灌排控制措施结合沟塘湿地改善水稻灌区排水水质的模拟分析 . (Chinese)

Simulating the effects of irrigation and drainage control combined with ditch-pond wetlands on drainage water quality improvement in a rice irrigation area. (English)

JIANGSU Sheng (China); CHINA

Nitrogen loss with the farmland drainage from the rice irrigation areas has been a great threat to the aquatic environment of the receiving water in southern China. In this present study, a systematic investigation was made to clarify the effect of different irrigation and drainage control strategies at the field level on ammonia nitrogen (NH3-N) reduction. The potential of pollutant reduction with the ditches and ponds was also proposed to meet the requirements of water quality in small agricultural watersheds. A case study was set in the Yanyun Irrigation Area along the Grand Canal in Yangzhou City, Jiangsu Province, China. A field experiment was performed on rice irrigation and drainage in two rice growing seasons. Some parameters were measured, including the soil properties, groundwater depth, and nitrogen concentrations of drainage flow in the field. The field hydrological model-DRAINMOD was used to simulate the different water management scenarios. A conceptual model was developed to predict the nitrogen reduction in the ditches and ponds under different irrigation and drainage management scenarios. The results showed that the annual drainage depth was as high as 1 162 mm, accounting for 59% of the total irrigation and rainfall depth under the current irrigation and drainage practice. The surface runoff depth accounted for 51% of the total drainage, including a 20% uncontrollable portion caused by storm events. A controlled irrigation strategy (or irrigating only when the field water level drops to -25 cm and no significant rainfall was forecasted) was significantly adopted to reduce the drainage and nitrogen losses: 55% reduction in the drainage and 59% reduction in the NH3-N, compared with the conventional practice. However, the controlled drainage presented a relatively low effect on drainage reduction and surface runoff. The total drainage remained nearly constant for the controlled drainage with the fixed irrigation volume under the current irrigation practice, indicating an increase in the surface runoff from 51% to 75%. The total drainage was reduced from 519 mm to 435 mm (16% reduction), whereas, the surface runoff increased from 35% to 56% under the controlled irrigation practice. The NH3-N concentration in the surface runoff (2.85 mg/L) was higher than that in the subsurface drainage (1.80 mg/L), indicating the significant increase of NH3-N losses with the increased surface runoff. Therefore, there were much higher volumes of irrigation and drainage water in the nitrogen reduction, in terms of the ditches and ponds distribution in the small agricultural watershed of the study area. Only when the drainage volume was significantly reduced using the controlled irrigation and drainage practice, the current ditches and ponds (15% of the farmland area) can be expected to gain lower ammonia nitrogen concentration level for the national water quality standards of Class V for surface water. Nevertheless, there is also an adverse impact on the NH3-N for the shallow depth (60 cm) of the drainage ditches, due to the increase in the surface runoff in the study area. In conclusion, the controlled irrigation of paddy fields can be more effective to reduce drainage and nitrogen losses. Consequently, the reduced amount of drainage water and the integrated management or optimization of the existing wetlands of ditches and ponds can greatly contribute to the water quality of rice irrigation areas in small agricultural watersheds. [ABSTRACT FROM AUTHOR]

针对水稻灌区农田排水氮素输出影响水环境的问题，该研究以大运河扬州段沿运灌区为例，在大田监测的基础 上，运用田间水文模型——DRAINMOD 模拟分析不同田间灌排控制措施的减排效果，并探讨利用农田周边沟塘湿地净 化排水，达到灌区小流域不同水质目标的水管理方案。结果表明，在研究区目前常规灌溉（定额为 9 600 m3 /hm2，合水 深 960 mm）和常规排水（排水沟深 0.6 m，等效间距 50 m）模式下，农田单位面积上的年均排水总量高达 1 162 mm，是 灌溉量与降雨量之和的 59%；其中地表径流占比 51%，仅有 25%是由降雨造成的不可控部分。采取理想的避免地表径流 的干湿交替控制灌溉措施（年均灌溉量 320 mm）可以显著降低排水量和氨氮的输出，相较于常规灌溉模式，可削减 55% 的排水量和 59%的氨氮输出。研究区农田控制排水削减排水总量的效果较差，且在一定程度上增加了地表径流。由于地 表排水中氨氮浓度（2.85 mg/L）高于地下排水（其浓度为 1.80 mg/L），地表排水比例的提高会增加排水对氨氮的输出。 从研究区小流域范围内沟塘湿地分布考虑，目前灌溉与排水量均过高，现有沟塘湿地不足以发挥作用；只有通过控制灌 溉措施显著减少排水量以后，才有可能利用现存的湿地面积将排水中的氨氮浓度降低到地表水水质标准Ⅴ类水。因此，该 研究建议在合理控制灌排水量的基础上，通过整合、优化灌区现有沟塘湿地资源来有效改善研究区农田排水水质。研究 可为类似地区农田排水污染控制提供理论依据 [ABSTRACT FROM AUTHOR]

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