A Land-Use Optimization Approach For Agricultural Nonpoint Sources Pollution Control In The Three-gorge Reservoir Area

In recent years, as water environment pollution problem becomes more and more serious and point sources(PS) pollution control level increases, the non-point source(NPS) pollution has gradually become a major pollution source of water pollution and the key deciding whether the water environmental problem could be solved effectively. Meanwhile, among which the agricultural NPS pollution caused by farmers' agricultural production and rural living has been recognized to the main source of water pollution, which covers wider range with greater uncertainty and is more difficult to calculate simulate, control and manage. NPS pollution is a landscape-scale phenomenon with various origins, including agriculture, silviculture, mining, and construction, etc. It is driven by multiple factors and includes diffuse pollution which is exclusively a result of human land abuse and land use changes. Generation, transport, and transformation of NPS pollution is closely related to human activities and natural factors. And among them, land use is one of the most important affecting drivers that determine the generation and transport of NPS pollution. Land abuse is one of the most direct forms of human activities, while irrational land-use activities and management mode will lead to soil erosion and nutrient loss with surface runoff, therefore, the large area of NPS pollution to watershed is formed. Meanwhile, small watershed is the headwaters of the water bodies, such as reservoires, rivers and lakes. Its soil erosion can lead to siltation in the river channel, and its soil nutrition loss caused by surface runoff and sediment can make water quality impairment and even eutrophication. All these become one of the main sources of NPS pollution, therefore, it has a significance to control and manage the generation and transport of NPS pollutants. Considering this, it is important to build best management practices(BMPs) based on rational land use through examining the effect of land use on NPS pollution systematically and analyzing the relationship between NPS pollutants and land use in a watershed comprehensively.The Three-Gorges Project(TGP) is one of the largest hydropower projects in the world, which has attracted worldwide attention for its potential influences on the ecological environment. It is ranked as a key project for the improvement and development of the Yangtze River, which has formed a gigantic multi-operation business pattern for flood prevention, power generation, navigation, etc. However, as water levels were driven up, current velocity and diffusivity were dramatically reduced, and the retention time of pollutants was prolonged after the water was cut off. Thus, the TGP also had a profound impact on the environment, such as non-point source pollution. With the completion and operation of the TGP, the influence of it exerting on the ecological environment gradually emerges. The Three-gorge Reservoir areas(TGRA) which is affected greatly by the TGP, because of the special ecological environment and the immigration pressures, and unreasonable human land use activities, the agricultural NPS pollution becomes more and more serious and is seriously restricting sustainable development of agricultural economy and rural environment. And from the point of view of the generation, transport, and transformation mechanism on NPS pollution, it mainly depends on land use/land cover change model. Therefore, it is urgent to strengthen research on the control of NPS pollution, especially studying NPS pollution based on different land use pattern and putting forward land use optimization management mode and measures for controlling NPS have an important theory value and practical significance in protecting water quality.Taking account into the generation and, on the basis of hypotheses, Wangjiagou watershed in the core zone of the TGRA has been selected as a study site. A large number of basic data were collected through the extensive on-site investigation, and through integrated technique means of field monitoring, location experiment and geographic information system(GIS), this study discusses the relationship between watershed land use and the NPS pollution. By establishing basin optimization model of harmonization between environment and economy, this study discusses the best management pattern of land use. Accordingly, this study put forward the theoretical assumption which including an control idea and a control paradigm, the former is land optimization(planning) control in the whole and the latter is BMPs control based on rational land use in the part. To be specific, a paradigm for intercepting and controlling NPS pollution which using the rural residential area-slopping land-paddy field ecosystem-hydro-fluctuation belt system to intercept and reduce NPS pollutants.The main content and result achievements are as follows:(1) The foundation database of Wangjiagou watershed in the Three Gorges Reservoir area NPS pollution research is establishedAccording to research needs, this part carried out a comprehensive investigation and analysis on natural environment and socioeconomic situation of Wangjiagou watershed, and combining on-site investigation, using the GIS software and so on as platform, to establish DEM. And land use type map is acquired from topographic map investigation.The soil type map is acquired by digitizing the Wangjiagou watershed 1:1000 soil type map. Consequently, the foundation databases for the NPS pollution research.are provided.(2) Spatial variability of soil nutrients and its response to land use pattern in small watershed of the Three Gorges Reservoir areaThe geostatistics combined with GIS method was used to analyze and determine spatial variability of surface soil nutrients(0-20cm) which included pH, soil organic matter(SOM), total nitrogen(TN), total phosphorus(TP), total K(TK), available nitrogen(AN), available phosphorus(AP), available K(AK) in Wangjiagou watershed of the TGRA. The results showed that:(1) From the view of soil nutrients statistic characteristics, The soils were widely deficient of OM, and the content of soil AN was relatively higher; The variation coefficients of soil nutrients were graded as AP>AK>TP>OM>AN>TP>pH>TK, and all these presented moderate spatial dependence; the semivariograms of pH, TP and AK in topsoil were best described by spherical model, that of soil SOM, TN and AP submitted exponential model, and those of TK and AN submitter linear model; soil pH have intense spatial self-correlation, and the spatial variability was determined mostly by structural factors, such as parent material, soil type, climate and water conditions, and other soil nutrients have moderate spatial self-correlation, and the spatial variability was determined mostly by random factors, such as crop type, fertilization level and land use management measures. (2)pH tended to increase gradually from southeast to northwest, and most areas in this watershed presented acidity and subacidity; the highest OM content was found in the southeast of this watershed and other areas decrease gradually from center to around; TN decrease gradually from center to around; the highest TP content was found in the northeast, northwest and central areas with shifts eastward, and other areas increase gradually from center to around; TK presented crushing characteristics and had the highest content in the northwest and southeast; AN tended to increase gradually from northeast and southeast to south; the highest AP content was found in the northwest and southeast, and other areas had no significantly variation; AK tended to decrease gradually from the center to northeast and southwest, and in most areas maintained high level. (3) Soil nutrients were significantly affected by different land use types and their spatial positions. Higher values of pH, TP and TK were found in garden land, and its SOM, AN and AK lower than other land use types; higher values of SOM, TN and AN from upland, and AP lower than other land use type; forest land had higher values of AK, but it also had lower values of pH,TN,TP and TK; higher values of AP was found in paddy field, and meanwhile its other soil nutrients maintained intermediate position.(3) Temporal and spatial variation of non-point nitrogen pollution and its response to land use pattern in small watershed of the Three Gorges Reservoir areaSurface water nitrate contamination has aroused nationwide concern recently. In order to reveal temporal and spatial variation of non-point nitrogen pollution, this part take Wangjiagou watershed as an example, based on the data from investigation and monitor from September,2008 to November,2009 under natural rainfall condition in this watershed, temporal and spatial variation characteristics of different forms of nitrogen concentrations were studied. The results showed that:(1) Nitrate contamination in this watershed has been very seriously, and the maximum concentration of N was 45.27mg/l which far exceeds the "V" standard of surface water quality, and this was related to typical agricultural farming area; (2) From the point of spatial scale, the variation rule of average nitrogen concentrations was, well>ditch>pond, and was associated with different land use management practices around the water bodies; (3) From the point of seasonal variation, top nitrogen producer concentrations and load was autumn which in the drizzle but which summer which have sufficient rainwater; there is the power regression relation between nitrogen export load and runoff amount, and maximum export load happened at November and October, and the highest TN export load could reach to 1233kg; (4)And the nitrogen output form was mainly dissolved nitrogen, but in some month was particles nitrogen, Rainfall and human land use activity are important factors for influencing the nitrogen export.(4) Land use pattern optimization analysis in small watershed of the Three Gorges Reservoir areaNPS pollution is a landscape-scale phenomenon with various origins, and they are driven by multiple factors and includes diffuse pollution which is exclusively a result of human land abuse and land use changes. That is, there is a strong relationship between different land use and NPS pollutants. Therefore, the mitigation of NPS pollution should be fully considered rational land use and scientific land management practices. Considering this, this part uses Wangjiagou watershed as investigation demonstrate-on,on the base of analysis on the present status of land use, studies the configuration and quantity characteristics of land use and the problems in existence of land use, and puts forward a land-use patterns optimization approach for controlling agricultural NPS pollution in the macroscopic scale. The results showed that:(1) The cultivated land, unused land and rural resident land will reduce and other land types will be increase, and the rates of forest land and garden land could reach to 55.36%; (2) In order to maintain ecological balance and protect water quality in this watershed, this paper attempts to construct a landscape functional network based on least-cost modeling and combines with ecosystem services and spatial interactions of landscape types, analyzes the spatial difference of the ecological function in Wangjiagou watershed and further discusses the landscape pattern optimization proposal.(5) Critical sources area identification of agricultural non-point phosphorus loss in small watershed of the Three Gorges Reservoir areaAgricultural nonpoint phosphorus(P) pollution is an important cause of eutrophication and water quality deterioration in many water bodies. Identifying critical sources areas that at high risk for P loss to surface water in watershed and concentrating management efforts on these minimal portions of lands are better measures than implementing general strategies over a broad area. A modified catchment scale phosphorus ranking scheme was developed for agricultural areas in Wangjiagou watershed. The result showed that:highest risk areas, take up about 44.9% of the watershed, and middle risk areas contain 45.6%, and lowest risk areas contain 9.5%, and the highest risk areas mainly spread near the downstream parts of main water systenms to the reservoir and from upland. These areas should be regarded as critical source areas to be treated in priority, and then best management practice(BMPs) should be implemented in a high priority to minimize P loss to sensitive watercourses.(6) Research on BMPs of agricultural non-point sources pollution in small watershed of the Three Gorges Reservoir areaMany researchers advocate BMPs to reduce nutrients(N,P) and sediment loss in agricultural areas. Considering the eco-environment vulnerability around the TGRA, and the generation and transport characteristics of agricultural NPS pollution in the TGRA, and with combining, ameliorating and designing a series of techniques and methods relating to the researches on agricultural NPS pollution control, the paper, adopting the comprehensive control thought of combining source control, process block and terminal regulation, designs a series of BMPs based on the rural residential area-slopping land-paddy field ecosystem-water-level-fluctuating zone to intercept and reduce agricultural non-point pollutants in the study area. The results showed that:(1) agrotechnical measures, such as straw mulching can effectively reduce sediment loss from effectively reduce, but it increased the loss amounts of N and P; (2)PAM optimum utilization technique can reduce loss amounts of sediment by 80%-90%, loss amounts of TN and TP by 20%-30% and 25%-35% separately; (3)Rice paddy field ecosystems could effectively control the inorganic nitrogen in sewage and rural sewage phosphorus within just 1 week; (4)Marsh gas tanks for rural households and dispersed livestock and poultry manure have good control effect on N and P, and have good economy benefits; (5) Riparian reed vegetable buffers of water-level-fluctuating zone can decrease the loss of N and P, and the load of TN reduced by 30.00%, and that of TP by 65.00% at least.This paper is a progress report of an ongoing research project from which the expected final product will be an control idea and a control paradigm which also named an integral technology system to control NPS pollution in the TGRA. And because the research results of this paper is just only the beginning, there still remains much work to be done systematically and comprehensively based on the previous research achievement so as to form an systematic control idea and an integral technology system to control NPS pollution in the TGRA.