Construction and effect of water-saving and pollution prevention irrigation system

Abstract: Unreasonable farmland irrigation and fertilizer management practice results in the huge loss of nutrients in the field, which is one of the main factors of agricultural non-point source pollution. In order to find out compatible measures of saving water, increasing production and reducing pollution in the paddy fields, a "four lines of defense" system was proposed in this paper, which was composed of field, grass ditch, pond wetland and main ditch. For the purpose of analyzing integrated purification effect, experimental research was systemically conducted in Qingshitan reservoir irrigation area in Guilin, Guangxi and Guangxi Center Station of Irrigation Experiment. Through optimizing the management of irrigation and fertilizer mode, it was expected to find the optimal management pattern of water and fertilizer. This was mainly done by analyzing field yield, irrigation amount, water drainage and discharge loads of total nitrogen (TN) and total phosphorus (TP) under the situation of different irrigation systems, nitrogen fertilizer application frequencies and distributions. Representative demonstration area was built in situ condition. Measuring weirs were installed respectively at the inlets and outlets of grass ditch, pond wetland and main ditch, and water samples were tested to obtain the TN and TP concentrations. Based on these data, the emission laws of TN and TP load were analyzed. The results showed that optimal management pattern was the practice of intermittent irrigation, fertilizer application four times at a ratio of 3:3:3:1 keeping the total amount constant. Compared with traditional practice, it could save 36.5% of irrigation water, reduce 20.2% of drainage water, and reduction rates of TN and TP loads reached to 24.0% and 19.8% respectively, while its effect on rice yield was not significant. It achieved the prevention and control of agricultural non-point source pollution from the source. The other three components of "four lines of defense", namely grass ditch, pond wetland and main ditch, also reflected the purification effect; and the purification rates of TN and TP loads of grass ditch were 21.8% and 25.9%, pond wetland were 21.7% and 20.9%, and main ditch were 16.3% and 14.4%, respectively. This article gave some suggestions on optimization design of each component of "four lines of defense". The research displayed that TN and TP loads in drainage were reduced by 61% and 60% respectively after the purification of the "four lines of defense" system. There is a great purification effect to reduce nitrogen and phosphorus loads from farmland discharged by the "four lines of defense" system, realizing the goals of reducing farmland nonpoint source pollution and recovering its water environment.