基于增强回归树的流域非点源污染影响因子分析

Influencing factors of non-point source pollution of watershed based on boosted regression tree algorithm

地表水的非点源污染在点源污染不断得到控制的前提下已经成为水环境污染的首要问题.非点源污染影响因子的复杂性及不确定性一直是流域非点源污染研究的重点和难点.本文利用SWAT(Soil and Water Assessment Tool)模型,以辽河子流域汎河流域为例,模拟了2003—2012年的非点源污染状况,对其空间分布状况进行了分析,并应用增强回归树的方法定量分析各种影响因子(坡度、土地利用类型、高程和土壤类型)对该流域非点源污染的贡献率.结果表明:在汎河流域,非点源污染呈现较高的空间异质性,其中总氮的空间分布差异较大,总磷的空间分布差异较小.坡度因子与载体泥沙、总氮和总磷均呈极显著正相关关系(P<0.01),对泥沙和总磷有显著影响,其贡献率分别为46.5%、38.2%;土地利用因子对载体泥沙、总磷的负荷量有重要影响,其贡献率分别达到27.2%、35.3%;高程较低、坡度较缓的耕地地区易产生较高的总磷负荷量;褐色土壤最易流失总磷,而草甸土易流失总磷,且易受泥沙侵蚀.本研究利用增强回归树模型克服了流域非点源污染影响因子的复杂性,可加深对非点源污染产生机制的理解.

Non-point source（ NPS） pollution has become a key water pollution problem under the condition of point source pollution was controlled. The complexity and uncertainty research of NPS pollution influential factors has always been important and difficult. This paper simulated NPS pollution of the Fanhe River watershed in 2003-2012 by the soil and water assessment tool（ SWAT） and analyzed its spatial distribution. Meanwhile,the boosted regression tree（ BRT） method was proposed to quantitatively analyze the corresponding influential factors including land use,soil,elevation and slope. The results showed that NPS pollution in the Fanhe River watershed had high spatial heterogeneity. The spatial distribution of total nitrogen（ TN） had greater difference than that of total phosphorus（ TP）. The three pollutants,TN,TP and sediment,were all positively related to slope gradients（ P〈0.01）. The slope gradients played the strongest role in determining the sediment and TP output with the contribution rate of 46.5% and 38.2%,respectively. Land use had important influence on sediment and TP loads,with the contribution rate of 27. 2% and 35. 3%,respectively.TN was produced abundantly in low-elevation and steep-slope locations and with cultivated land use.Cinnamon soil was most vulnerable to the TN load while meadow soil took the second place in terms of soil erosion and TP load. The paper overcame the complexity of influential factors for NPS pollution by BRT,and deepened the understanding of NPS pollution mechanism.