Experiment and analysis on flow rate of improved subsurface drainage with ponded water

An improved subsurface drainage with characteristics of less land occupied and reasonable flow rate drained was presented to overcome the disadvantage of small flow rate of conventional subsurface pipe drainage and adapt to the new challenge of agricultural drainage. Based on soil column experiment, the performance of improved subsurface drainage was discussed in saturated and unsaturated soil. During the experiment, five factors of filter width, soil medium, surface ponding depth, outflow condition and groundwater depth were considered. Then, drainage equation for improved subsurface drainage flow rate under conditions of surface ponding and homogeneous saturated soil was first proposed. The results indicated that improved subsurface drainage performed better in both saturated and unsaturated soil with surface ponding. Flow rates of improved subsurface drainage equaled to 2-3 times of that in conventional subsurface pipe drainage when filter width varied from 2 cm to 6 cm in completely saturated fine-sand medium. In addition, the greater the hydraulic conductivity gaps between filter and soil in a specific range, the more effective the improved subsurface drainage is. Besides, in case of large groundwater depth, improved subsurface drainage was still effective. When groundwater depth was 2 times of drain depth, improved subsurface drainage flow rate was comparable to 2 times of that in conventional subsurface pipe drainage in saturated fine-sand medium. The calculated flow rates matched well with the observed ones. The research results can provide technical support to the application of improved subsurface drainage. Meanwhile, it will be beneficial to enrich agricultural drainage technologies in China, which is of practical significance to ensure food safety and promote sustainable development of irrigated areas.