Reusing Effluent Water in Drainage Ditches for Irrigation in Hilly Regions

【Objective】 Most hilly regions in China are short of freshwater resources and recycling the effluent water in their drainage ditches is a way to relieve this pressure and improve water use efficiency. This paper investigates how reusing the effluent water for irrigation affects leaching of nitrogen (N) and phosphorus (P) from soils. 【Method】 In-situ experiment was set up in a field to measure the change in water flow and N and P concentrations in the ditches and the ditch buckets. We calculated the ratio of recycled water volume to the volume of water pumped for irrigation (i.e., regression rate), as well as the change in N and P pollutant loads and their determinants. 【Result】 The water had been drained and reused for irrigations for 24 cycles during the growing season, and the total regression rate reached 89.93%. The loads of total P, total N, nitrate nitrogen and ammonia nitrogen during the growing season were 0.28, 3.27, 2.35 and 2.35 kg/hm2, respectively. The load reductions of P and N were correlated with the ratio of their concentrations in the effluent and in the irrigation water. The reduction in total P and ammonia was significantly correlated with the regression rate. The reduction in total N and nitrate was significantly correlated with irrigation and rainfall in the second day after the irrigation. Nitrate reduction rate was also significantly correlated with temperature. 【Conclusion】 The cycles of drainage and its reuse for irrigation not only saves water but also improves utilization of water and fertilizers, thereby reducing the risk of N and P pollution to the downstream.