| Since 1970', the influence of the nonpoint source (NPS) pollution to the environment has been revealed, and become the main source of damaging the water quality. Because of the uncertainty and extensity, it is impossible to inspect all the nonpoint source pollution in the field. The evaluation of the NPS pollution is a cross-discipline cooperation research, involve several disciplines such as statistical analysis, 3S technology, hydrology and agrology, etc., and these disciplines need some organic integrated to suitable for the study of the NPS pollution. The model of the NPS pollution is a complex simulated tool, which combined to the advanced theory and technology of those disciplines shown above. Through imitating natural nonpoint source pollution, it simplifies the hydrologic, physical and chemical procedure of the generation, delivery and transformation of the nonpoint source pollutant. It can be applied to make optimum measure for managing and controlling the nonpoint source pollutant entering the water body, and fulfil the aim to slow down or dispel the influence of the pollution to water quality, realize the sustainable utilization of natural resources.Natural environment would be changed after Three Gorges Dam founded. Since its special geography location and agro-structure, hazard of agricultural nonpoint source pollution in Three Gorges Area would be increase. In this study, comparison analysis, statistics and modeling were used to evaluate Three Gorges Area' agro-NPS pollution, including generation of pollution, pollutant loads, and process of NPS pollution. The results showed: 1 Character of NPS pollution in Three Gorges AreaAgro-NPS pollutants are main pollutants in Yangtse River water body, especially for suspended sediment, 90% sediment in Yangtse water body were come from cropland loss. The main terrains of Three Gorges Area were hill country. Because slop-croplands take a big portion of the total cropland and have low vegetation cover, Soil loss was facilitated to generate in Three Gorges Area. Soil loss is dominant factor of soil nutrient loss in Three Gorges Area. Serious soil loss made productivity of slop cropland decreased or lost, even made hammada in karst area. For the Three Gorges Reservoir, sediment carried by runoff increased suspend sediment load in the Reservoir water body; excessive sediment deposited in the Reservoir would be decreased the Reservoir's capability; nutrients also transported by soil and became water pollutant. 2 Varieties of water and soil environment(1) After the water storage of Three George Reservoir, the water surface of reservoir area has widen 2-3 times, depth of water increased to 2 times, the speed of water stream decreased to 1/4-1/5. The detention time of pollutant prolonged, which was 6.7days from LuoHuang to BeiShi before the Reservoir founded, but 148.4days under 175m water level after it finished. The extension of pollutant's detention time aggravates the water pollution. Eutrophication would even take place in some first class branches of reservoir area.(2) The periodic water fluctuations of Three Gorges Reservoir area promote the weathering progress of the parent rock in the riverbank.(3) The bank soil corrosion by the reservoir stream has aggravated the soil erosion.(4) The soaking of the soil by the reservoir water has a significance effect to the soil material dissolution, which not only reduce soil quality, but also cause water pollution after entering water body.(5) River bank area has buffering effect to nonpoint source pollution as a large-scale artificial wetland. The water plant inside can suppress the flood; prevent the soil erosion by the torrential rain and stream. As the activity region of the transportation and transformation of the energy and material in the reservoir area ecosystem, it has a strong function to hold the material from the farmland runoff and nonpoint source pollution, facilitated to the detention, collection and degradation of the organic matter, significant to improving water quality, purifying sewages.(6) The precipitation or absorption of organic matter and some solid particles in water can supplement soil organic matter, and improve the soil physical property. Nutrition element of sewage, such as N, P, can increase soil nutrients, improve the fertility of the soil to a certain extent, after absorption and transformation by soil.(7) There are similarities in agricultural environment and condition of nonpoint source pollution between Taihu Lake and Three Gorges Reservoir. But, NPS pollutants in Taihu Lake were mainly generated by paddy filed. In contrast, in the Three Gorges area, these generated because of the erosion of upland and hill. Only in Chongqing area, the loading of total surface runoff was 24.94 million tons in 2002. It's indicated that agricultural nonpoint source pollution would become a serious problem in Three Gorges area.3 Chemical fertilizer N, P pollution load in Three Gorges reservoir areaThe regress equation between the intention of N pollution source of reservoir area and application amount of fertilizer is:y = -6.3921 + 0.3127xAccording to the equation, it can be predicted that N fertile pollution source intensity of reservoir area is 24,352.94t.The regress equation between N loss of surface runoff and application amount of fertilizer is:y = 9.6999 + 0.0918xAccording to the equation, it can be predicted that N loss of surface runoff is 7,160.92t in 2003. The regress equation between the intention of P pollution source of reservoir area and application amount of fertilizer is:y = 7.9056 + 0.1258xAccording to the equation, it can be predicted that P fertile pollution source intensity of reservoir area is 2,755.5t.The regress equation between P loss of surface runoff and application amount of fertilizer is:y = 982.56 + 98.969xAccording to the equation, it can be predicted that P loss of surface runoff is 1.166.4L The pollution of N, P fertilizer has a little effect for water quality, only contributes 0.043mg/l, 0.004mg/l to the actual pollution of water body of the Yangtse River, respectively; reach the first-class national standard of earth's surface water quality (N< 0.2mg/l, P< 0.01 mg/1). 4 livestock's dung pollutionAccording to the statistics for many years, the annual average dung amount of Reservoir area is 12,520,000t/a, among them the biggest is the pig's dung, accounting for 82.51% of total, secondly is big animals', accounting for 10.54%; poultry's accounting for 4.61%, sheep's accounting for 2.34%, respectively. The dung contains large amount pollution elements, such as nitrogen, phosphorus, etc. The livestocks of reservoir area excreted N 126,012.9t/a, P 8,915.26t/a. The annual N runoff amounts oflivestock dung in reservoir area are 12,112.94t, and P is 454.28t, which have the tendency to increase progressively year by year. The amounts of annual N, P runoff of livestock dung are relatively high in Banan, Filling, Fengdu, Kaixian, Zhongxian, Yunyang, which are main region polluted by livestock.5 Sediment pollution of Three Gorges reservoir areaThe soil is corroded seriously in the reservoir area of Three Gorges reservoir area. Except Jiangjin, Banan, the erosion modulus of ChongQing section is among 475-6,800t/km2, average erosion modulus is 4,515t/km2; corroded area is 43,500km2. The total erosion amount reaches 196 million ton per year, 65% of it is coming from the slope-cultivated land. Three of the biggest areas of soil sand producing are Yunyang, Fengjie, and Wushan, accounting for 56.5% of the total sand yield in Chongqing. The runoff and the sediment delivery of reservoir area are distributed unevenly, mainly concentrated on May - October, especially the hydrometric station of Cuntan and Yichang reported May - October accounting for 97% of the whole year. The sediment delivery amounts in each reservoir sections of Chongqing are range from 3,810 to 6,323,406.5t, and the sediment amount into the Three Gorges Area is 140 million ton per year, accounts for 26% of total sediment in the Reservoir. Before the water storage of the reservoir, the sediment delivery amount is 530 million ton per year. After the storage, the sediment delivery amounts reduced largely, decreased 57% to 2002, and 80% to the average of many years earlier, because of the sediment deposits in the reservoir area The sediment generated by two main branches, Mingjiang River and Jialin River are large, and increased largely even after the water storage at 135m water level of the reservoir in 2003. Because the speed of rivers is reduced after the water storage of Three Gorges Reservoir, the sediment delivery is reduced, result in sediment deposition in the reservoir, and endanger the security of the dam.6 GEOWEPP simulations in watershed of Three Gorges ReservoirStudy area was distinguished 135 hillslopes and 64 channels. Hillslopes were independence relatively. Channels were composing by 0-3 hillslopes. Channels were compound stepwise and entering taken on water body in last class channels from outlet.Annual runoff volume was 169,161.9m3 in study area. Because of difference of soil, slope length, slope width, vegetation, net runoff volume had great difference. Most hillslopes' runoff volumes were less than 3000m3, and only 9.6% of hillslopes' runoff volume larger than 3000m3. Hillslopes 1,50,131,132,133,135 didn't produce runoff, so these hillslopes also didn't produce soil loss, sediment deposition, and sediment delivery.The ranges of soil loss yield of hillslopes were 0.5-1,712.53 t/a, except 6 hillslopes that didn't producerunoff. Total soil loss in study area was 20,938.13t/a. The average intensity of soil loss was 3,709.15t/km . 25.99% of study area soil loss intensity was strong or higher. 5.85% of study area soil loss intensity was higher than 15,000t/km2. In there area, topsoil loss 15mm. Serious soil loss weakens soil productivity, and became important pollutant.Sediment deposition yield were small, just 37% of hillslopes had deposition, and annual deposition yield was 726.02t. Single hillslopes deposition yield were range from 8.16 to 76,477.7kg/a. Sediment deposition produced in few hillslopes. There were 10 hillslopes' deposition yields occupied 46.35% of total deposition yields.Except 6 hillslopes that didn't produce runoff, other hillslopes' annual sediment yields were between 0.5 kg and 1,697,630.85kg. Annual total sediment yields of study area were 20,212.08t. Intensities of produced sediment on hillslopes were between 0.03 t/(km2-a) and 20,080.18t/(km2-a). The intensity of sands producing is range from 0.03 to 20,080.18t/(km2a). The average intensity is 3,580.53t/(km2-a), the intensity under 500t/(km2-a) account for 32.6% of the whole area, 500-2,500t/(km2a) accounts for 28.1%, 2,500-5,000t/(km2a) accounts for 13.3%, above 5,000t/(km2a) accounts for 25.9%, respectively.By the GEOWEPP simulated, annual sediment discharge from outlet was 164,037.4m3, with 13,405.6t sediment uptake and the load of sediment in water was an 82g/l. Annual sediment delivery yield of whole watershed was 2,360t/km2.Sediment Delivery Ratio (SDR) is an index to reflect capability of sediment delivery in watershed. Evaluated SDR by GEOWEPP in study area was 0.267.7 Simulation of the agricultural nonpoint source nutrients pollution in the Three Gorges reservoir drainage area by GEOWEPP.According to the nutrient content of soil, the enrichment ratio (Er, the ratio between the nutrient content of sediment in runoff and the nutrient content of the topsoil), and the sediment amount carried byrunoff, it can be estimated:N = CxErxSAccording to the soil loss of each hillslope simulated by GEOWEPP, calculated out the total organic matter loss is l,014.75t/a, total N loss 36.32t/a, and total P loss 10.74t/a; the intensity of loss is 179.76,6.43, 1.90t/(km2-a), respectively. The sediment load at the outlet isl3405.6t/a, therefore TN, TP of the whole water body of the drainage area is calculated as 23.23t/a, 5.65t/a, the loss intensity is 4.11 and 1.00t/(km2-a), respectively.According to result of study, some suggestions shown below were proposed to control the non point...
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