基于水文和土壤侵蚀模型评估植被过滤带净化径流中氮磷的效果

为了方便快速地评估单次径流条件下植被过滤带对污染物氮、磷的净化效果,该文设计了地表径流的模拟放水试验,分析了泥沙与颗粒态污染物的相关性,依据土壤混合层概念简化了土壤污染物的输移,建立了植被过滤带对污染物作用效果的估算模型。结果表明:径流中颗粒态氮、磷含量与泥沙含量有显著相关性(P0.05),据此建立线性相关方程,估算出植被过滤带对颗粒态污染物的净化效果,其86%以上样本的颗粒态氮、磷质量浓度模拟偏差均在±20%之内;溶解态氮、磷的迁移分两部分,即发生于坡面地表径流中和土壤混合层中,分别根据质量平衡原理建立方程对溶解态污染物的迁移进行了模拟,验证得到83%以上样本的溶解态氮质量浓度模拟偏差在±20%之内,67%以上样本的溶解态磷质量浓度模拟偏差也在±20%之内,研究表明,可以基于土壤混合层概念和水文及土壤侵蚀模型对植被过滤带净化效果进行评估。

Assessment on retention efficiency of vegetative filter strips to nitrogen and phosphorus in surface runoff based on hydrology and soil erosion model

Abstract: Vegetative filter strips can effectively reduce nitrogen and phosphorus load or concentration from agricultural land to surface water. However, the efficiency of vegetative filter strips vary with soil, hydrology and vegetation condition in different rainfall-runoff cases through long-term monitoring, so it is important to assess retention efficiency of vegetative filter strips under single rainfall-runoff condition. Mathematical model is an important tool for the planning and design of vegetative filter strips. Thus, an ease-to-use assessment model is presented to better assess nitrogen and phosphorus retention effect by vegetative filter strips under single runoff condition in the northwest area of China with serious soil erosion. The simplified model proposed involves 3 parts: coupling sub-model, linear correlation sub-model and mixing zone conceptual sub-model. The coupling sub-model of the vegetative filter strips model (VFSMOD) and the modified universal soil loss equation (MUSLE) is made to simulate purification effect of suspended solids (mainly means sediments), because sediment retention efficiency is the combined effect of the sediments entrapped process and the soil erosion or sediments yield process in vegetative filter strips itself. According to significant correlation with sediments and those particle pollutants on runoff, the linear correlation method is used to calculate removal effect of particle nitrogen and particle phosphorus by vegetative filter strips. Infiltration is regarded as the only mechanism to remove dissolved pollutants, namely vegetative filter strips reduce dissolved nitrogen and phosphorus volume by diffusion and infiltration into soil. Therefore, based on mechanism research of the interaction of dissolved pollutants in topsoil and surface runoff, and through simplifying pollutant transport in deep soil, the mixing zone conceptual sub-model is utilized to assess the effect of vegetative filter strips on dissolved nitrogen and phosphorus. Furthermore, the mass dynamic balance equilibrium within overland flow and soil mixing zone is considered accordingly; in overland flow, mass balance includes diffusion, infiltration and soil losses, and in soil mixing zone, it is regarded as uniform mixing and its change covers diffusion and infiltration from runoff, denitrification, leaching, soil losses and adsorption etc. Among them, the coupling model can predict hydrology and suspended solids transport well by considering runoff sluicing impact on sediments from vegetative filter strips in surface soil, and the results show that the relative deviations between simulated and measured concentrations of suspended solids are within ±20%, and the determination coefficients between simulated and measured values is 0.98. The simplified particle nitrogen and phosphorus transport algorithm is presented by the linear correlation method based on field plot experiment data, and the good prediction in runoff and sediments also resulted in good prediction of particle nitrogen and phosphorus transport by vegetative filter strips, since adsorbed nitrogen and phosphorus are the main components of nitrogen and phosphorus in the areas with serious soil erosion. Similarly, the results also show that the relative deviations between simulated and measured concentrations of particle pollutants are within ±20%. It is obvious that a good prediction of dissolved nitrogen and phosphorus is obtained by the mixing zone conceptual model, the input and output of the mass balance system of vegetative filter strips consider biological and chemical processes of dissolved nitrogen and phosphorus in soil mixing zone, and furthermore the detailed analysis and proper simplification for vegetative filter strips system are presented. The relative deviations are within ±20% between simulated and measured concentrations of dissolved nitrogen and phosphorus. Thus, the combined nitrogen or phosphorus modeling approach successfully predicts runoff, sediment, nitrogen and phosphorus transport in the areas with serious soil erosion, which provides the reference for controlling runoff and pollutants transport using vegetative filter strips.