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

Abstract: Modern 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 model-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.