1 The water-holding function of soil can not be ignored.
Soil is a medium with biological activity and pore structure developed on the land surface of the earth. Its main function in the ecosystem is to provide material support, water, nutrients and even air for plants. Because of the porosity of soil, it can not only store water, but also store water. The water storage capacity of soil depends on a series of factors. Among them, vegetation is the most important, because there is a deep ground cover under the forest, a grass felt layer on the grassland, and an organic layer with good water retention and permeability on the topsoil. Therefore, the soil covered with plants can play a role in retaining precipitation and regulating runoff [3]. According to research, the precipitation blocked by forest land accounts for about 30% of annual precipitation, and 5% ~ 30% of surface runoff can be converted into soil water or groundwater. The water storage capacity of grassland is between forested land and uncultivated land (bare land). The maximum water holding capacity of the surface soil of Chinese fir forest is 80 ~ 90 mm, and the precipitation below 10 mm can be completely absorbed by the undergrowth. It is observed that the runoff coefficient of farmland with 95% vegetation coverage is 0.23, while that of farmland with 15% vegetation coverage is 0.59. Generally, well-developed forests (forest, shrub, grass and litter layer) are only 0. 1% of agricultural land, or even lower. According to 1 hm2, the field water-holding capacity of 30 cm thick ground cover and topsoil is 1000 ~ 1500 t, and the water-holding capacity of forest soil is equivalent to a water-holding capacity of 1 10,000 ~10.5 million. It can be seen that the function of soil absorbing and storing water mainly depends on the surface cover and soil organic layer. The process of soil erosion begins with the destruction of vegetation and the thinning or disappearance of surface cover, which leads to the surface soil being washed away, the soil layer becoming thinner and the soil physical properties becoming worse, which leads to the decrease of soil water storage, the increase of surface runoff and the intensification of soil erosion. At the same time, vegetation has rapidly evolved into a grass and shrub community. Compared with forest community, the structure and species composition of grass-shrub community are greatly simplified, the self-regulation ability of ecosystem is reduced, and the most important thing is the deterioration of soil conditions on which plants depend, thus reducing productivity. Due to the erosion of topsoil, the compact and barren subsoil rises relatively or is directly exposed. If there is no external energy and material input, it is slow to restore vegetation only by natural conditions themselves. If we continue to cut down vegetation, it will become grassland or bare land or rocky hillside, completely losing the use value of agriculture and forestry. "Without skin, there is no hair", and the loss of ground cover and soil organic layer under the forest means the disappearance of a large number of reservoirs. Once the flood comes, the forest soil that could have become a "mainstay" will be destroyed at a loss.
Water control should be combined with soil control and soil protection.
Soil is a fragile thin layer on the earth's surface. The formation of soil is very slow, and it may take 20 thousand years to form 1 m soil layer. The location and quantity of soil are fixed and cannot be introduced from other places. Therefore, it can be considered that soil is a non-renewable resource and must be cherished. There is a close relationship between soil and water. As early as the Spring and Autumn Period and the early Warring States Period, our ancestors knew and mastered the dialectical relationship between water and soil. According to "Biography of Wu in the Spring and Autumn Period", in the era, there were nine years of floods, and people moved to mountainous areas to live. At this time, Tang Yao hired agronomists to help people plant in mountainous areas (i.e. hole planting and nest planting) to conserve water and soil and fertilizer. According to Records of Food Goods in Hanshu, Xia Dynasty was ordered to control water 13 years, leveling soil and water, dividing soil nationwide, cultivating according to local conditions, and opening canals for irrigation and drainage [4]. This idea of combining water control with soil control still has positive guiding significance. From 65438 to 0998, the north and south of our country suffered a serious flood, which warned us from another aspect: we should control water and soil at the same time. The combination of water control and soil control is conducive to consolidating water conservancy achievements. As we all know, the flood of the Yangtze River 1998 is second to that of 1954, and the flood in some reaches is less than that of the 1950s 104 m3/s, but the flood level is several tens of centimeters or one or two meters higher than that of 1954. This phenomenon is directly related to excessive logging, vegetation destruction, soil erosion and the increase of river sediment in the basin. At present, the riverbed of the Yangtze River is rising at an annual rate of 1 cm, and the Yangtze River is in danger of becoming a "suspended river". It is a fact that the flood level in Jingjiang flood season is more than 8 meters above the ground on both sides of the river. Due to the siltation of rivers and lakes, the riverbed at the bottom of the lake rises, the water capacity decreases, the capacity of flood peak regulation and storage decreases, and the flood discharge is not smooth. Even if the amount of water is not large, it will form a higher water level, resulting in a situation of "light rain, high water level and great flood". Draw lessons from the positive and negative experiences of flood control in ancient and modern rivers. It is put forward that soil management and protection should not be forgotten after the disaster, that is, while strengthening water conservancy construction and improving flood control ability, more attention should be paid to rational use and optimal allocation of land and comprehensive land management.
3 Main ways of soil control and soil protection
3. 1 Adjust the land use structure, return farmland to forests and grasslands on steep slopes, and restore vegetation.
The most prominent problem in the upper reaches of the Yangtze River is excessive deforestation, which increases the area and intensity of soil erosion and soil erosion. The forest coverage rate dropped from 30% to 40% in the early days of the People's Republic of China to 10% now, and the area of soil erosion reached 352,000 square kilometers. The total annual soil erosion has increased from 654.38+0.3 billion t in the past to 65.438+0.5 billion t at present, accounting for more than half of the total erosion in the whole basin (2.4 billion t), equivalent to 360,000 hm2 of cultivated land every year. The Ministry of Forestry has decided to prohibit the felling of natural trees in the upper reaches from 1998.09. 1, and to close hillsides to plant trees. By the year 2000, it is undoubtedly correct that cultivated land with steep slopes above 25 degrees should be returned to forests. In order to achieve the goal of building shelter forest in the upper reaches of the Yangtze River, on the one hand, we should pay attention to scientific afforestation and greening. First of all, macro-planning should be carried out on the layout of forest species to avoid blindness and waste of investment. Secondly, we should pay attention to afforestation methods, prohibit all-cultivation afforestation, advocate belt afforestation, and prevent soil and water loss and air pollution. Thirdly, it is necessary to emphasize the artificial multi-layer vegetation of mixed conifers and broadleaves, and the combination of arbor, shrub and grass, and select and match tree species according to the slope and soil thickness. On the other hand, returning farmland to forests on steep slopes is a difficult problem, because in some poor mountainous areas, almost all sloping farmland exceeds 25 degrees. In order to solve the food rations that people depend on, it is suggested to develop agroforestry first and gradually return farmland to forests.
3.2 Taking small watersheds as units and changing sloping farmland into terraces, an eco-agricultural engineering system for soil and water conservation was established.
The sloping farmland in the Yangtze River basin is concentrated in the middle and upper reaches, especially in the vast hilly areas of the upper reaches. The soil erosion of these farmland is very serious. According to the investigation in typical areas, the annual erosion of cultivated land accounts for 60% of the total annual soil erosion in this area, which seriously restricts the life of reservoirs and the improvement of land productivity. However, in eastern Sichuan, from 199 1, the "prevention first, comprehensive planning" proposed by the soil and water conservation law was implemented; Comprehensive management, according to local conditions; Strengthening management and paying attention to benefits ",taking small watersheds as a unit, insisting on transforming sloping farmland into basic farmland as a breakthrough, has achieved initial results. According to multi-point investigation, its practice can be summarized as: (1) act according to local conditions and capabilities. In gentle slope areas, high-grade farming such as grain and grass can be implemented, and long slopes can be changed into short slopes to slow down runoff; On the slope where citrus is planted, ditches and large nests can be planted. (2) According to the topographical features, the sloping farmland is changed into terraced fields. The first condition is to do a good job in plot planning and determine the height, top width and slope of the plot. (3) Harnessing the slope water system to turn harm into benefit. In order to avoid the collapse of the bank caused by the rapid flow of water on the slope after rain, the terraced fields must be planned and carried out simultaneously with the reconstruction of the slope water system. (4) Accelerate the pace of fertilization and adopt supporting high-yield techniques. It is necessary to apply more organic fertilizer, appropriately increase green manure intercropping, or increase the application amount of organic fertilizer by using high-yield furrow combined with soil accumulation ridge and furrow method. (5) Pay close attention to the utilization after transformation and improve the multiple cropping index. Terrace soil layer is thickened, texture is improved, and water and fertilizer are enhanced. Combined with improved varieties popularization, standardized cultivation, formula fertilization and pest control, not only the planting area in Koharu can be expanded, but also the paddy-upland rotation can be implemented.
3.3 adjust measures to local conditions, promote agriculture and forestry, and develop diversified economy.
The hilly and mountainous areas in the Yangtze River basin account for 85% of the total land area, which is characterized by large ups and downs, many slopes and thin soil layers. The utilization of sloping land should not only preserve soil and improve fertility, but also prevent soil erosion. In addition to terraced fields, the best way is to develop agriculture and forestry, because agriculture and forestry can resist corrosion and make rainwater fully penetrate and stay in the soil in large quantities. The main feature of agroforestry is to allocate different land uses in a business unit, such as the ecological model of fruit and tea farmers in subtropical areas, and use their different biological characteristics to form three-dimensional planting. At the same time, the distribution of roots is different, which can fully absorb and utilize soil nutrients and water. Litter can increase the accumulation of soil organic matter, improve the ecological environment of iron-rich and aluminum-rich soil, and also help young forests and young fruits to interplant crops (peanuts, dry grain, green manure, melons, fruits, etc.). ) to improve soil coverage and achieve the purpose of soil and water conservation [6].
3.4 Rebuilding homes and transforming low lake fields should be combined with harnessing rivers and lakes.
The middle and lower reaches of the Yangtze River are the most concentrated, serious and frequent areas in the whole basin. Because the ground elevation is generally lower than a few meters to a dozen meters, there are more than 3,600 kilometers of Yangtze River trunk dikes and more than 30,000 kilometers of branch dikes for protection. When the flood season comes every year, the flood control situation is very severe. After the disaster, we should adhere to a high starting point, re-plan the overall land use planning, combine the construction of new cities with the transformation of old cities, combine the comprehensive management of low lake fields and the construction of ecological environment with the management of rivers and lakes, return farmland that is not conducive to flood storage and discharge to lakes, strictly control the construction of towns and flood diversion areas and the population size, rationally distribute and save land, and formulate a comprehensive management and development plan that integrates economic, social and environmental benefits, so as to make the affected areas.
3.5 to prevent "hidden erosion" of paddy soil
There are three types of soil erosion. One is gradual erosion, such as sheet erosion and gully erosion. The second is sudden erosion, such as gravity erosion. Third, invisible and imperceptible "hidden" erosion. The first two are mainly distributed in sloping farmland, hillside and valley slope, while the latter mostly occurs in paddy fields. According to the investigation, not only the sloping land, but also the paddy field in the plain, a large number of clay particles are brought into the river by series irrigation or hydroponics harrow. In mountainous and hilly areas, the clay particles carried away by the water flowing out of the field per unit area can account for 0.4% of the total topsoil; In plain areas, soil erosion often occurs. Taking Suzhou as an example, the annual total siltation of rivers can reach 654.38+270 million m3. The Yangtze River Basin is the most concentrated and largest paddy field in China. In the long run, rivers and lakes will be silted up, which will bring serious consequences. We have to pay attention to and guard against it.
The Yangtze River Basin is a huge and complex eco-economic system. Located in the subtropical monsoon climate zone of East Asia, the precipitation varies greatly from year to year and the regional distribution is uneven, which leads to frequent floods. In addition, agricultural and forestry production is a process of economic reproduction and natural reproduction, which has strong dependence on natural conditions, long production cycle and slow investment recovery, which determines the long-term and arduous task of water and soil control. Drawing lessons from the past, we should not forget to control and protect the soil in water control. We should strengthen the post-disaster water conservancy construction, emphasize the rational use of land according to local conditions and optimize the allocation of resources. Soil control and soil protection should focus on the whole basin system, unified planning, returning farmland to forests and grasslands in the upper reaches, changing slopes into terraces and developing compound agriculture and forestry. The middle and lower reaches should seize the opportunity to carry out post-disaster reconstruction from a high starting point, return farmland to lakes, comprehensively control low lake fields, prevent "hidden erosion" of paddy fields, and prevent floods and disasters before they happen.