. The Old Zhaoniu He is to the east, the New Zhaoniu He to the west and the Shentun New He to the south. The Panzhuang main canal, transporting water from the Huang He, passes through the western part of the area (Figure 1). In terms of the relationship between water conservancy projects and the natural environment, Yucheng may be regarded as a microcosm of the Huang-Huai-Hai Plain after the proposed south-to-north water transfer. Hence, experimental research on improving the saline soil here has practical and theoretical significance for the problem of the salinization of the soil and its prevention in the areas to be irrigated by the water transfer.
Twenty fixed excavation sites were established for the collection of soil salinity samples. Since 1974 samples have been regularly collected in June and November from the six soil layers at a depth of 0 to 200 cm. Since April 1979, nine of the sites have taken one sample a month. There are 19 boreholes for observing the water table. Since 1973, one observation has been taken every 10 days. Observations began in 1978 on most of the other research items in the areas of hydrometeorology, agriculture and foresty.
VARIATIONS IN SOIL SALINITY OF THE EXPERIMENTAL AREAS
Seasonal Variation in Soil Salinity
Because of the influence of the monsoon climate, every year there are two periods when salts accumulate in the soil, and one desalination period. During the dry period of intense evaporation from March to June, salts accumulate in large amounts in the surface soil. During the rainy season from July to September, the water and salt move downward and the soil sheds salt for the main part. With the decrease in precipitation and increase in evaporation between September and November, water and salt move upward and salts accumulate in the surface soil again. Figure 2 shows the seasonal variation in soil salinity. It is clear that the key to improvement of the soil is to reduce its salinity during the two periods when salts accumulate.
Year-to-year Variation in Soil Salinity in June
Soil salinity was higher in June 1979 than in June 1974 at only two of the 14 collection sites; it was lower at the other 12 sites. Comparing June 1978 with 1974, salinity was higher at only one of 20 sites and lower at the remaining 19. During these four years the aggregate mean rate of desalination was 44 per cent, with an average loss of 3.3 metric tons of salt per hectare in the cultivated layer. The yearto-year tendency for June soil salinity to decline indicates that the improvement measures adopted at Yucheng can stably reduce soil salinity during the accumulating period in the spring.
Year-to-year Variation in Soil Salinity in November
The year-to-year variation in November soil salinity is complex and shows alternating accumulation and losses of salts. This indicates that the improvement of the saline soil in the experimental area is still somewhat limited in reducing salinity during the autumn accumulating period.
Vertical Variation in Soil Salinity
Soil salinity decreases with depth. In June, the salinity in the 0-10 cm layer averages 1.61 as much as that in the 150 to 200 cm layer. In November, the ratio is 1.38, showing a stronger return of salts in the surface soil during the spring than in autumn. In 1974 and 1978, however, the June ratio of the salinity in these two layers was 1.64 and 1.24 respectively, illustrating that amelioration measures had effectively controlled the return of salts to the surface soil during spring.
MEASURES FOR IMPROVING SALINE SOIL IN THE EXPERIMENTAL AREA
There are 1,050 pump wells in the experimental area, nearly one per 8 ha. There are about thirty deep wells of about 100 m depth and the others are mostly 50 to 60 m deep. The mean yield of each well is 60 to 80 m³/hr. In general, pump irrigation is concentrated from March to June and secondarily from October to November.
Pump wells serve to improve saline soil in three main ways: (i) the extraction of groundwater for irrigation keeps the salts down; (ii) the lowering of the water table decreases the evaporation of phreatic water and mitigates the return of salt to the soil; and (iii) the vacating of underground reservoir capacity increases the infiltration of rainfall which helps leach out salinity during the rainy season. Of the three functions, the lowering of the water table is the most important.
Open Channel Drainage System
The open channel drainage system includes a complete set of five levels of ditches. The 3 main ditches (the Old Zhaoniu He, Shinu He and Fengchan He) are 4.0 to 5.0 m deep. The 14 branch ditches have a total length of 16 km, are 3.0 to 3.5 m deep and about 2,000 m apart. The 80 subbranch ditches, with a total length of 135 km, are 2.5 to 3.0 m deep and 500 to 1,000 m apart. The 206 farm ditches total 155 km in length, are 1.5 to 2.5 m deep and are 500 m apart. There are 3,660 field ditches with a total length of 806 km, a depth of 1.0 to 1.5 m and a separation of 100 m.
These open ditches can drain the surface water and a part of the groundwater and with-them the salts which are dissolved therein. Draining the groundwater can lower the water table and reduce phreatic evaporation and salt accumulation. When the water level is reduced, rainfall can infiltrate deeper and strengthen the role of precipitation in leaching. The amount of drainage at the Wangzifu flow gauging site on the Fengchan He during July and August 1978 was 620,000 m³, with a mean water salinity of 0.65 per cent and a total removal of 409 tons of salt. The rainfall was 259 mm. The catchment area of the gauging site is 22 km², so an average of 186 kg/ha of salt was removed.
The drainage of salt through open channels is preconditioned on a certain amount of rainfall and runoff. Rainfall is concentrated in July and August storms. In other months rainfall is scarce and the flow ceases in the ditches. The relatively great year-to year variation in rainfall during July and August necessarily leads to different effects on the leaching of soil salinity and on the return of salts in the autumn. During the rainy season in a rich rainfall year there is extensive soil leaching and open ditches remove a large amount of salt, reducing the return of salt in the autumn, so that the salinity of the soil in November (S11) is always less than in June (S6) before the rainy season (e.g., 1974,1976 and 1977). On the contrary, when the rainfall in July and August is excessively small, there is relatively little leaching of the soil or draining of salts, so the salinity of the soil in November is always larger than before the rainy season (e.g., 1975, 1978 and 1979) (Figure 3). Thus there is an inverse relationship between the soil salinity in the autumn and the rainfall in July and August. This is the main factor in the wide year-to-year variation in November soil salinity.
Developing Soil Fertility
To change the inadequate soil fertility in the experimental area, the area planted to green manure has been extended to about 2,000 ha a year. The focus has been on increasing the use of organic fertilizer in conjunction with greater application of nitrogenous and phosphate fertilizer.
Developing soil fertility has changed the soil structure, increasing its porosity and moisture storage capacity. This has speeded up and extended the infiltration of surface water and the downward movement of water and salt while slowing and restricting groundwater evaporation. This has been an important measure in reducing the accumulation of salts in the surface soil and strengthening the effectiveness of improvements in water control (Yang et al, 1979).
After increasing the application of organic fertilizer and ploughing under sesbania at the Xiaosiliu and Wangzifu experimental sites, the organic content of the 200 cm soil layer increased from 0.7 per cent to 0.8-0.9 per cent, total nitrogen from 0.05 per cent to 0.6-0.7 per cent and effective phosphate from 7-8 ppm to 911 ppm. The volume weight of the soil decreased from 1.41-1.47 to 1.24-1.33 and the soil salinity from 0.53 per cent to 0.42 per cent.
In recent years, over 4 x 106 trees, 2.5 x 106 bushes and over 250 ha of fruit trees have been planted throughout Yucheng, forming a network of trees throughout most of the farmland. The rate of forest cover has increased markedly from less than 10 per cent to 18 per cent. From initial observations at Yucheng by the Shandong Provincial Forest Science Institute, the field tree network reduces the force of the wind by 10 to 20 per cent during the spring dry period. At the same time, it lowers the daily average temperature by 0.7 to 0.9°C, raises the relative humidity by 5 to 15 per cent and reduces evaporation from water surfaces by 10 to 18 per cent. In addition, the transpiration from the trees plays a biological drainage role and can lower the water table. The effect of a field tree network on the microclimate and groundwater shows that afforestation is a positive measure for improving saline soil.
Affected by the water cycle, water and salt in the soil are in a process of dynamic change. The increase or decrease in surface soil salinity in a given period is mainly determined by the direction of movement of water and salt, but the magnitude of change depends chiefly on the velocity of movement in a certain direction. Vertically, in general, the salts accumulate when upward movement (evaporation) predominates and are removed when downward movement (infiltration) rules. Horizontally, salts accumulate when water is supplied and are removed with drainage.
Hydraulic and engineering measures adopted by Yucheng, such as pump wells, open channel drainage and land levelling, have been able to change the direction of the water cycle in a portion of the area. The several links of pumping, irrigation and drainage are effective measures for improving saline soil because they increase the capacities of the downward movement of water and salt and of horizontal discharge, drop the water table and reduce the amount of water moving upward, for example, in the evaporation of phreatic water. Developing soil fertility and afforestation can improve the soil structure, increase soil porosity, improve the ecological environment of the farmland, decrease evaporation and reduce the pace of salt accumulation, so they are important measures for improving saline soil.
However, if water control projects make the water supply to a region increase greatly in a horizontal direction, causing the amount and speed of upward moving water to increase via a rise in the water table and heightened evaporation of phreatic water, it could have unfavourable effects on salinity control.
Yang Shouchun, Huang Zhaoyuan et al. 1979, "The function and effect of well irrigation and drainage or the combined use of wells and canals in improving saline soil", Proceedings of the Symposium on the Improvement of Saline Soil, Shandong Science and Technology Press, pp. 121-140.