Drainage optimization of paddy field watershed for diffuse phosphorus pollution control and sustainable agricultural development

Abstract Rice cultivation consumes abundant irrigation water and fertilizer, which has negative environmental effects. Therefore, water and fertilizer use efficiencies need to be enhanced while ensuring yields, to guarantee sustainable production and reduce diffuse pollution. Paddy rice fields are usually connected with ditches and ponds in the dominant rice planting regions in southern China. However, the influence of drainage optimization on sustainable rice production from source (paddy fields) to process (ditches and ponds) is still unclear. Based on a typical rice cultivation region, a two-year field experiment and a long-term watershed simulation were conducted to evaluate the comprehensive effectiveness of optimizing drainage of paddy field watershed. The results showed that 89.23 % of diffuse P loss from paddy fields via surface runoff occurred during the early growth stages, and particulate P was the main form. Increasing the maximum ponding water level (Hmax) of paddy fields reduced surface runoff without yield loss, and diffuse P loss from paddy fields was decreased by more than 27.18 % after increasing Hmax from 50–80 mm to 60–180 mm at various rice growing stages. In addition, the currently unavoidable diffuse P loss from paddy fields could be effectively intercepted by ditches and ponds. When the Manning’s roughness coefficient of ditch was increased from 0.1 to 0.35, the reduction rate of watershed P loss increased from 1.18 % to 16.02 %, while the reduction rate increased from 10.15 % to 16.52 % when the drainage area of ponds was increased by 50 %. Overall, more rainfall, drainage water and diffuse P can be intercepted by drainage optimization in the paddy field watershed, which would decrease irrigation water requirement and increase fertilizer use efficiency. These results highlight the significant effects of optimizing drainage by increasing the earthen levees of paddy fields to 120–180 mm, maintaining vegetation of ditches, and expanding drainage area of ponds. The proposed measures require more farmers’ labour for maintenance. If extensive implementation can be realised, for example with government's incentive policies, this could represent a big step forward in the effective diffuse pollution control and sustainable agricultural development in China and other rice planting countries.