Environmental Effects of Exploitation and Utilization of Groundwater Resources

The development and utilization of groundwater resources in this area has not only achieved great economic and social benefits, but also caused some environmental effects, mainly including the continuous decline of groundwater level, wetland shrinkage, vegetation degradation, land desertification and soil salinization.

First, the groundwater level continues to decline.

The intermittent observation data in recent 50 years show that the groundwater level in Anxi-Dunhuang basin and Yumen-Zhen Shi basin has a large-scale overall downward trend (except the spring overflow zone), and the overall downward rate is greater in the south than in the north and greater in the east than in the west; The rate of decline is increasing year by year.

The water level of No.2002 observation well in the south of Changma Gobi decreased by 6.59m from 1987 to 1999, with an average annual decrease of 0.51m; In 2003-2004, the water level decreased by 2.92m, with an average annual decrease of1.46 m; From 2003 to 2004, the water level of observation well S97-2 in the east of Changma Gobi decreased by 5.28 meters, with an average annual decrease of 2.64 meters ... In recent 50 years, the groundwater level in Changma Gobi decreased by 5 ~ 10 meters. After the completion and operation of Changma Reservoir, the decline rate rose sharply. In 2004, the groundwater level of YT004 hole of Huangzhawan Supply and Marketing Cooperative in fine soil plain of alluvial fan in Changma decreased by 1.07m, with an average annual decrease of 0.14m; . Hedong Township Supply and Marketing Cooperative YT0 14 hole 1997 ~ 2004, the groundwater level decreased by 1.33m, with an average annual decrease of 0.166m; From 2007 to 2004, the groundwater level in YT023 hole of Zhen Shi Township Water Pipe Institute decreased by 0.48 meters, with an average annual decrease of 0.06 meters. Since 1997, the groundwater level in most areas of Yumen-Zhen Shi basin has generally decreased by 0.2 ~ 1.33 m, with an average annual decrease of 0.025 ~ 0. 177 m except for a slight increase in the groundwater level in Ziqiao area. In recent 10 years, the groundwater level in the fine soil plain of changma irrigation area has decreased by 0.6 ~ 3.00 meters.

During the period of 1987 ~ 1999 in Shuangta Irrigation Area of Anxi-Dunhuang Basin, the groundwater level decreased slightly more than that in Changma Irrigation Area in the same period, and its spatial change was the largest in the periphery of Anxi County (and Nancha) in the middle, with a decrease of 1. 19m in the east. The water level in Xiaowan Farm (A 1 observation well) decreased by 0.2 1m, with an average annual decrease of 0.1/kloc-0. County (observation well A2) decreased by 0.95m, with an average annual decrease of 0.48m；; The area of Nancha (observation well AD09) decreased by 0.48m, with a decrease of 0.24m In recent years, the water level in Shuangta Irrigation Area decreased by 0.9~4.0m (Figure 9- 1), and in recent 20 years, the water level in Shuangta Irrigation Area decreased by 2 ~ 8m.

Figure 9- 1 Multi-year dynamic curve of Wulian (A7) observation well in Shigong Farm, Anxi County

From 1950s to 2004, the water level of Yueyaquan Lake in the south of Danghe Irrigation Area in Anxi-Dunhuang Basin decreased by 9 ~ 10m, and the maximum water depth decreased from about 10m in 1950s to about 1.2m at present. The water level dropped by several centimeters or more every year from 1970s to 20-30cm now, which made the famous Crescent Spring face extinction. The water level in Qizhen and Tiejiabao decreased by 4.99~5.90m at 1966 ~ 1980, and decreased by 1997 ~ 65438 in 2003, with an average annual decrease of 0.15 m; ; From 1999 to 2004, the water level in Wu Dun township (observation well D3) in the east decreased by1.88m, which was 0.38m. In 2003-2004, the groundwater level in Yumenguan-Mamitu area decreased by 0.03m-0.09m, which was not significant and basically stable. In recent years 10, the water level in the south of Danghe irrigation area dropped by 0.64~4.0m, and in recent 50 years, the local water level dropped by 5 ~ 10m (Table 9- 1).

Fig. 9-2 Comparative photo of the change of crescent spring area (left 1947, right 1997)

Table 9- 1 Statistics of Groundwater Level Decline in Yumen-Zhen Shi and Anxi-Dunhuang Basin Irrigation Areas

The main reasons for the decline of groundwater level in this area are the decrease of groundwater recharge and the increase of exploitation, and other factors such as rising temperature and the trend of climate drought have also played a role.

The aquifer in the southern piedmont zone is very thick and the hydraulic gradient of groundwater is large. After the local groundwater recharge is reduced, the recharge of groundwater system obeys the law of hydrostatics in a certain period of time, that is, part of the storage capacity in the high water level zone in the southern basin is converted into the recharge in the northern basin, which is the main reason why the piedmont water level drops greatly and the fine soil zone becomes more stable.

Second, wetlands are shrinking.

According to the internationally recognized definition of Wetlands Convention and its classification of wetland types, the main types of wetlands in China include lake wetlands, swamp wetlands, river wetlands, estuarine wetlands, coastal beaches, shallow waters, reservoirs, ponds, rice fields and other natural wetlands and artificial wetlands. Their common feature is that their surfaces are covered or filled with water all the year round, which is the transitional zone between land and water. Wetlands play a very important role in regulating climate, purifying environment, maintaining biodiversity and ecological balance, and providing resources. They are known as "kidney of the earth", "Cradle of Life" and "Gene Bank of Species", attracting worldwide attention.

The lake wetlands in this area are distributed in Yumenguan and Houkeng in the lower reaches of Shule River. Swamp wetlands in Anxi-Dunhuang Basin are mainly distributed in the periphery of lake wetlands such as Wanyao, Mamitu, Houkeng, Yumenguan and Dafangpan in the groundwater overflow zone in front of Danghe Fan. In Yumen-Zhen Shi basin, it is isolated in the upper reaches of Shuangta Reservoir, the depression between Changma Fan and Zhen Shi Fan, and the groundwater overflow zone of Lucaogou in the piedmont of the south side of the northern section of the mountain. Huahai Basin is distributed in some areas of Beishihe River Basin. Ponds are mainly distributed in the middle and lower reaches of Sandaogou-Shidaogou.

In recent decades, due to the continuous decrease of groundwater resources, groundwater level and spring water resources, the wetlands in this area have shrunk in a large area. Compared with the 1950s, the overflow zone in Sandaogou-Bulong area moved down by 1000 ~ 2000 m, and the wetland area along the spring ditch decreased with the overflow zone moving down. Before 10, ziqiao township in Anxi county was rich in water resources. In addition to ensuring agricultural irrigation, 2.5 million cubic meters of water is used to irrigate grasslands every year. By July 2004, almost all the water in ponds and dams had dried up. Now with the large-scale reclamation (a considerable part of which is developed wetlands and grasslands), surface water is far from meeting the requirements of agricultural irrigation. In this case, a large amount of groundwater was exploited, and the groundwater level dropped by 0.6 ~ 4 m in 10 year, resulting in some springs drying up, lakes disappearing and wetlands shrinking. In 1990s, there were more than 320 springs in Yumenguan-Wanyao area, and the vegetation grew well. In the spring of 2005, it was greatly reduced, and most of Populus euphratica and Tamarix in Wanyao and Mamitu areas died, and vegetation such as reeds around the wetland grew poorly (Figure 9-3 and Figure 9-4). At present, the area of Dunhuang wetland has been reduced from 3.75 million mu in 1950 to less than 300,000 mu now, and the groundwater level is still declining, and the trend of wetland shrinkage has not been effectively curbed.

Figure 9-3 Growth of Reed, Populus euphratica and Tamarix chinensis around Wanyao Wetland in 2005

Figure 9-40,000 Kiln Area Shrinking Wetlands

TM interpretation shows that in 1980s, the wetland area in the whole region was 443.72km2, but it shrank to 358. 1km2 at the end of 1990s, with a decrease of 85.62km2 and a reduction rate of 20%. (Table 9-2).

Table 9-2 Statistics of Wetland Area in the Middle and Lower Reaches of Shule River in Different Periods of the 20th Century

Third, vegetation degradation.

The decline of regional groundwater level and the sharp decline of spring water resources directly led to the decline of vegetation ecosystem in this area. In recent decades, due to the drying up and migration of rivers, the forest belts and shrubs originally developed along the banks of Shule River and Danghe River have rapidly degenerated or even disappeared, and swamps and water-loving vegetation widely distributed along the spring overflow zone have declined with the decrease of spring water, replaced by xerophytes, sandy land and saline meadows. In 1950s, 495,000 mu of shrubbery grew in Anxi West Lake, but by the early 1980s, only 6,543.8+mu was left, and it was bare and dying. The natural Populus euphratica forest of about 1.95 million mu originally grown on both sides of Shule River in Shuangtababao-Wangganzi is not much left. The Populus euphratica forest around Yanzi Lake has all died and become a wind erosion land. Nearly 600,000 mu of grassland in Shawo, Anxi has been completely desertified, and licorice meadow has been completely destroyed. The area of Caohu Lake in Ziqiao Town is 90% less than that in 1950s. The reeds in Lucaogou reached 2m in 1950s, and all of them have died. In the past 30 years, the grassland degradation area in the middle reaches of Shule River alone reached 654.38+0.305 million mu, accounting for more than 80% of the grassland area. The area of degraded grassland in the whole basin exceeds 6.5438+0.995 million mu.

In addition, the deterioration of water quality is also one of the main reasons for vegetation degradation. As can be seen from Table 9-3, the TDS of groundwater is less than 3 ~ 3.5g/L, and most plants grow well. When it is greater than 6 ~ 10g/L, the plants are sparse and degenerate; When it exceeds 10g/L, the plants wither and die.

Vegetation degradation caused by water quality deterioration mainly occurs in the middle and lower reaches of rivers. With the gradual increase of TDS in groundwater, plants also degenerated from salt intolerance to salt tolerance, from dense growth to sparse growth, and then withered and died, leaving a large salt shell at last. For example, Halonuoer, Halachi, Boluo Lake, Ganhaizi and Tangyihu in Dunhuang all died of vegetation due to the high TDS of groundwater, and salt accumulated on the surface to form large salt shells.

By the end of 1998, TM images showed that the area of woodland, grassland and cultivated land in the whole region was 199.2km2 (Table 9-4), 1569.5km2 and 149 1.96km2, respectively, higher than that in the last century.

Table 9-3 Relationship between Growth Status of Main Plants and TDS of Groundwater (unit: g/L)

Table 9-4 Statistics of Oasis Landscape Area in Shule River Basin in Different Periods of 20th Century (Remote Sensing Interpretation)

Fourth, land desertification.

According to the disaster mechanism, the desertification land in this area can be divided into two types: quicksand burial and wind erosion desertification. According to the degree of desertification, it can be divided into mild desertification, moderate desertification, severe desertification and severe desertification. Buried quicksand is distributed around the fixed and semi-fixed sand dunes in Kumutage, Changshaling, Xishawo and the northern part of Huahai Basin, and most of them are serious desertification. Wind erosion and desertification areas are mainly distributed in the west of Yumenguan, north of Dunkou, north of Huangdunzi Farm, west of West Lake, Suoyangcheng-Tuhulu, Qiduntan and the middle of Huahai Basin.

The magnitude of buried quicksand disaster is related to the size of sand dunes, the moving direction of quicksand and the development state of vegetation. The closer to the desert, the larger the area of sand dunes and undeveloped vegetation, and the greater the disaster of quicksand burial; On the contrary, the farther away from the desert, the smaller the area of sand dunes and ridges, the more developed the vegetation, and the lighter the disaster of quicksand burial. Wind erosion and desertification are widely distributed in this area, mainly concentrated in Gobi and desert areas with underdeveloped vegetation outside oases. The general distribution law of wind erosion and desertification disaster degree is that the east is stronger than the middle, the north is stronger than the south, and the barren area is stronger than the oasis area.

Land desertification is closely related to the decline of groundwater level and vegetation reduction caused by excessive reclamation and large-scale development and utilization of water resources. Since the construction of Shuangta Reservoir, the inflow from the lower reaches of the river has been reduced or even cut off, and the oasis has shrunk or died as a whole. The abandoned old river course, including the fine soil deposition of Tailu Lake, which belongs to the river, has been transformed into wind erosion land by wind erosion, and the river course has become a flowing sand dune. For example, the mobile sand dune about 2m high in Shule River between Xiaowan Farm and Anxi County is a typical example. Similarly, due to the decline of water level and the degradation and decline of vegetation, the wind erosion in this area is strengthened and the process of land desertification is accelerated. At present, the mountainous desert in Changsha moves westward at a speed of 2m every year. Since 1990, Beiqiaozi has been buried by sand, and the farmland forest has reached 799.5 mu. Since 1950s, the sand dunes in Huahai Basin have moved forward by 50 ~ 80m, and 40% of the land needs replanting twice every year to basically maintain seedlings. Shuangta-Suoyangcheng and Anxi Huangdunzi Farm have evolved into serious desertification areas. In addition, the recent resettlement project broke ground in a large area, destroying and disturbing the original landform, loosening the fine particles on the surface and forming new sand sources. In some areas, such as Huahai Bijia Beach and other places, due to the decrease or inadequacy of immigrants, large areas of cultivated land have gradually become deserts due to no cultivation. In the immigrant development zone of Qiduntan Township, Anxi, sand dunes and small wind-eroded sandy land nearly 2 meters high appeared on the originally barren grassland.

According to the interpretation of TM images and comparative analysis of relevant data, the total area of desertification land in this area was 3039.59km2 in 1980s and 3066. 12km2 in 1990s (Table 9-5), with a slight increase of 26.59km2, but the distribution area of different types of desertification land in different river basins changed greatly in different periods. Compared with the 1980s, the sandy land area in the whole region has been increasing, with the growth rates of Yumen-Zhen Shi and Huahai Basin being 38.9% and 64.2% respectively, while the Anxi-Dunhuang Basin is basically unchanged. The saline-alkali land area in Yumen-Zhen Shi and Huahai Basin increased by 39.8% and 13.5% respectively, while the saline-alkali land area in Anxi-Dunhuang Basin decreased by 10%. The increase of sandy land area is mainly caused by desertification in soil desert zone, and the increase of saline-alkali land area is mainly caused by desertification in saline-alkali land.

Table 9-5 Statistical Table of Desertification Land Area in Shule River Basin in Different Periods of the 20th Century (Remote Sensing Interpretation)

Verb (abbreviation of verb) soil salinization

(A) the distribution and characteristics of saline soil

There is little precipitation, strong evaporation, shallow water level and large area of salinized land in the middle and lower reaches of Shule River. The degree of salinization in different areas is quite different, but the overall performance is that the soil salt content increases gradually from south to north and from east to west along the surface slope and river flow direction. According to the soluble salt analysis results of soil samples (506 groups of samples from 86 sites) collected in field investigation in 2004 ~ 2005, combined with the previous soluble salt analysis data, the average salt content of the soil at the depth of 0 ~ 0 ~100 cm in Shule River Oasis was divided into non-saline soil, saline soil, light saline soil, moderate saline soil, heavy saline soil and extra-heavy saline soil according to the degree of salinization. In addition, the ions of various soluble salts in soil are harmful to crops to varying degrees. Under the same conditions, chloride salt is more harmful and sulfate is less harmful. According to the ratio of Cl- to S concentration, the chemical types of soil can be divided into four categories (Table 9-7).

Table 9-6 Classification Standard of Saline Soil Types

Figure 9-5 Map of Soil Salinization Degree in Oasis Area of Shule River Basin

Table 9-7 Classification Standard of Chemical Types of Saline Soil

Non-salinized soil: mainly distributed in the front of Changma alluvial fan south of Huahai Town, Yumen Town, Sandaogou Town and Bulongji Township, and agricultural irrigation areas of Xiaowan Farm, Nancha-Sigong and Dunhuang-Huang Qu. Groundwater depth is greater than 3 ~ 5m, TDS is less than 1g/L, groundwater runoff is faster, and vertical alternation of water and salt is relatively weak. The vertical salt content of soil is basically the same as that in the middle and lower parts (Figure 9-6A). Cl-—S is the main chemical type of soil.

Saline soil: mainly distributed in Qiduntan, Huanghua Farm 9- 10 Team, Yanxi Rock Neck Branch and West Lake. The buried depth of groundwater is 1 ~ 3m, and the TDS is1.0 ~ 3.0g/L. Groundwater runoff is slow. The soil salt content is higher vertically above 1m, and gradually decreases below 1m (Figure 9-6B). The soil chemical type is mainly S-Cl- type.

Figure 9-6 Vertical Distribution Types of Soil Salinity in the Middle and Lower Reaches of Shule River

Slightly saline soil: scattered in the area north of Beiqiaozi-Dianzi River, Xiangyang Township and Huangdunzi-Tianshuijing. Groundwater depth 1~3m, local less than 1 ~ 3m, TDS 1 ~ 3g/L, local greater than 3g/L ... The groundwater runoff conditions are poor. The soil salt content is higher in the surface layer and below 1m, and lower in the middle, showing type I (Figure 9-6C). The soil chemical types are mainly S-Cl- and Cl-—S-types.

Moderately saline soil: mainly distributed in most natural oases in the north of Huahai Basin, northeast of Huangzha Bay, Hucongquan-Xiacaozi, Wanggarze-western Hubei, north of Dunkoukou and west of Yumenguan. Groundwater depth 1 ~ 3m, or 3 ~ 5m, and tds2 ~ 3g/L or more. Groundwater runoff is slow, mainly by evaporation. Soil salinity is high in the top, middle and bottom layers, and it is inverted "mountain" in other areas (Figure 9-6D). The soil chemical types are S-Cl- and Cl-. The surface layer has a 2 cm thick salt shell.

Severe saline soil: it is mainly distributed along the Caolu River at the southern foot of the northern mountain, on the west side of Tangyi Lake in Dunhuang, in kharrazi at the eastern edge of Kumutage Desert, and in the area from Huasanjing to Lucaojing in Huahai Basin. The buried depth of groundwater is less than 1m or 1 ~ 3m and tds2 ~ 4g/L, and the groundwater is basically stagnant. The vertical distribution of soil salt presents a "ladder" shape, and the surface salt is high, which gradually decreases downward (Figure 9-6E). The chemical types of soil are mainly-Cl- and Cl-. There is a 2 ~ 5 cm thick salt shell on the surface.

Extra-severe saline soil: mainly distributed in western Hubei, Tangyi Lake and northwest Wanyao, with sporadic distribution in the middle of Huahai Basin. The buried depth of groundwater is 1~3m, or less than 1 ~ 3m, TD S3 ~ 5g/L, and the groundwater is mostly in stagnant state. The salt content in the top soil layer is very high, and it gradually decreases downward in a "step" shape (Figure 9-6F). Cl- is the main chemical type of soil. The surface layer has a salt shell more than 5 cm thick.

Saline soil has a strong inhibitory effect on the growth of crops. According to relevant data, the general yield of light and moderate saline soil is only 60% ~ 70% of that of non-saline soil, while only salt-tolerant plants such as red willow and Achnatherum splendens can grow in severe saline soil. Most of the severe and extra-severe saline soils are difficult to grow, showing a decadent landscape.

(B) the impact of the development and utilization of groundwater resources on soil salinization

The influence of groundwater resources development and utilization on soil salinization is mainly manifested in two aspects: first, a large number of high salinity groundwater (TDS > 3g/L) is exploited for irrigation, which accelerates the salt accumulation speed of surface soil and aggravates the soil salinization degree under the strong evaporation and concentration; Second, a large number of surface water and spring water resources are introduced into shallow groundwater area (flooding raises groundwater level), and poor drainage leads to secondary salinization caused by salt accumulation in the upper soil layer or aggravates the salinization of primary saline soil.

1. Exploitation of brackish water for irrigation will aggravate soil salinization.

After Shule River, Danghe River and other rivers leave the mountain pass, the salt brought into the area is 665,000 t/a (calculated by TDS0.50g/L). These salts migrate with surface water and groundwater from the upper reaches of the river to the lower reaches, and from high places to low places. Due to evaporation and concentration along the way, soil particle adsorption and other effects, the TDS of surface water and groundwater gradually increased. For example, after Shule River comes out of the mountain, it passes through Ma Yin Farm and Shuangta Reservoir in Anxi County, and finally reaches Wangganzi Forest Farm. The TDS is 0.50g/L, and the surface water TDS increases to 0.70g/L, 1.08g/L, 1.4 1g/L and 2.48g/L respectively. The trend of groundwater TDS increasing from east to west is more obvious. The TDS of groundwater, which has a direct influence on soil salt accumulation, is 0.6 1g/L, 0.6 1g/L, 1.76 ~ 4.22g/L and 2.80 ~ 6.655 in Sandaogou, Hedong, Bulongji, Huancheng and Xihu townships, respectively. Under normal circumstances, the higher the TDS of surface water and groundwater, the greater the degree of soil salt accumulation. Comparing different TDS with groundwater depth 1 ~ 3m, we can see its influence on soil salt accumulation (Table 9-8). When the TDS of local sewage is 1.0 ~ 3.0g/L, the average salt content of 1.0 ~ 2.0m soil layer is 9.54 ~ 53.83g/kg, and the soil is saline soil ~ moderately saline soil. When the TDS of local sewage is 3.0 ~ 5.0g/L, the average salt content of 1.0~3.0m ~ 3.0m soil layer is 32. 13 ~ 133.27g/kg, and the soil is light saline soil to heavy saline soil.

Table 9-8 Statistical table of the relationship between groundwater salinity, water table depth and soil salt content

It shows that the utilization process of water resources is the process of soil salt accumulation and soil development to salinized soil. Long-term exploitation of salt water and brackish water for irrigation will accelerate the salt accumulation on the soil surface and further aggravate soil salinization.

2. Irrigation causes the groundwater level to rise, and the degree of soil salinization intensifies.

Some areas only pay attention to the introduction of surface water, while the irrigation technology of water-saving projects is backward. In some irrigation areas, in order to wash and suppress salt, the irrigation quota is constantly increased, resulting in the average irrigation quota of soil improvement areas of 1500 ~ 2500 m3/ mu. Although a large amount of field leakage can temporarily reduce the salt content in the soil, it artificially raises the groundwater level. With the increase of evaporation intensity in spring and summer next year, the soil salt content will rise sharply, which will aggravate the soil. For example, in Xiangyang area of Anxi, after many times of flooding, due to the rising groundwater level and the barrier of the lower clay layer, the soluble salt could not leak out, and concentrated on the surface through atmospheric evaporation, resulting in secondary salinization, a large number of salt spots appeared on the surface, and 80% of the sown wheat seedlings died. In addition, due to the rise of groundwater level, the evaporation in the primary saline soil area increased, which further aggravated the degree of salinization. For example, in September 2004 and August 2005, the comparison results of soil salt content of 0 ~ 30cm and 0 ~100cm in the same position and depth in Dunhuang Andun Highway and Yumen-Zhen Shi area showed that the average soil salt content in 2005 was generally higher than that in 2004 (Table 9-9).

Table 9-9 Comparison Table of Soil Salt Content in Different Years

3. Dam leakage, groundwater level rise and soil secondary salinization.

Since the founding of New China, Changma Irrigation District has built 14 plain ponds of different sizes along Erdaogou-Shidaogou to intercept the overflowing spring water. These water conservancy facilities have played a very good role in regulating water volume and preventing the early morning. However, with the passage of time, the water storage capacity of the reservoir is getting smaller and smaller, and the water quality is getting worse, which leads to a sharp rise in the groundwater level around the reservoir and the deterioration of water quality, and promotes the development of cultivated land in the direction of salinization. For example, about 1 .5 million mu of land around Sandaogou1and No.3 Tangba has been salinized to varying degrees, and some areas have even been abandoned due to serious salinization.

From the interpretation results of TM images, it can be seen that the salinized land area in Yumen-Zhen Shi Basin was 397.95km2 in 1970s (Table 9- 10), 445. 14km2 in 1980s and 479.28km2 in 1990s, an increase of 8 1.33km2 compared with 1970s. The salinized land area in Huahai Basin was 235.24km2 in 1980s and 240.37km2 in 1990s, which was relatively stable. The reason is related to the improvement of local salinized soil. The area of salinized land in Anxi-Dunhuang Basin was 579.57km2 in 1980s and 609. 14km2 in 1990s, an increase of 29.57km2 or 5.3% compared with 1980s. This increase is mainly due to the shrinking of the wetland west of Yumenguan and its transformation into salinized land. The area of salinized land in the whole region was 1259.95km2 in 1980s and 1328.79km2 in 1990s, which was 5.4% higher than that in 1980s.

Table 9-10 Statistical Table of Salinized Land Area in Shule River Basin in Different Periods of 20th Century (Remote Sensing Interpretation)

(3) Prevention and control of soil salinization

The formation of saline soil in this area is mostly the result of long-term natural factors, but the intensification of primary saline soil salinization and the generation of secondary saline soil are closely related to the change of water resources distribution pattern and the relationship between water and salt caused by unreasonable utilization of water resources by human beings. Therefore, the prevention and control of soil salinization in irrigation area must be based on agricultural irrigation and water saving, and the dynamics of water and salt in the area should be regulated. The main measures are as follows:

1. Vigorously promote water-saving irrigation technology to effectively reduce the groundwater level.

Advanced and reasonable irrigation technology is adopted in the middle and upper reaches, instead of flooding irrigation, irrigation quota is adjusted according to agricultural water consumption, surface water supply is controlled, and groundwater infiltration recharge is reduced; Increase the exploitation of groundwater in the middle and lower reaches, develop vertical shaft drainage instead of drainage, or dig drainage ditches (mainly in shallow buried areas with TDS greater than 3g/L), and reduce and control the groundwater level so that it is 2.5 ~ 3.0m below the critical depth. ..

2. Increase vegetation coverage, so that malignant surface evaporation can be transformed into benign plant transpiration.

Afforestation and planting salt-tolerant alfalfa can not only beautify the environment, but also reduce the groundwater level and prevent salt from accumulating on the soil surface. According to the observation data in Aksu, Xinjiang, the transpiration of an adult Populus davidiana forest belt in 200 days is 8364m3, and that of an adult Elaeagnus angustifolia forest belt in 200 days is 30 150m3. The average transpiration of each tree is 6m3, which is equivalent to the drainage of a drainage ditch. Planting alfalfa has many functions, such as increasing green manure, reducing ground evaporation and soil desalination. The experimental observation data of Huanghua farm show that after planting alfalfa for 3 years, the groundwater level can be reduced by 0.93m, and the average desalination is 33% in the depth of1.0 m.

3. Adopt agricultural technology to reduce the salt accumulation on the soil surface.

Agricultural measures mainly include: intensive cultivation, deep tillage, leveling land and pressing sand, increasing chemical fertilizer, planting green manure, improving soil and planting salt-tolerant crops. They can eliminate soil hardening, increase porosity and reduce capillary action, and have a significant effect on preventing soil surface salt.

At present, due to the decline of regional groundwater level, soil secondary salinization only occurs sporadically in some areas. At present, the treatment of saline soil is mainly to improve the reclaimed saline-alkali land. "Shule River Immigrants Comprehensive Development Project" has a large reclamation area. The newly reclaimed land is 408 1 1,000 mu, more than 70% of which belongs to medium saline soil to heavy saline soil. It is difficult to improve the soil because of its high salt content. Therefore, in the early stage of improvement, water conservancy measures can be taken to reduce the groundwater level, and then agricultural management and biological measures can be taken to further dilute the soil to achieve the ultimate goal of improvement and governance.