Bin Dong, Yuanlai Cui, Zhi Mao

College of Water Resources and Hydropower, Wuhan University
 

The overuse of irrigation water and chemical fertilizer and low rates of water and fertilizer use efficiency in rice production of China with consequences for serious loss of nitrogen and phosphorus and non-point source pollution of agriculture have undoubtedly caused serious damages to farmland water environment and farmland ecology. In order to control, reduce and even solve the non-point source pollution problem in farmland and to restore the farmland water environment, the new concept, “Four Lines of Defense”, which consist of four components, i.e. on-farm water saving irrigation (WSI) in paddy field, field drainage ditches, constructed wetlands and ecological trunk channels has been presented and developed. In order to link up the new theory with practice, the field experiments for each component have been conducted in rice-based irrigation systems, south of China, and the effect on water saving, crop yield and the pollution mitigation and its mechanism of each component and the overall treatment system are studied.
On-farm WSI practices, as the first line of defense can have around 5% yield increasing in average, and on the other hand, reduce discharge of the nitrogen and phosphorus by 20~30% through WSI practices, controlling discharge and combining with efficient fertilizer utilization. The non-point pollutants discharged from farmland firstly enter into field drainage ditches - the second line of defense, and then are removed partly through the screen and purification of grass in drainage ditches before entering into constructed wetlands. The second defense line can mitigate nitrogen and phosphorus by 35~50%. Constructed wetlands which are converted from the popular water storage facility - irrigation ponds in rice-based irrigation system are the third line of defense. It can mitigate nitrogen and phosphorus by 40~60% through physical filtration, settling, chemical adsorption, botanical absorption, micro-biological degradation. The last line of defense, ecological trunk channels can mitigate nitrogen and phosphor by 15~50% through collecting the discharged water from constructed wetlands and then discharge to all kinds of drainage channels for further treatment and purification. The overall defense lines can reduce the nitrogen and phosphorus 60~70% from paddy field drainage water. Because the drainage water quality is improved significantly, it can be reused in irrigation system, so the whole system can further save water 20~30% on the base of its original effect of water saving.
The new concept of "Four Lines of Defense" has been applied and demonstrated at mezzo scale in rice-based irrigation system. This new system possesses the advantages of efficient water and fertilizer use efficiency, high crop yield, low construction and operation cost and high efficiency in pollutant removal, and produces good environmental, economic, and social benefits as well.