| China has applied the most chemical fertilizer to the farmland,while the fertilizer' utilization ratio is low.Large amount of the nitrogen and phosphorus have been carried into receiving lakes and rivers through rainfall-induced runoff,thereby making a significant contribution to eutrophication.Natural drainage ditch,because of its perennial bare soil,has a limited nitrogen and phosphorus interception ability for rainfall runoff.Therefore,it is significant to transform the natural drainage ditches to ecological ditch and investigate the removal effect and mechanism of ecological ditches for nitrogen and phosphorus,which is of theoretically and practically significance for the prevention and control of agricultural non-point source pollution.In this task,a typical paddy rice field in Hejia Greek basin in the western of Tai lake in Yixing city Jiangsu province was chosen.A natural drainage ditch in this area has been modified into agricultural ecological ditches.The modified ecological ditch owned the function of water discharge as well as ecological intercept of nitrogen and phosphorus in rainfall runoff.Three rainfall runoff events with different rainfall intensity has been monitored to investigate the interception effect of agricultural ecological ditches on nitrogen and phosphorus in rainfall runoff.The spatial and temporal distribution of total nitrogen and phosphorus in ditch sediment has also been analyzed.Besides,the three-dimensional hydrodynamic and water quality model of agricultural ecological ditch has been established using EFDC(Environmental Fluid Dynamics Code)model.Sensitivity analysis and calibration of model parameters have been firstly undertaken and model verification was also carried out using the experimental data monitored from a rainfall runoff event.Combined the model equation and the model output,interception mechanism of ecological ditch on nitrogen and phosphorus were analyzed.The results suggested that(1)In the three different rainfall events,the average removal rate of TN(total nitrogen)and TP(total phosphorus)by agricultural ecological ditch were 31.5% and 40.9% respectively;(2)the order of removal rate of ecological ditch on different nitrogen and phosphorus was NH4+-N(ammonia nitrogen)> PN(grain nitrogen)> NO3--N(nitrate nitrogen)and PP(particles phosphorus)> DP(dissolved phosphorus),and the removal rate of PN and PP showed down trend along with the ditch runoff increasing;(3)the total nitrogen and total phosphorus of ditch sediment content in the growth period of rice showed a trend of lower after the first increase,which showed ecological ditch has certain self-purification ability and sustainability to the interception of nitrogen and phosphorus.;(4)the average relative errors of EFDC model simulated results were all less than 15%,which verified the reliability of the model. |
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