江苏沿海滩涂农田高降渍保证率暗管排水系统布局

Subsurface drainage system layout to meet high assurance rate of waterlogging control in coastal mud-flat farmlands in Jiangsu

为确定适于江苏沿海滩涂地区农田高标准降渍要求的暗管排水系统布置方案,该文根据江苏东台1953-2015年的逐日气象资料,以棉花为例,雨后3 d内将地下水埋深降至地表以下80 cm为排水降渍指标,运用DRAINMOD模型模拟了降渍保证率为95%的暗管排水系统布置方案,并在此基础上分析了相应的田间水文效应,以及补充灌溉对产量的作用。结果表明:在满足95%保证率的高标准降渍要求下,棉田的最大暗管间距与暗管埋深的模拟值大致呈对数函数关系;8月进行补充性灌溉后,满足降渍指标的暗管排水布局可获得的棉花多年平均相对产量为84.3%~86.3%;随着暗管埋深的增大,总排水量模拟值逐渐升高,生育期地下水埋深下降明显,花铃期受渍期间地下水埋深小于30 cm的累计值逐渐降低。综合考虑研究区除涝降渍、排水洗盐、作物产量、环境保护和经济效益等多重目标,推荐埋深120~150 cm、间距15~20 m为研究区适宜的暗管排水系统布置参数。研究成果可为沿海地区滩涂农业综合开发及生态环境保护提供理论依据。

Modem agricultural development in the coastal mud-flat area of Jiangsu, China faces the challenges of poor drainage water management; the existing open ditch systems cannot meet the drainage requirement for water table control and waterlogging prevention due to the concentrated rainfall during the crop growing seasons. In order to examine the subsurface drainage system layout plans based on the high probability of waterlogging prevention in the farmlands, a model simulation study using the field hydrology modeI-DRAINMOD was conducted for cotton growing condition in Dongtai, Jiangsu, China. The applicability of DRAINMOD was tested with reported data of water table drawdown from a locally conducted subsurface drainage experiment. Long term simulations were subsequently conducted based on daily weather data from 1953 to 2015 in the study area, and the soil drainage and crop factors. The waterlogging control objective was based on lowering water table depth to 80 cm below soil surface in 3 days after rainfall events, and DRAINMOD simulation results for different subsurface drainage system layout plans were evaluated on the assurance rate of 95%. In addition, the effect of supplemental irrigation on crop yields were examined through simulations of crop yields with different irrigation strategies. The simulation results showed that： 1） DRAINMOD could predict water table depths reasonably well for the study area and the relative error between simulated and measured water table depths was 4.7%; to meet the high assurance rate of 95% for cotton, the required subsurface pipe spacing presented a logarithmic relationship with the subsurface pipe depth that ranged from 80 to 200 cm （P〈0.01）; when supplemental irrigation was applied in August, the predicted average relative yields of cotton were in the range of 84.3% to 86.3% under the proposed drainage system layout. 2） For the optimum design of subsurface drain depth at 150 cm and spacing at 20 m, with good surface drainage condition, the simulated subsurface drainage accounted for 87.3% of the total drainage, the surface drainage alone could not meet the drainage requirement for waterlogging reduction in the study area. 3） DRAINMOD predicted values of the soil excess water index （SEW30） in the flowering and boll-forming stage of cotton were in the range of 27.15 to 47.49 cm.d, which were much lower than the values that would lead to 10% to 15% reduction of the cotton yield based on existing studies. 4） For a severe 3-day storm event that produced 369.9 mm rainfall in 3 days in the simulated period, DRAINMOD simulation results showed that shallow and narrowly placed drains could lower water table more effectively during the early days of drainage, while the deep and wider drains could lower water table to a greater depth during the later time period. 5） For the coastal area, lowering water table depth from 150 cm to 200cm may result in salt water surface rise of 11.96 m, posing a potential threat of saltwater intrusion. Because lowering water table to an excessive depth may lead to high soil moisture and nutrient losses, the subsurface drainage pipes should be placed as shallow as necessary to achieve the desired water table control level from both the economical and the environmental perspective. Considering multiple objectives of waterlogging reduction, salinity control, maximum crop yields, environmental protection and economic benefits, the authors recommend the drain depths of between 120 cm and 150 cm and the spacing between 15 m and 20 m as the appropriate design parameters for the study area. Findings from this study may provide technical reference for agricultural drainage system installation in the coastal mud-flat area.