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9. The origin and transformation of wetland on the south-eastern slopes of the
greater Khingan range (Da Xingan Ling)

Chen Yongzong,Li Baoging,Ren Hongzun,and Xie
Youyu
Institute of Geography, Chinese Academy of Sciences, Beijing

Abstract

Most of the wetland on the southern slopes of the Greater Khingan Range (Da Xingan Ling), in Heilongjiang Province, has not been developed. The wetland is characterized by swampmeadows and meadows. The study area on the south-eastem slopes of the range has had a periglacial environment since the late Pleistocene, when broad valleys and narrow streams were formed. which contributed to wetland development. Both biological and engineering measures can be taken to drain and reclaim the wetland. Reclamation has caused changes in hydrological conditions. temperature, and soil structure and fertility.

In this chapter the term "wetland" includes swamps, swampmeadows, and meadows. Widely distributed in south-eastern Inner Mongolia and Heilongjiang Province, most of the wetland has not been developed. The wetland on the south-eastern slopes of the Greater Khingan Range (Da Xingan Ling), in Heilongjiang Province, is characterized by swamp-meadows and meadows, and occurs mainly in the valleys. It is flat in landform, rich in water resources, and has thick and fertile soils. Reclamation of the wetland could not only raise the level of agricultural production of the region but also contribute to the development of forestry and animal husbandry.

Fig. 9.1 . Location of the Study Area

TABLE 9.1. Temperature, Precipitation, and Evaporation Characteristics of the South-eastern Slope of the Greater Khingan Range, Heilongjiang Province

Location Temperature (T), Precipitation (P), and Evaporation (E) Month Annual mean Data period
Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
Butha Qi 1909-1944
T(C) - 19.7 - 15.1 - 6.4 4.2 12.2 18.3 20.8 19.3 12.1 2.4 -6.4 - 17.7 2.0 1952-1972
P(mm) 3.0 3.5 5.0 15.9 30.5 775 157.0 114.8 66.8 17.5 5.8 2.3 499.6 1952-1972
E(mm) 17.7 31.4 75.1 153.6 232.1 208.6 181.9 174.0 131.8 93.8 40.1 18.7 1,358.6 1952-1972
Arun Qi
T(C) - 20.5 - 16.3 - 6.1 3.1 12.5 18.4 20.7 18.6 11.8 2.6 - 9.3 -18.2 1.4 1957-1972
P(mm) 1.7 1.8 4.5 14.2 31.7 52.1 156.1 121.1 49.6 13.7 2.6 2.2 451.8 1957-1972
E(mm) 12.1 24.6 78.4 182.0 277.7 262.9 190.0 155.4 130.1 96.0 35.3 11.9 1,456.4 1957-1970
Jalaid Qi
T(C) -174 -14.1 -14.1 6.3 14.6 20.1 22.4 20.6 14.1 4.7 -6.6 -15.4 3.8 1959-1972
P(mm) 1.6 1.6 4.6 11.3 21.5 51.6 148.3 97.9 56.0 16.2 3.1 1.7 415.4 1959-1972
E(mm) 17.3 36.7 95.1 216.7 307.9 288.0 225.6 180.1 157.6 119.7 43.2 16.5 1,704.4 1959-1970
Xuguit Qi                            
T(C) -25.0 -22.1 -133 -09 8.1 137 16.1 14.1 7.0 -1.6 -13.7 -22.0 -3.3 1956-1970
P(mm) 3.6 31 4.7 18.8 32.9 686 1524 114.4 49.3 13.1 5.2 3.8 469.9 1956 1970
E(mm) 7.1 15.4 47.3 104.8 183.4 181.9 131.0 117.7 93.1 64.9 21.8 75 975.9 1956 1965
Morin Dawa Daurze Zizhiqi
T(C) -22.0 -20.3- 9.8 23 9.4 15.6 19.3 17.1 9.9 0.7 -12.1 -22.5 -1.0 1957-1970
P(mm) 3.6 3.7 5.8 13.5 39.4 66.8 136.3 137.4 57.4 14.8 4.1 5.5 488.3 1957-1970
E(mm) 0.9 18.5 53.0 125.8 211.9 343.4 153.4 127.3 101.2 73.8 23.7 16.2 1,257.0 1957-1966

This chapter discusses the origin and evolution of the wetland, in the Greater Khingan Range, the measures to be taken for its reclamation, as well as the changes in moisture, heat, and soilfertility conditions after reclamation in the Butha, Arun, and Jalaid areas on the south-eastern slopes of the range (fig. 9.1).

Origin and Evolution of the Wetland

The origin of the wetland in the study area is related mainly to climatic, hydrological and geomorphological conditions. The wetland possesses the following common features: accumulation of surface water (or excessive soil moisture content), apparent gleying in the soil profile, and a thick layer of intertwined roots.

The study area lies north of 46N, and is located between the Pacific Ocean in the east and the hinterland of Eurasia in the west. Owing to the influence of the south-east monsoon in summer, and the Mongolian and Siberian high-pressure system in winter, plus the orographic effect of the Greater Khingan Range itself, the region experiences a short, warm, and rainy summer and a long, dry winter. The temperature. precipitation and evaporation characteristics of the study area are shown in table 9.1. The mean annual temperature is about 0C, and five to seven months have an average temperature < 0C. The soil is frozen for 150-210 days. and the seasonally frozen earth is over two metres in depth. Island-like permafrost occurs in the northern part of the mountains. Because the wetland soil is frozen for so long, microorganic activities are retarded and only partial decomposition of plant residues occurs. It is also difficult for root-systems to penetrate deeply, hence a layer of intertwined roots has developed. In this region, the mean annual rainfall ranges from 415.4 to 499.6 mm, and the mean annual evaporation is between 975.9 mm and 1704.4 mm. Over 67 per cent of the rainfall is concentrated in July, August, and September. Since most of the precipitation is from rainstorms, more than 80 per cent of the annual stream discharge occurs during the period July-October, and the valleys are frequently flooded. The geomorphic features of the region do not favour drainage, thus facilitating the development of wetland.

TABLE 9.2 Relationships between Discharge Capacity of Unmodified Rivers and Peak Flood Discharge

River Station Discharge capacity
of natural channel
(m/sec)
Flood discharge
P = 20% P = 5%
Dasuerchi Leitianxang 2-3 67.0 129.0
Shaligou Sunjiatun 2-3 49.7 510.0
Handahan Taipinzhang 7.9 244.0 310.0
Erlontou He Bayianggoule 9.3 220.0 440.0
Tumen He Fengtun 13.9 190.0 310.0
Harzhulu He Lujiazi 4.5 94.0 196.0

Source. Report of the Wetland Group. Heilongjiang Land Resources Survey Team, 1975.

One geomorphic feature of the Greater Khingan Range is that the broad flood-plains in the valleys have narrow river channels, the width of which is normally only 1.6-0.3 per cent that of the floodplain. The flood-plains are over 400-500 m wide, even if the drainage basins are only 100 km in size. On the basis of the fossil data of the late-pleistocene groups of rhinoceroses and elephants from Heilongjiang Province and eastern Inner Mongolia,1 on the spore-pollen analyses of Picea and Abies, and on the palaeo-periglacial geomorphology of the region, it is clear that during the late Pleistocene the Greater Khingan Range had a periglacial environment, which has continued to the present. Consequently, the broad valleys and narrow streams typical of periglacial valleys and which provide suitable sites for wetland development were formed. Under such circumstances, the large amount of clastic rocks produced by freeze-and-thaw weathering and the thin layer of loess-like soil accumulated on the slopes became the sources of the sub-clay and clay in the wetland. The infiltration capacity of the clay and sub-clay is only 7-25 mm/day, which contributes directly to the excessive water of the wetland.

The meandering river channels in the broad valleys have a dissection depth of only 1-2 m, and the coefficient of sinuosity is mostly over 1.5. The longitudinal gradients of the rivers range between 0.2 and 0.5 per cent, and the coefficient of roughness is 0.04 0.05. The relationship between channeldraining capacity and peak flood discharge of different frequencies (table 9.2) indicates that the present channels are incapable of discharging large and medium flood-waters, thus causing inundation of the valleys. Both longitudinal and latitudinal gradients of the valley flood-plains are small, and dense aquatic foliage develops during the flood season, thereby impeding water flow. Flood-water overflowing the river banks can move only slowly on the plains, where it may remain several days or even weeks in a single flood period. The freezing season of this region begins in October/November and lasts untie the end of March/April. Condensation during the freezing period, the impermeable layer of the frozen soil below the surface during the early thawing period, as well as the shallow groundwater in the river valleys, are all factors responsible for the waterlogging of the wetland.

Three layers of loose sediments generally exist in the valleys below the south-eastern slopes of the Greater Khingan Range (fig.9.2). The lower part is 10-20 m thick and consists of yellowbrown sub-clay interbedded with rock fragments. The middle part, composed of fluvial sand and gravels, is 3-10 m thick. The upper part, 1 -2 m thick, contains dark-grey to black and yellowishbrown sub-clay and clay. Above the upper part is a layer of intertwined roots 0.1-0.3 m deep. X-ray-diffraction analysis of clay minerals and spore-pollen analysis of samples collected from subclay and clay strata of the upper part indicate that hygrophilous plants of herbaceous Cyperaceae predominate whereas aquatic swamp plants are deficient. This indicates a gradual change from a fluvial depositional environment characterized by sand and gravels to the present wet-meadow and grassland environment. The clay minerals are characterized by hydro-mica, which indicates that they were formed during a cool period and

later were subjected to weak weathering. C14 dating by the Geography Department of Beijing University of samples of darkgrey and black sub-clay from a depth of 1.23 m in wetland on Zhonghe People's Commune of Butha Qi, indicates that the subclay is 7,540 + 170 years old. The sample is comparable in age (early Middle Holocene) to the large areas of black soil in this region, the age of which has been confirmed by archaeological data.2 It is also comparable to the age of Holocene strata peat from Pulandian. Liaodong Peninsula, Liaoning Province.3. Above the Pulandian Holocene peat strata are grey and black sub-clay strata containing palaeo-lotus seeds. These strata of 1,040 + 210 to 2,000 years B. P. are comparable to the layer of intertwined roots of the wetland of the region, and to the age of peat (the upper part being 2,000-2,500 years old) in the vicinity of Khabarovsk, USSR, investigated by Neishtadt.4 Therefore, the wetland of the Greater Khingan Range began to develop in the middle Holocene (about 7,000 years B.P.), but its more active period of development has been in the last 2,500 years (since late Holocene times), with peak development during the so-called "Little Ice Age, " during the thirteenth to nineteenth centuries. In recent years the wetland has been drying up as a result of cyclical climatic fluctuations characterized by alterating cold and warm as well as dry and wet periods. The impact of human activities has also contributed to the drying process.

Major Measures for the Transformation of the Wetland

Excessive moisture, inadequate soil maturity and low ground temperature are the three disadvantages of wetland from the perspective of agricultural use. Excessive moisture is the key problem, from which the other two stem. Since moisture derives mainly from storm- and flood-waters, improvement in the rainstorm and flood-discharge capacity of the wetland is the principal method by which it will be transformed. Both biological and engineering measures can be taken to drain the wetlands On the south-eastern slope of the Greater Khingan Range engineering measures should be taken.

Two such measures basic to wetland improvement are flood control and the drainage of waterlogged fields. The former requires dredging the existing channels, broadening and deepening channel beds, increasing the flow capacity of the channels, and building dikes along the rivers and reservoirs in the upper valleys. Drainage measures include mainly digging ditches in the wetland and increasing its vertical infiltration capacity. The shortterm goal of channel improvement should be the prevention of the major floods that occur once each decade, and the long-term objective should be to prevent those which occur only once every 20 years. The draining of one day's storm-water in two days, at a rate of 1.5 I/sec/ha, for those rainstorms that occur once every five years, should be the target for draining waterlogged wetlands. If an improved river is used as the main channel to drain water from floods that are recurrent every five years, water levels in the main channel should not flood the branch channels, which would result in the prolonged submergence of the crops in the cultivated wetland.

Engineering systems for flood control and drainage of the valley wetland include the main drainage channel and its dikes, drainage ditches and their branches, upstream reservoirs, as well as ditches to prevent the water from flowing off the slopes and onto the cultivated land.

The moisture condition of the wetland is positively related to the groundwater table (fig. 9.3). To lower the groundwater level and to reduce the soil moisture content of the wetland, the designed depths of the main drainage channel and its branches should be greater than the depths of the clay and sub-clay strata. Normally 2 m is suitable. These transformation measures and the engineering design standards suggested for the wetland have been proved effective.

Fig. 9.2. Profile of Quaternary Deposits in Dasuerchi He

Changes in the Wetland after Reclamation

Hydrological Changes

Prior to reclamation, the wetland in the river valleys served as a floodwater retention area. This function disappeared after reclamation, which, in turn, has altered the water balance of the drainage area. Water balance in a closed drainage system may be expressed by:

Y=X-ZDW (1)

where Y= runoff (mm)
X= precipitation (mm)
Z = evaporation (mm)
DW= water storage of the drainage basin.

The impact of reclamation on precipitation is not known, and this relationship is not considered in the analysis of water balance. The magnitude of Y depends on the values of Z and D W After the surface water had been drained, the D W value decreased and surface runoff increased correspondingly. The magnitude of the Z value is related

to water-supply conditions. Water was abundant before reclamation and its evaporation rate approximated that of a large water body. Before reclamation the amount of water consumed by evaporation on the wetland accounted for 70 per cent of the total annual precipitation. After reclamation, water supply was reduced and evaporation decreased correspondingly. As a result, the difference between the two was about 20 per cent, and runoff from the wetland was greater than that prior to reclamation.

Reclamation has brought about changes in soil structure, improvement in the vertical drainage capacity, and an increase in the infiltration capacity of the wetland. The recharge of the groundwater by precipitation may be calculated by the following empirical formula:

V= 0.79CA-0.923 (T-t) (2)

where
V= groundwater recharge by one precipitation event (mm)
A = average rainfall intensity (mm/min)
T= duration of one rainfall (min)
t = total time of absorption (min)
C = coefficient of recharge efficiency (0.1 may be used because of low groundwater level of the wetland).

When the soil moisture content Wo<=20%,
t = 34.6A-0.962. When Wo>20%, t = 25.1 A-0-892.

Fig. 9.3. Relationship between the Level of Groundwater and Soil Moisture Content of Wetland

Based on the calculation of precipitation infiltration capacity of the wetland in Zhonghe People s Commune, Butha Qi, during the period June-September, 1976 (a moderately dry year), by using equation (2) and assuming that the Wo of the pre-reclamation wetland has reached saturation, the infiltration coefficient is 0.1. This means that the infiltration capacity of precipitation after reclamation increased by 10 per cent. Equations (1) and (2) indicate that both surface runoff and subsurface runoff have increased since reclamation, a factor that should be considered when locating water-management projects in the lower reaches of large-scale reclaimed wetlands.

Soil moisture drops sharply after the wetland has been reclaimed. Figure 9.4 shows the percentage of soil moisture at different depths before and after reclamation. The soil moisture content in the 10-20 cm layer is still rather high since there remain in the near surface layer of the newly reclaimed wetland large amounts of plant residues with a strong power of absorption. A comparison made in 1975 of soil moisture in the newly reclaimed wetland at the Fuxing People's Commune, Arun Qi, with that of the sloping field, shows the former is 5-15 mm higher at a depth of 0-40 cm, in spring, and 20-50 mm higher in the period June-August. The longer the period since reclamation, the more are the changes that occur in the morphology of the topsoil, and ultimately the phenomenon of high soil moisture content would disappear. Drought, in fact, is still the main reason for the declining crop yields on the reclaimed wetland.

In the study area, drought occurs mainly in the spring. According to observations made in 1975 on reclaimed wetland at the Zhonghe People's Commune, Butha Qi, the total amount of evaporation on the maize field in May and June was 182.3 mm, and the average annual precipitation for the same period was 108.0 mm (table 9.1), the latter being 59.2 per cent of the former. An examination of the precipitation frequency curve of May and June for the region shows that assured rate of natural precipitation is only about 7 per cent, so the cultivated wetland after many years of reclamation should be irrigated. irrigation water should come first from reservoirs. In areas without reservoirs, groundwater may be utilized. But since the amount of water in the relatively shallow (3-10 mm) sand and gravel aquifer of the wetland is limited, both surface and groundwater should be used.

Changes in Ground Temperature

The ground temperature of the wetland before reclamation is generally lower than that of the slope field, because the heat capacity of the former is higher. The heat capacity of soil is a function of both the moisture content and the mechanical composition of the soil. If the moisture content of clay increases from 30 to 80 per cent, then its heat capacity would increase from 0.45 to 0.706.6 i.e. as the moisture content of clay is increased to 50 per cent, an extra 0.291 calorie of heat would be needed to raise the temperature of a unit of soil by 1C. In addition, the ability of the wetland to absorb solar radiation and its heat conductivity differ from those of the slope field, differences that can also cause ground temperature differences. The ground temperature of a tract of wetland measured on 27 May 1975 at the Fuxing People's Commune, Arun Qi, where the soil moisture rate was 20-30 per cent, was 9.4C at the depth of 60 cm. But at the same depth where the soil moisture approached saturation the ground was still frozen. Based on comparative observations made in June at two localities where the groundwater levels were the same but the thickness of the sub-clay strata were different, the ground temperatures for a 1.3 m thick section of sub-clay were 3.2C,4.4C, 3.8C, and 3.3C lower than the temperatures of a 0.7 m thick sub-clay section at the depths of 5,10,15 and 20 cm, respectively. Because of the low heat conductivity of the intertwined root layer, the ground temperature of wetland with such a layer is usually 0.5-0.8C higher at 08.00 h than that without the layer. But at 14.00 h the temperature of the wetland with such a layer was 0.8-1.6C lower than that without the layer.

After reclamation, the surface water of the wetland disappears, the groundwater level becomes lower, the soil moisture content decreases, the property of the upper soil layer changes, and the ground temperature rises correspondingly (table 9.3 and figure 9.5). The value differences tend to become larger with increasing depth (table 9.3). The longer the period since reclamation and the beginning of cultivation, the higher the ground temperature. The ground temperature of wetland reclaimed for 25 years is 1.2C higher than that reclaimed for 7 years, and 4.4C higher than that reclaimed 4 years previously.

Fig. 9.4. Changes in Soil Moisture of Wetland Before and After Reclamation

TABLE 9.3. Ground Temperature Changes Before and After Reclamation of Wetland

Type of land Depth (cm)
0 5 5-10 10-15 16-20
(C) (C) (C) (C)
Wetland before        
reclamation (T1) 13.5 10.5 8.0 6.3
Reclaimed wetland(T2) 17.0 14.8 12.5 11.9
Difference (T1-T2) - 3.5 - 4.3 - 4.5 - 5.6

Source: Based on measurements by Hou Guangliang

Greater Khingan Range

Although the ground temperature of wetland rises after reclamation, it is still slightly lower than that of the slope field (table 9.4). The study area is located in the zone transitional between the continental and the frigid continental zone. Heat deficiency is thus the main factor limiting the normal growth of crops, and the low ground temperature of the wetland is unfavourable for crop development. Experiments conducted from June to September '1974 at the Zhonghe People's Commune indicated that the ground temperature of the wetland may be increased by 1.2-1 .8C by adding sand to improve the characteristics of the soil, and it may be increased by 1.2-1.5C with the application of cattle- and horsemanure as fertilizer. Ground temperature may also be increased by ridge tillage and by properly arranging the directions of the ridges. However, the problem of how to raise the ground temperature of a large area of wetland still remains to be solved.

Changes in Soil Fertility

Fig. 9.5. Ground Temperatures of Wetland Before and After Reclamation

TABLE 9.4. Ground Temperatures (C) of Reclaimed Wetland and Slope Field (Based on observations at Fuxing Commune, Arun Qi, 1977)

Type of land Month and depth (cm)
June July August
5 10 5 10 5 10
2-3 years after reclamation(T1) 17.7 16.3 22.8 20.4 19.2 18.3
Slopefield (T2) 20.8 20.0 25.3 24.0 22.4 22.3
Difference (T1-T2) - 3.1 - 3.7 - 2.5 - 3.6 - 3.2 - 3.0

TABLE 9.5. Soil Fertility Changes on Reclaimed Wetland after Reclamation

  Years after reclamation
1 4 7 25
Organic matter content (%) 10.45-12.00 11.32-11.43 6.30 4.60
Total nitrogen content (%) 0.43-0.54 0.49 0.26 0.17

Source: Report of the Wetland Group, Land Resources Survey Team, Heilongjiang Province,
1975

The potential soil fertility of valley wetland on the southeastern slope of the Greater Khingan Range is very high. The organic matter content usually is over 10 per cent, and the maximum may reach 20 30 per cent. The total N contents is more than 0. 26 0.53 per cent and the total P is over 0.13 per cent. Prior to reclamation, development of the potential productivity of the wetland is limited by excessive moisture, low ground temperature and incomplete decomposition of the plant residues in the soil. But after reclamation the potential soil fertility is gradually released as a result of changes in moisture and heat conditions. Such conditions, however, vary with the length of time under cultivation.

The differences of organic matter and total N content in wetland soil, at 20 cm. after various lengths of time since reclamation are listed in table 9.5. It can be seen that the organic matter and total N contents differ little during the first four years after reclamation, when the soil is in the process of becoming mature. But seven years after reclamation the organic matter content is reduced by 1.3 per cent compared with that of the uncultivated wetland nearby, the reduction being 17.1 per cent of the original content, and total N declines by 0.04 per cent, the reduction rate being 13.3 per cent. The organic matter content 25 years after reclamation decreases by 1.7 per cent compared with the content after seven years, the reduction ratio being 27 per cent, and total N decreases by 0.09 per cent. the reduction ratio being 34.6 per cent. Similar changes in wetland soil fertility also occurred at the Bayangaole People's Commune, Jalaid Qi, where changes were not marked during the first four years after reclamation, but after 15 years of cultivation the organic matter decreased by 3.6 per cent, the reduction ratio being 40 per cent, and total N decreased by 0.13 per cent, with a reduction ratio of 32 per cent.

A comparison of the Butha Qi data with those of the Bayangaole People's Commune indicates that the more southerly the location the greater is the decline in soil fertility. This phenomenon could be related to latitudinal temperature differences. That soil fertility decreases appreciably after reclamation suggests that the cultivated former wetland should be fertilized to prevent the decline.

Analysis of the changes in available nutrients (mainly nitricnitrogen, ammonium-nitrogen, rapidly available phosphate and potassium) in wetland soil after reclamation indicates that the longer the period after reclamation, the poorer the supply of such nutrients (table 9.6). The amount of easily hydrolyzable N decreased over time, except for rapidly available phosphate, which increased after reclamation. After four years of cultivation, the easily hydrolyzable N increased appreciably, especially nitricnitrogen. Seven years after reclamation the nitric-nitrogen content increased greatly compared with the nearby uncultivated wetland. However, nitricnitrogen content decreased 25 years after reclamation compared with that seven years after reclamation, but it was still higher than that of the uncultivated wetland. These changes show that nitrification was strengthened with the maturation of the reclaimed wetland.

TABLE 9.6. Changes in the Available Nutrients of Wetland Soil after Reclamation

  Years after reclamation
1 4 7 25
Easily hydrolyzable nitrogen (%) 0.0092 0.01 0.007-0.014 0.0071-0.01
Nitrate nitrogen (%) 0.001 0.004 0.0055 0.0035
Ammonium nitrogen (%) 0.002 0.0018 0.0007 0.0007
Rapidly Available phosphate (ppm) 6.0 6.0 0.4 8.5

Source: Report of the Wetland Group, Land Resources Survey Team, Heilongjiang
Province, 1975

Some of the ammonium-nitrogen and the nitrogen contained in the potential nutrients were transformed into nitric-nitrogen, thereby enhancing the supply of nutrients. Later, with the decrease in potential fertility, the supply of nitric-nitrogen worsened. This further indicates the importance of the properly timed application of fertilizers on former wetlands after a certain period of cultivation. Experiments showed that spring-wheat production could be increased by applying superphosphate, peat, nitric-acid phosphatic fertilizer, and superphosphate plus nitric-ammonium.

Experience demonstrated that good results could be obtained by applying phosphate fertilizer. It is also important to apply such trace elements as copper, boron, zinc, and molybdenum. Yields of spring wheat grown on the reclaimed wetland at the Fuxing People's Commune, Arun Qi, increased by 6.2-45.1 per cent after application of these trace elements, of which boron and molybdenum gave the best results.

Conclusions

1. The wetlands on the south-eastern slope of the Greater Khingan Range are a product of the interplay among water, heat and landform. The periglacial geomorphological features have played a decisive role in the formation of the valley wetland. Water on the wetland derives mainly from precipitation and storm floods, and the extremely weak capacity of the flood-plain to drain storm-water is responsible for its accumulation on the surface. The formation of the valley wetland had its origin in land swamps beginning in the early Middle Holocene (7,000 years B.P.), and the period of its most rapid development has been since the Late Holocene (2,500 years B.P.). At present, the wetland is desiccating.

2. The main measures that should be taken to improve the wetland are the drainage of surface water, the prevention of flood-water from inundating the valley wetland, and the dredging of river channels to facilitate discharge. Engineering projects for flood control should be designed to prevent flooding by storms which occur every ten years or so, and flood-drainage projects should be designed to drain floodwaters brought by storms that recur approximately every five years. Practical experience has shown that both criteria. if followed, can yield good results.

3. Both surface and underground runoff of the wetland increases after reclamation. Soil moisture quickly decreases and crops must be irrigated. The ground temperature of the wetland is lower than that of the slope field, and gradually increases with the length of time after reclamation. Temperature may be raised by mixing sand and manure with the soil. During the initial period after reclamation, the organic matter and total N contents do not change greatly, but after seven years of cultivation they begin to decrease. The cultivated former wetland should be adequately fertilized because the quickly available P keeps increasing after reclamation. Nitrification increases steadily in the initial stage of cultivation but decreases after 25 years.

References

1 Zhou Mingzehn, et al., Ouaternary Mammal Fossils in North east China, 1969. Special Publication Series A, No.3. Science Press, Beijing (in Chinese).

2 Provincial Museum of Heilongjiang, "A Study of the Neolithic Site at Angangqi, " Kaogu [Archaeology] (1974): 99-108 (in Chinese).

3. Institute of Geochemistry. Chinese Academy of Sciences, Guiyang, "`A Preliminary Report on the Chronology of the Holocene Geology of Southern Liaoning Province," Diqiu Huaxue [Geochemistry] 4 (1974) (in Chinese).

4. M.E Neishtadt,"History of Forestry and the Palaeogeography in Holocene USSR, " 1957, pp. 165-170 The Academy of Sciences, Moscow (in Russian).

5 A.D. Brudastov, Drainage of Mineral Areas and Marshlands. Vol. 1, 1957. Hydrology Publishing House, Beijing (Chinese ed.).

6. D.G. Williasky. Pedology. vol. 1, 1954, pp. 237-238. Higher Education Publishing House, Beijing (Chinese ed ).


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