Experiment And Study Of Pollution Mitigation By New Irrigation And Drainage System In Rice-based Irrigation Districts

Focuses on the problem of agricultural non-point source pollution in the rice-based irrigation area in South China, Two experimental sites of irrigation and drainage system with "Four Defense Lines" for pollution mitigation were established in the rice-based irrigation area located in the Yangtze River and Pearl River Basins respectively. The field experiment for each link of the "Four Defense Lines" system is engaged. The analysis and simulation are conduct on the basis of the results of the experiment. The effect of the pollution reduction and its mechanism of each link and the comprehensive system are studied. And then, the methods for establishment of the new irrigation and drainage system with pollution mitigation function are presented. The main research contents and conclusions are as follows:(1) The experiment and research of the composition of" Four Defense Lines’ system and the effect on the pollution mitigation were engaged. The " Four Defense Lines " system which composed by water-saving irrigation in paddy field, field grass ditch, pond wetland and ecological trunk ditch has been constructed in Hubei Zhanghe and Guangxi Qingshitan irrigation districts. According to the results of the experiment, the effect on improving the drainage water quality from paddy field with "Four Defense Lines " system is evaluated.The first defense line—Water-saving irrigation:Through field water-saving irrigation, controlling discharge and combining with high utilization rate of water manure, the first defense line can reduce discharge of the nitrogen and phosphorus by20%-30%. The second defense line—field grass ditch:The non-point pollutants discharged from farmland firstly enter into field grass ditch, and then it is removed again through the field grass ditch before enter into pond wetland. The second defense line can mitigate nitrogen and phosphor by35%～50%. The third defense line—Pond wetland:Water is purified by field grass ditch and enters into the pond wetland, further reducing the contamination again through physical filtration, settling, chemical adsorption, botanical absorption, micro-biological degradation. The third defense line can mitigate nitrogen and phosphor by40%～60%. The fourth defense line—Ecological trunk ditch:Through collecting the discharged water from pond wetland and then discharge to all kinds of drainage receiver, ecological trunk discharge ditch re-purifies the water discharged from the wetland. The fourth defense line can mitigate nitrogen and phosphor by15%～50%. The" Four Defense Lines "comprehensive system can reduce nitrogen and phosphor60%～70%form paddy field drainage. Because the drainage water quality is improved significantly, it can be recycle-used in irrigation area, so the comprehensive system can further save water 20%～30%on the base of its original effect of water saving.(2) In order to provide the reference for the management of the pond wetland, the influence of sediment in pond wetland on reducing pollution under the conditions of different soil, water depth and water quality has been experimented and studied.The flow turbulence will accelerate the release of phosphorus from sediment. Unstable water-soil environment and the content of dissolved oxygen can affect the growth of microorganisms and the rate of nitrification/denitrification, good aerobic-anaerobic environment in the root zone is very important for maintaining the purification capacity of wetland. For different water quality indicators, the appropriate water depth of wetland is different. As for total phosphorus, the purification effect is better when the water depth is45cm, for total nitrogen, the better water depth is25cm. When the nutrients in sediment accumulate to a certain extent, secondary pollution may occur, so the periodic dredging to ensure the treatment capacity and stable operation of wetland is very important.(3) DrainMOD model was applied to simulate the paddy fields displacement and nitrogen loss form the experimental farmland in Zhanghe irrigation district during the growing season of middle rice. Its adaptability in the humid regions in South China was verified. Based on the parameter calibration and model improvement, DrainMOD model was utilized to simulate the process of the drainage discharge and nitrogen loss from paddy fields during the drainage in different hydrologic years, different irrigation manners and amounts of fertilizer.The ammonium nitrogen loss during the surface drainage form paddy fields mainly occurred in the early and middle growth stages of rice, while the nitrate nitrogen loss mainly occurred in the middle and late growth stages of rice. The upper limits of ammonium nitrogen load and nitrate nitrogen load during the drainage of paddy field are designed to be14.1kg/hm2and6.6kg/hm2respectively in the wetland area design. Fertilization had a significant influence on the ammonium nitrogen and nitrate nitrogen loss from the surfaces of paddy fields, and the ammonium nitrogen and nitrate nitrogen loss along with the drainage from paddy fields increased with the increase in amount of fertilizer.(4) Water quality at the outlet of wetland was simulated by using pond wetland water quality models for TN, NH4+-N, NO3--N and TP. The reliability of model was verified by the comparison of measured and calculated values. The selection and optimization design of pond wetland for controlling agricultural non-point pollution was studied in combination with practical operation situations of the pond wetlands in Zhanghe irrigation district and Qingshitan Irrigation district in the multiple years.The simulated values gained with the first-order kinetic model for TN, NH4+-N, NO3--N and TP of pond wetland are closer to the measured values, with an error less than30%, so the simulated result is of high reliability. The displacement of the farmland and pollutant content in the drainage that were simulated with the DrainMOD model were taken as the inputs of wetland water quality model, and then the two models were coupled to form a comprehensive system to simulate and forecast the quality of water from the pond wetland. The results showed that the coupled model had good simulation effect, and only the basic data such as soil and meteorological data of the region is needed to simulate and forecast the purifying effect of the drainage water, which provided theoretical basis for the "Four Defense Lines " comprehensive system.(5) The technology concept of "Four Defense Lines" was applied in the establishment of new irrigation and drainage system, current research achievements were concluded in terms of the technologies for removal of pollutant from the farmland drainage in ecological drainage ditches and pond wetland of irrigation districts. The theories, methods, and efficiency of this new irrigation and drainage system with pollution mitigation function in rice-based irrigation districts were discussed. This new system possesses the advantages of low construction and operation cost, high efficiency in pollutant removal, good drained water quality, and simple operation. It also considers the operability and economical efficiency, and produces good environmental, economical, and social benefits.