不同排水等级沟道氮磷截留效应及综合评价OACHSSCD

Agricultural drainage ditches,as the direct carriers of farmland return flow,serve dual functions of waterlogging control and ecological purification.The cross-sectional dimensions of ditches determine their drainage class and are closely related to the efficiency of nitrogen(N)and phosphorus(P)retention efficiency.However,the underlying mechanisms by which cross-sectional dimensions affect N and P retention remain unclear,hindering the optimal design of ecological drainage systems.To clarify the relationship between drainage class and N/P retention,and to identify the optimal unit within the ecological-drainage system,this study selected three drainage ditches with different cross-sectional dimensions(small ditch D-B,medium ditch D-T,and large ditch D-M)in Anxin County,Huang-Huai-Hai Plain.Through in-situ field monitoring,the dynamics of N and P concentrations and removal efficiencies were systematically analyzed.Principal component analysis(PCA)was applied to extract a comprehensive removal capacity indicator,and the entropy-weighted TOPSIS model was employed for multi-index comprehensive evaluation,considering water conveyance capacity,removal efficiency,and land occupation.The results showed that:(1)N and P removal efficiencies decreased with increasing drainage class.The average removal rates of total nitrogen(TN)and total phosphorus(TP)in D-B reached 34.73%and 25.73%,respectively,whereas those in D-M were only 3.30%and 3.33%.(2)In earthen agricultural drainage ditches,N and P removal was primarily governed by cross-sectional dimensions.D-B exhibited a lower hydraulic loading rate(HLR,0.26 m/d)and longer hydraulic retention time(HRT,0.67 h),leading to a larger water-substrate interaction area.Moreover,the appropriate water depth and flow rate in ditches of smaller drainage classes facilitated vegetation growth,which further enhanced N and P removal.(3)The entropy-weighted TOPSIS comprehensive evaluation indicated that D-B achieved the highest comprehensive score(Ci=0.5186),making it the priority unit for drainage system configuration.Under the premise of ensuring drainage safety,replacing large ditches with smaller ones increased the total land occupation of ditches but significantly improved the N and P purification capacity of the system.In conclusion,moderately increasing the proportion of small cross-section ditches represented an effective approach to achieve synergistic optimization of drainage safety and non-point source pollution control.