1. Soil submergence
(1) The process, necessary conditions and formation stages of soil submergence
1) The process of soil submergence is a complex soil formation process in agronomy which consists of interconnection and mutual influence of biochemical and chemical processes; and it is a process of agro-geochemical soil formation in agro-geology. In short, it is to make the soil subcultivation and become subcultivation soil.
2) The necessary conditions for soil subcultivation are sufficient moisture and abundant organic matter.
3) The formation process can be roughly divided into four stages. First of all, the decomposition of organic matter is easy to decompose part of the anaerobic decomposition under flooding conditions consumes a large amount of oxygen in the soil, the redox potential decreases drastically to form a strong reduction conditions in the soil, resulting in a series of organic and inorganic reducing substances and a series of intermediates; secondly, the intermediates (including acids, low-molecular-weight carbohydrates, sulfide and phenolic compounds, etc.) affect the dissolution of iron and manganese minerals in the soil and activation, but also can be dissolved and activated with iron and manganese. It can also form water-soluble bond with iron and manganese, generate organic and inorganic reducing substances, such as CH4, H2S, H2 and NH4, etc., and reduce iron oxide and manganese oxide to form different forms of ferrous iron and manganese, etc.; Thirdly, while iron and manganese activation, there is dissolution and migration of SiO2 and Al2O3, as well as changes of salt ions, such as Ca, Mg and K, etc.; finally, it leads to changes of clay particles and possible clay minerals. Finally, it leads to the change of clay particles and may produce the erosion of clay minerals, which makes the soil texture deteriorate to make the soil sticky and heavy and reduce the permeability.
The above four stages make the soil become submerged soil, and there are five main changes to the soil: First, the greenish-gray color shown by ferrous and manganese submerged soil becomes the typical color of submerged soil, which is known as the "submerged color"; second, some trace elements are activated to increase the soluble state; and third, the production of toxic gases and compounds; Fourth, the clay particles and salt-based ionic changes increase the viscosity of the soil, permeability deterioration; Fifth, change the soil configuration.
(2) Characteristics of submerged soil and its relationship with the water table
The understanding of the characteristics of submerged soil and its impact on agricultural production has been finalized in the agronomic field, so only the research information of the former in the Dongting Lake area is quoted.
Previous researchers conducted a systematic study on the latent fertility of the soil of Dongting Lake Yueyang Junshan Farm, Huarong Xinhe Township and Honghu Xiaogang in Jianghan Region, and the relevant characteristics of the pointers are listed in Table 4-17,4-18, according to the Hunan Academy of Agricultural Sciences, the yield of latent fertility paddy fields in Dongting Lake Region is 7024kg/hm2, and the yield of retention fertility paddy fields reaches 8524-10024kg/hm2. 17% to 30% reduction in latent fertility paddy fields.
Table 4-17 Nutrients in latently fertilized and retention (de-latent) soils
There are two points to be noted about latently fertilized soils: first, the active manganese and organic matter are higher than that of retention soils, and only the effective P is lower, so from the point of view of the composition of nutrients, latently fertilized soils are no worse than that of retention soils, and so the reason for the reduction of yield is not the factor of nutrients, but may be mainly due to deterioration of the texture of the soil and increase of toxic and reducing substances; secondly, it is the fact that latently fertilized soil has no more nutrients than retention soil. Secondly, the degree of latent fertilization, redox potential and active reducing substances increase with the rise of the water table. Therefore, there is a close relationship between the degree of latent fertilization and the water table. This is clearly reflected in the changes in groundwater level and soil configuration (Figure 4-4; Photos 4-8,4-9).
Table 4-18 Pointer characteristics of submerged and retentive (de-submerged) soils
Figure 4-4 Groundwater level versus soil configuration in the Dongting Lake-Jianghan area
A-ploughing layer; P-ploughing subsoil; W- Retention layer; G-submerged layer (the dotted line between the profiles is the water table)
(3) Development trend of soil submerged fertilization after the opening of the Three Gorges Reservoir
Soil submerged fertilization in the Dongting Lake area is relatively serious. According to the second soil census in 1970, there are 181,800hm2 of latent fertile paddy soil in Dongting Lake area, accounting for 36.8% of the latent fertile paddy soil area in the province. Among them, 80,000 hm2 in Changde City, 52,800 hm2 in Yueyang City, and 49,000 hm2 in Yiyang City. Since the 1980s, the Yangtze River Commission has carried out a series of studies for the construction of the Three Gorges Project, in which Cai Shuming [4] published a monograph on the submerged fertile soils of the plain area of the river network of the Dongting Lake in 1997, and superimposed soil types on the color composite image of the Landsat-Tin artifacts in 1989 (scale 1∶250,000), and the soil type of the soil of the Dongting Lake area was determined by the color composite image of the Landsat-Tin artifacts. (scale 1∶250,000), superimposed on the soil type map and land use map to produce a wetland distribution map; then through analysis, and field investigation and verification in typical lots, the soil was classified into swampy soil, submerged rice soil, retention rice soil, and seepage rice soil according to the soil moisture condition and wetland characteristics, and the submerged rice soil was calculated to be 209,400 hectares by the area, which is nearly 30,000 hectares more than that of the Second Soil Census. This is nearly 30,000 hectares more than the second soil census. The argumentation before the opening of the Three Gorges Reservoir has already been mentioned, that the water level of Dongting Lake will rise, and the groundwater level will rise with it, so the latent fertile rice soil will increase to 426,000 hectares. In fact, after the opening of the Three Gorges Reservoir, the water level of Dongting Lake and the groundwater level will be greatly reduced during the dry period, which will be very conducive to the de-submerging of the soil, and thus the submerged soil will be reduced. However, after the opening of the Three Gorges Reservoir, Dongting Lake is in a situation where the rate of sediment deposition is less than the rate of tectonic sedimentation and evolves in the direction of expansion, and because Dongting Lake has been surrounded by dykes and cannot be expanded, the only way is to increase the depth of the water or elevate the water level, which naturally causes the elevation of the groundwater level, which in turn exacerbates the degree of latent fertilization of the soil.
In short, the Dongting Lake soil submergence is an important agricultural geo-environmental problems affecting agricultural production, the development trend of soil submergence is not the Three Gorges Reservoir before the opening of the opening of the argument that the opening of the heavier; but from the near future is in a mitigating trend, but from the long term, it may also be tend to be more serious. Therefore, this is an important issue that needs to continue to strengthen the research.
Photo 4-4 The Yangtze River water can't enter the Hualong River at the mouth of the string transfer in January 2006
Photo 4-5 The Lotus River has been cut off in November 2005
Photo 4-6 The Hudu River has been cut off in December 2005
Photo 4-7 The Songzhi River has been cut off in November 2005
Photo 4-8 The submerged Yuhuawan field in Anxiang Local water level is less than 30cm
Photo 4-9 Groundwater level of Qianyuhuawan field in Nanxian County is less than 35cm
2. Changes in schistosomiasis epidemic areas
Schistosoma eggs with complete shape and clear contents were found in the liver and intestinal tissue sections of female corpses unearthed from the No.1 Han tomb of Changsha's Mawangdui in 1972, which indicates that more than 2,000 years ago, in the Han Dynasty, in Changsha there was an Schistosomiasis is prevalent in Hunan [19], and the earliest case of schistosomiasis found in Hunan was reported in 1905 in the Chinese Medical Journal of Changde Zhoujiadian, an 18-year-old dysentery patient with "Schistosoma japonicum" eggs detected in the feces [20], and since then in 1920 there have been constant reports of people found in Changde, Yiyang, Yueyang, Huarong, etc. [21], and the first case of schistosomiasis found in Changde, Yiyang, Huarong, etc. [22]. Since then, by 1920, there were reports of schistosomiasis patients in Changde, Yiyang, Yueyang, Huarong and other places [21]. The above indicates that schistosomiasis was prevalent in Dongting Lake area a long time ago, and to this day, Dongting Lake area is still the most serious epidemic area of schistosomiasis in China, and the distribution area of the host of schistosomiasis, the nailsnail snail, accounted for 48% of the total distribution area of the country [22]. Among them, Yueyang, Changde, Yiyang and other three cities at the end of 2000 has not yet reached the schistosomiasis epidemic control standards of counties, cities, districts, fields there are 25, the epidemic area population of 3,286,600 people, the patient nearly 100,000, the area of the nail snail more than 1,700 square kilometers, can be seen in the dongting lake area of the seriousness of the schistosomiasis epidemic, which seriously deteriorate the dongting lake area of the agro-geological environment.
Schistosoma haematobium is a kind of endemic disease, and the distribution of the epidemic area is restricted by many factors, including the geological environment, and the epidemic area seriously affects the sustainable development of agricultural economy. Therefore, from the point of view of agricultural geo-environmental evaluation, the study of the change of schistosomiasis infected area is a content that can not be avoided. The study is based on the collection of information about the schistosomiasis infected area, and on this basis, the change of the schistosomiasis infected area is clarified based on the change of the geo-environmental environment of Dongting Lake Area in recent years, and is taken as one of the contents of the evaluation of agricultural geo-environmental environment of Dongting Lake Area. There are three main changes in the agro-geological environment related to the changes in the schistosomiasis infected area.
(1) The water level of Dongting Lake in the dry period after the operation of the Three Gorges Reservoir dropped considerably to extend the distribution range of the snail
Schistosoma must be parasitized in the snail, and the snail with schistosoma parasitism is called the infected snail. Nail snail reproduction is a determining factor is the eggs laid must have mud skin fruit, otherwise it can not survive, so in the mud skin can not be formed underwater and dry ground snails can not reproduce and extinction. Therefore, the Dongting Lake area snails can only be distributed in the dry water level and flood level between the continental beach, because they can be formed between the mud skin with snails breeding conditions for survival. According to years of statistical data, generally in early April before the flooded continental beach and the end of June has not been flooded continental beach no snail distribution. As mentioned before, after the Three Gorges Reservoir operation, before April, February, January and the previous year's December, November water level is greatly reduced, the continental beach is not flooded and there is a large area of exposure, which gives the nails snail to expand the survival and reproduction of the space, thus expanding the scope of the infected area.
(2) after the Three Gorges Reservoir operation of sediment siltation is greatly reduced, so that the wetland ecosystem of the reed beach to the reed beach and grassy beach evolution, and expand the schistosomiasis infected area
Hunan Institute of Parasitic Disease Control [23] by the Dongting Lake system upstream and downstream is divided into four districts, each district to choose the two lakes to investigate the nail snail and the infected snail beach, the results of the survey in 1988 (Table 4-19) shows that In 1988, the survey results (Table 4-19) showed that there was no distribution of infected snails in reed beaches, and in 1998, another survey (Table 4-20) also showed that there was no distribution of infected snails in reed beaches, and both surveys only had distribution of infected snails in grass beaches and reed beaches, which means that grass beaches and reed beaches are schistosome infected areas. After the Three Gorges Reservoir is in operation, the amount of siltation in Dongting Lake will be greatly reduced, which is only (1/8)~(1/5) of that before the operation, and the tectonic sedimentation will still be carried out according to the original rate, and the ecosystem succession in Dongting Lake will be reversed (see Fig. 4-5), that is, the reed beach will be evolved to the grass beach, which will become the infected area of the schistosome with infected snails.
Table 4-19 Density of infected snails in vegetation on different continental beaches in 1988
(3) Expansion of schistosomiasis infected area by the "4350 Project" of returning farmland to the lake
The Hunan Institute of Schistosomiasis Prevention and Control and the Schistosomiasis Prevention and Control Offices of Yueyang City, Changde City and Yiyang City, Hunan, conducted a study on the "4350 Project" of returning farmland to the lake in 1988. "
The Hunan Institute of Schistosomiasis Control and the Schistosomiasis Control Offices of Yueyang City and Yiyang City, Hunan Province, investigated the spread of snails in the dykes and lakes returned to the lakes by the "4350 Project.
1) 41 out of 206 double retreated dykes and lakes (retreated fields and retreated people) were investigated, 7 dykes and lakes had nails before retreated, and 9 dykes and lakes had nails after retreated; the area of nails before retreated was 210.34 hectares, and after retreated it amounted to 1,079.74 hectares, which was 5.13 times as much as that before retreated.
Table 4-20 Relationship between schistosomiasis and water level elevation and vegetation in Dongting Lake Area
2) Comparative study of 4 retired lakes and Xiyang village of Li County, which has not been retired, with Haokou of Li County, Zhongshan village of Xiangyin, Qingshanwan of Hanshou, and Huarong Integratedwan. 4 retired lakes (waste) had a significant increase in the density of live snails (P<0.05), and the unretired lakes did not have any significant increase (P>0.05). The density of live snails increased significantly (P>0.05) in the four retired lakes and no significant increase (P>0.05) in the non-retired lakes.
3) Democracywan in Ziyang District, Yiyang, was destroyed in 1999, and a survey of snails was conducted in Zengming and Minglang villages. The densities of live snails and positive snails increased by 84.62% and 73.17% in Zengming Village and 70.62% and 83.74% in Minglang Village, respectively.
The above information shows that the return of land to the lakewan and the collapse of thewan due to the "snail with the attachment to the water drift into the" increase, greatly aggravate the schistosomiasis epidemic.
According to the Three Gorges Reservoir operation, the water level of Dongting Lake is lowered during the dry water period, the rate of sedimentation is less than the rate of tectonic sedimentation and the wetland ecosystem reverse succession occurs, as well as the implementation of the "4350 Project" of returning the land to the lake, the schistosomiasis epidemic in the Dongting Lake area will be changed and tends to be more serious.
3. Interference with natural succession of wetland ecosystem
(1) Meaning of ecosystem succession
Ecosystem is a dynamic system, and ecosystem dynamics contains two aspects of succession and evolution. The evolution of an ecosystem refers to the change of the system over a long period of time on a large scale, which is the result of the heterogenous process of the influence of long-term changes in the external environment, such as geology and climate, as well as the autogenous process of the formation and emergence of new species of biological communities **** the same action; the ecosystem succession includes the change of the living system and the non-living system over a short time scale, which occurs over a relatively short period of time due to the conditions affecting the system of the change so that one ecosystem type is replaced by another.
(2) Ecosystem succession of Dongting Lake wetland
The ecosystem succession of Dongting Lake wetland is divided into the ecosystem succession of artificial wetland and natural wetland. This book focuses on the ecosystem succession of natural wetlands, and the succession diagram is shown in Fig. 4-5. From the diagram, it can be seen that the non-living system of natural wetlands is the water body - low continental bank - middle continental bank - high continental bank; and the living system is the corresponding aquatic vegetation - meadow vegetation -forest vegetation. Their succession is mainly determined by two factors when other conditions are basically the same: one is the relative relationship between tectonic deposition and sedimentation in the natural state; the other is man-made economic construction.
In the natural state, when the rate of tectonic subsidence is less than the rate of sedimentation, the Dongting Lake ecosystem is undergoing a positive succession, the water body for the continental beach instead of the continental beach, the continental beach is constantly silted up, from the white mud beach and the low continental beach and the middle continental beach and the high continental beach; and at the same time, its biological changes, in short, is the shrinkage of the dongting lake to the wetland marshy evolution, this evolution has lasted for nearly 200 years until the Three Gorges Reservoir This evolution has continued for nearly 200 years until the Three Gorges Reservoir operated. After the operation of the Three Gorges Reservoir, the rate of tectonic sedimentation is greater than the rate of sediment deposition, and the wetland ecosystem will undergo a reverse succession. Ecosystem succession in a natural state, whether positive or negative, is a slow and gradual process that is adaptable to living systems without ecological catastrophe.
Figure 4-5 Schematic diagram of ecosystem succession in natural wetlands of Dongting Lake Area
The impact of man-made economic construction on ecosystem succession mainly refers to some large-scale projects that can change the natural conditions in a considerable range, for example, the construction and opening of the Three Gorges Reservoir. After the opening of the Three Gorges Reservoir, as mentioned before, the water level of Dongting Lake dropped severely during the dry season, the water body shrank or even dried up, and the continental beach was exposed, so the boom of continental beach afforestation emerged, and the wetland ecosystem underwent a completely man-made succession. This kind of succession is realized in a short period of time, life activities are difficult to adapt, so it will cause ecological disaster.
(3) the development of poplar planting is not according to the laws of nature of the wetland ecosystem of man-made succession
Dongting Lake area to develop poplar planting is to increase agricultural income and industrial structure adjustment, but must comply with the laws of wetland ecosystem succession, and thus the planting area of poplar should be moderate. Now on the dongting lake wetland poplar planting area and its impact on several issues from the perspective of agro-geological environmental assessment is analyzed.
1) There are three kinds of opinions on poplar planting area:
One is that the area where poplar can be planted by senior engineer Li Yanifu of Hunan Forestry Department[24] through investigation and calculation is:
1.29 million mu of Pingwan flooding (Bawan, Xiaowan, Dawan); 2 million mu of arable land not suitable for planting crops; 2.5 million mu next to the ditches, roadsides, and residences; 700,000 mu of low-yield reed beach; 28.5 million mu of water level; and 0.5 million mu of water level; and 0.2 million mu of water level. 700,000 mu; 1,000,000 mu of lakes above the water level of 28.5m; 800,000 mu of hills and barren forested mountains around the lake. Above *** total 8.3 million mu, if you remove some unexpected factors, at least 5.3 million mu can be planted poplar.
The second is the Hunan Forestry Academy of Sciences Yuan Zhengke researcher through the investigation and research [25] that can be planted poplar area are: the ditch, road, village next to the gap and empty ping 73.95 million acres; 45.45 million acres of high continental beaches; storage of floods 513,000 acres ofwan; 24 million acres of flatwan floodingwan. The above **** totaled 1.497 million mu.
Third, Changde City, Yiyang City, Yueyang City proposed poplar planting area *** counting 8.5 million mu.
2) has formed a poplar planting boom. According to the Hunan Provincial People's Congress, Hunan Forestry Department, the World Wide Fund for Nature in December 2005 to January 2006 organization of the Chinese Academy of Sciences, Hunan universities, Hunan departments and the World Wide Fund for Nature and other units of experts carried out a scientific investigation around the Dongting Lake (I participated in the scientific research): the Dongting Lake District, the wind of poplar planting from the 1990s, to the Lake District in 2002 Poplar planting area has reached 850,000 acres, has developed to 3 million acres, and there is a further spread and the development of the trend, such as in 2005, Yuanjiang City, plans to plant 300,000 acres of poplar, Changde City, the development of poplar included in the city's "five one million acres within the industrial restructuring plan. The rapid development of poplar planting in recent years is directly related to the lack of water in Dongting Lake during the dry season and the exposure of the island beaches, which is the result of the operation of the Three Gorges Reservoir, for example, in February 2005, poplar trees were planted in the grassy beaches of the southern Dongting Lake by ditch-digging bulldozers (Photos 4-10 and 4-11).
(4) Consequences of man-made succession
1) The root system of poplar trees must be above the water table, and once the dry season is over and the water table rises, the root system will rot, so poplar trees do not have deep roots (Photo 4-12,4-13), and when they grow and develop to a certain size, they will fall over, and the poplar trees will not be able to become a timber. For example, the South Dongting Lake in the southeast of the lake abutment continent and the south of the southeast of the continent, the south of the Meiping Lake Chau Beach, East Dongting Lake Huarong newbornwan, etc., because of the low terrain, the water table is high, the diameter of the poplar forests up to 8 ~ 14cm when a large area of the collapse (Photo 4-14).
2) It is completely artificial to change the naturally formed grass and reed beaches into forest beaches (Photo 4-15,4-16), and this artificial succession violates the laws of nature, destroys the ecology, and the poplar planting may not be able to achieve the expected economic benefits, which is just as the scientific research group of the Lake Dongting Lake has given to the Hunan Provincial Party Committee, the Provincial People's Congress, and the Provincial Government in its "Recommendations on the Conservation and Management of Wetlands of International Importance of Lake Dongting", the second article of which says: "The protection and management of wetlands of international importance of the Lake Dongting is not a matter of concern, but of the human rights. The second article of the "Recommendations" said: "The large-scale planting and flooding of poplar trees in the lake area has seriously damaged the integrity and continuity of the ecosystem structure of the lake wetland, resulting in the wetland ecosystem to the land ecosystem succession, threatening the survival of fish, aquatic animals and birds; at the same time in the continental wetland planting field, forests will cause the wetland vegetation community decline and even large-scale death, resulting in a single species of 'green green' wetlands. This will result in a 'green desert' landscape with a single species, making the biodiversity of lake wetlands decline. In particular, the unpredictable threat of potential pests and diseases (see Photo 4-17) can be devastating to the survival of native species. Meanwhile, the expansion of the forest and paper industry around Dongting Lake will also result in increased water quality pollution in the lake. Therefore, it has become urgent and important to strictly control the spread of poplar in the lake area, carry out structural adjustment of production and operation in the lake wetland, and protect and restore the ecological environment quality of the lake wetland. We suggest that the development and utilization of wetland resources in Dongting Lake should be planned without affecting the structure and function of the wetland as a prerequisite, and the development and utilization should be planned on the basis of protection. In the present situation, the 'three disallowances' rectification measures should be seriously implemented, i.e., immediately prohibiting the continued planting of poplar trees in the 'core area of the protected area, basic farmland area and flooding channel', and at the same time should also sternly prohibit the planting of poplar trees in the middle and low level beaches, especially the planting of poplar trees in the low level beaches in the form of It is also strictly prohibited to plant poplar trees in the middle and low level of the beach, especially to plant poplar trees or reeds in the low level of the beach by digging ditches and draining. The poplar trees that have been planted should be cut down or removed within a certain period of time in order to restore the natural landscape of lakes and wetlands". The provincial government attaches great importance to the proposal, Zhou Bohua May 9, 2006 instructions "on the Dongting Lake Wetland of international importance protection and management of a number of proposals" from a group of experts and scholars around the Dongting Lake field visits, read very inspired, please Comrade Taibo convene the Development and Reform Commission, the Forestry Department, the Department of Agriculture, the Agricultural Office, the Environmental Protection Bureau, and other units of the serious study, the adoption of the proposal to formulate measures to form a document, and effectively improve the Dongting Lake Wetland. Formation of documents to effectively improve the protection and management level of the Dongting Lake International Important Wetland".
4. land sand
Sanded land is sand siltation due to the "rapid fall of sand, slow fall of mud," formed, so the general distribution in the sand channel, flood channel on both sides of the intermittent "rosary", but also in the dike breach near the distribution.
The harm of sandy land is mainly manifested in the increase of stained fields, blocking waterways, schistosomiasis epidemic aggravation and land degradation, etc. According to the survey [26], the annual loss due to sand damage in the Dongting Lake area amounted to 155 million yuan, of which 893 villages were affected by sand damage with 68.6 thousand hectares of arable land (Photo 4-18), 18.8 thousand m2 of grassland (Photo 4-19), and roads and waterways suffered from sand damage. ) and highways and canals suffer from sand damage.
Photos 4-10,4-11 Ditching and planting poplar trees on grassy beaches of South Dongting Lake
Photos 4-12,4-13 Poplar trees not y rooted
Photos 4-14 Poplar trees planted on reed beaches collapsed
Photos 4-15 Grassy beaches artificially turned into forest beaches
Photos 4-16 Grassy beaches artificially turned into forest beaches
Photo 4-17 Poplar Tree Insect Pests
The source of sand that leads to sand damage is mainly the sediment brought in by the three mouths of the Jingjiang River. After the operation of Three Gorges Reservoir, the amount of sediment that flows into the lake and silts up through the three mouths of the diversion has been greatly reduced (see Table 1-5 and Fig. 1-8), and it is only (1/8)~(1/10) of that before the operation. Therefore, it is very favorable to reduce the land sanding in Dongting Lake area. However, due to the dry season, the water table of the continental beach decline is large, so that the land is dry, vegetation (crops) is not easy to grow or even can not survive, coupled with this period of the lake area windy, and thus in the wind often occurs in the "flying sand", the formation of mobile sand. In general, the Three Gorges Reservoir operation is conducive to the improvement of sandy land.
5. Agricultural rodent
The Oriental vole (Microius fortius) (Photo 4-20) is an important agricultural rodent pest in the Dongting Lake area. Generally speaking, the Oriental vole inhabits the lakeshore in the winter and spring (November to the following April) during the dry season, and its reproduction intensity is large during this period, and it can be 20d to reproduce a litter; in the late spring and summer, with the rise of lake level, the continental beach is gradually submerged, and the Oriental vole is gradually flooded. In late spring and early summer, as the water level of the lake rises, the lake is gradually inundated, and the vole migrates to the dykes by swimming, endangering crops (Photo 4-21), and at the same time, it may spread leptospirosis and epidemiological hemorrhagic fever and other diseases and epidemics [27].
Agricultural rodent damage caused by the eastern vole in the dyke is related to the continental beach in the lake outside the dyke in the dyke. In those fields where the lake outside the levee has a continental bank, the vole migrates into the field and harms the farmland, and in those fields where there is no continental bank in the lake outside the levee, the vole is basically not found to be active. The degree of damage to farmland caused by the vole is related to the number of breeding populations on the island beaches, which in turn is determined by the number of consecutive days that the beaches are exposed during the dry season in winter and spring. The more days of exposure, the longer the breeding period of the Oriental vole, the larger the population, and the more serious the damage caused by agricultural rodents.
Three Gorges reservoir operation before Zou Shao Lin and others on the Three Gorges reservoir operation after the eastern vole agricultural rodent pests have been studied [28], he cited the relevant literature: "the Three Gorges reservoir in January to June to increase the discharge flow, the water level rises, the continental beach exposed to the number of days and the area of exposure to reduce the July to September basically does not change the discharge flow, the continental beach exposed to the same situation; October to December to reduce the discharge flow, the water level increases, the continental beach exposed to the same condition; 10 to December to reduce the discharge flow, the water level, the number of days of continuous exposure, and the area of exposure to the same condition; the number of days of continuous exposure. In July to September, the discharge flow is basically unchanged, and the exposed area of the continental beach remains unchanged; in October to December, the discharge flow is reduced, the water level decreases, and the number of exposed days and area of the continental beach increase. It is believed that the operation of the Three Gorges Reservoir is unfavorable to the breeding of the eastern vole except for the period from October to December. However, in the three years after the operation of the Three Gorges Reservoir, the observation data proved that the water level was lowered in October, November, December, January, February and March, i.e., the whole dry period, and the number and area of open air on the continental beaches increased, which was very favorable for the breeding of the voles, and greatly favored the increase of their population, thus aggravating the agricultural rodent infestation. In fact, since the operation of the Three Gorges Reservoir, agricultural rat infestation has tended to become more serious, especially in 2005, when the most serious rat infestation occurred in the past 10 years. Rat extermination must not be organized under (Photo 4-22,4-23). It is reported [29], the Chinese Academy of Sciences Institute of Subtropical Agricultural Ecology, the Ministry of Agriculture and Hunan Provincial Department of Agriculture and other relevant departments to form an expert group, went to the Dongting Lake area to investigate the rat, found that the number of outbreaks of voles in the East for the last 10 years the most serious one, only the South Dongting Lake area of the rodent damage to the area of 68,000 acres, the dike on both sides of an average of every square meter there are rat holes 5 to 6, as many as 15, each hole hide rats 5 to 10 only, and the number of rats in the area of the dike. Through the rice paddies to catch up with the rats, the amount of rats is visually estimated to be 300 to 500 per mu, with more than 1,000 rats. The cause of such serious rodent infestation, experts believe that from 2003 to 2004, Dongting Lake is in a low-water state, the river beach, the lake beach area increases, is conducive to the breeding of Oriental voles. Obviously this is not unrelated to the reduction of water volume in Dongting Lake during the dry period after the operation of the Three Gorges Reservoir.
6. Impact on Dongting Lake flood control and levee engineering geology
The Three Gorges Reservoir after the opening of the flood control of the Dongting Lake area in general is very beneficial, but there are always two sides of things, and thus the operation of the Three Gorges Reservoir in a number of issues, the flood control of the favorable at the same time there is also unfavorable.
1) Three Gorges Reservoir operation after sedimentation rate will be much smaller than the rate of tectonic sedimentation, so will increase the capacity of Dongting Lake, which should be conducive to flood control, but in fact, not. Figure 4-6 is the dongting lake various periods of the water body diagram, can play a role in flood storage is the high flood level below the flood season water body and flood storage level below the flood storage water body; Although the rate of sedimentation is greater than the rate of sediment siltation will increase the lake capacity, but the increase in the lake capacity in the flood season has been filled with the water body is only an increase in the level of the water body only to the flood season it has been unable to play a role. Therefore, the increase in lake volume due to tectonic sedimentation over sedimentation is not meaningful for flood control for quite some time, but it is meaningful in the long run.
Photo 4-18 Sanding of arable land (South Dongting Lake)
Photo 4-19 Sanding of grassland (South Dongting Lake)
Photo 4-20 Oriental vole
Photo 4-21 Seedlings nibbled by Oriental vole
Photo 4-22 Rat tails handed in by masses of people exterminating rats
Photo 4-23 Battle of exterminating rats on the shore of Dongting Lake
2) Sediment siltation rate is less than the rate of tectonic sedimentation on the engineering geological stability of the levee has a negative impact, manifested in two points: First, the increase in the foot of the levee and the lake bottom of the height difference affects the stability of the levee; the second is the Jingjiang River due to the operation of the Three Gorges Reservoir occurs from the top down for a long time and long-distance along the course of scouring the riverbed in the scouring process, the foot of the levee is prone to be amassed, the levee, the stability of the shoreline engineering has an adverse effect on the stability of the dyke. The stability of the embankment and bank protection works is adversely affected. For example, in March 2006, Huarong County, the lower Jingjiang River, Tianzhi No. 1 section of the 600m serious bank failure endangered the Yangtze River embankment (Photo 4-24), one of the reasons for bank failure Yueyang City, Yangtze River Repair and Defense Engineering Section, Wu Wensheng, section chief that is the operation of the Three Gorges Reservoir, the discharge of clear water, the capacity of its sand hosting enhancement, so that the riverbed of the lower Jingjiang River scouring seriously due to the [30].
Figure 4-6 Schematic diagram of water bodies in Dongting Lake in various periods
Photo 4-24 Yangtze River embankment collapse
7. Agricultural water use in the dry season
Agricultural water use includes drinking water for people and livestock and irrigation water for farmland
1) According to the information provided by Zhu Xiang et al [31], in the Song Li area in the western Dongting Lake area, due to the dry season, the Songzi River cuts off its flow. , coupled with the river at this time the river flow is small, the water level is low, resulting in the Song Li area spring irrigation water shortage of arable land amounted to 47,300 hectares, about 410,000 people have difficulty in drinking water. And according to the provincial people's Congress in March 2005, Liu Shuai Environmental Resources Committee pointed out that Hualong County seat of more than 160,000 people, the daily water supply shortfall of 0.5 million to 10,000t, and to 0.3 million t / d increase; January 10, 2006 Hualong County Water Resources Bureau, deputy director of Liu Yangchun, senior engineers to provide the information, the Hualong County, along the county town of domestic water supply to the general to be 30,000t / d, the peak of up to 50,000t / d, and now can not be met at all!
The water supply can only be taken in the morning, midday and evening at regular intervals, and the shortage of water is particularly serious in the dry season.
The problem of agricultural water shortage during the dry season, the previous argument is to increase the flow of the Three Gorges Reservoir during the dry season, so that the Yangtze River and Dongting Lake water level rise and get relief. But as mentioned earlier, this argument and the Three Gorges Reservoir after the operation of the actual opposite, not only did not raise the water level, but also a substantial reduction, so the dry period of agricultural water more tense.
2) After the Three Gorges dam reservoir will form a huge volume of 39.3 billion m3 of water, the reservoir began to store water in October each year, a large body of water itself has a strong cold storage effect, coupled with the reservoir water temperature structure, the reservoir is mostly the bottom of the discharge of cold water, which makes the dam below the runoff water temperature is significantly lower than that before the dam was built. The low water temperature of the runoff from the reservoir will be unfavorable to crop growth and development when it is introduced into the farmland, which is also an unfavorable influence after the operation of the Three Gorges Reservoir.
The above seven aspects of the Three Gorges Reservoir after the operation of the new problems on the impact of agriculture, the root cause is the Three Gorges Reservoir to enable the Yangtze River according to the needs of power generation, shipping flow scheduling and change the hydrological conditions of Dongting Lake, the result is that the water level in the dry season is lowered and lead to a substantial decrease in the shallow groundwater table, and therefore, it is an agricultural geo-environmental assessment of the problem. The reality of Dongting Lake water level and groundwater level lowering is completely opposite to all the arguments made by the relevant departments before the operation of the Three Gorges Reservoir and the ecological problems arising therefrom. Therefore the agro-geological environmental evaluation of these problems is an innovation, more necessary and more meaningful.