面源磷负荷改进输出系数模型及其应用

ESTIMATION OF NON-POINT PHOSPHORUS LOAD USING IMPROVED EXPORT COEFFICIENT MODEL AND ITS APPLICATION IN WATERSHED MANAGEMENT

传统的输出系数模型是模拟较大流域面源污染的有效手段,但它假设同一土地利用类型的输出系数相同,忽视了营养物径流和截留过程中其他流域特征对输出系数空间分布的影响,难以为流域分区管理提供依据。在传统输出系数模型基础上,引入产污因子(CI)和截留因子(RI),校正地形、降雨、植被缓冲带对面源磷流失空间格局的影响。用改进输出系数模型模拟滇池流域面源磷负荷并将模拟结果应用于流域管理。结果表明,改进的输出系数模型能更好地模拟面源磷污染的空间格局,识别出面源总磷的关键风险区——湖滨平原地带以及与入滇河流邻近的山谷、台地。以2008年滇池流域面源总磷的模拟结果(352.5t)为基准,若单位面积磷肥施用量减少20%,面源总磷负荷将减少8.6%;若环湖公路内侧全部实现退耕还林或还草,面源总磷负荷将减少6.0%;若同时实施减少磷肥施用量和环湖公路内退耕两项措施,面源总磷负荷将减少13.0%。

Export Coefficient Model(ECM)is effectively used in generating reasonable estimates of nonpoint pollution load in large watershed.In its classical form,ECM assumes that export coefficients are homogeneous within each land use type,ignoring the impacts of spatial heterogeneity of other watershed characteristics on spatial distribution of non-point pollution.However,phosphorus,which is regarded as the critical nutrient for water eutrophication and algae gloom,is largely affected by runoff,terrain and vegetated buffer strips in its transfer process from land to surface waters.By integrating a weighting function of Contributing Index(CI)and Retention Index(RI)into ECM,the spatial heterogeneity of terrain,precipitation,and vegetated buffer strips were considered to represent the potential of phosphorus losses in a specific site of a watershed land to its receiving waters.The data needed to construct the improved Export Coefficient Model with CI and RI index(CIRI-ECM)are easily available and the calculation process can be done in geographic information system.Since the improved model has not changed the structure of ECM,export coefficient values from literatures can be used in CIRI-ECM if reasonably selected.The CIRI-ECM was applied in the Dianchi Watershed in China.Firstly,the CIRI-ECM was quantitatively tested in its clumped estimating capability with observed data and qualitatively assessed in its distributed representing effects with other research results.The results showed that the error of CIRIECM in estimating annual clumped phosphorus load of Dianchi watershed in 2008was-0.8%,smaller than that of ECM(-17.2%).Also,CIRI-ECM performed better in depicting the spatial pattern of phosphorus losses from land sources in the Dianchi watershed.Then,the CIRI-ECM was used to estimate watershed phosphorus load,to identify the critical source areas of phosphorous pollution from non-point sources,and to predict phosphorus load under particular management practices.The critical source areas of phosphorus pollution were identified as the farmland and the urban built-up land in the floodplain area of the Dianchi Lake where their downslope flowing paths lacked effective vegetated buffer strips,and the farmland in the valleys and the terraces that were hydrologically connected to the streams flowing into the Dianchi Lake.According to CIRI-ECM,the annual phosphorus load of the Dianchi Watershed from nonpoint sources in 2008 was 352.5tons.The load would reduce by 8.6%if the use of phosphorus fertilizers was reduced by 20%;the load would reduce by 6.0%if all the agricultural fields inside the lakeside road were turned into forests or grassland;the load would reduce by 13.0%if both of the above management practices were taken.Overall,by integrating Contributing Index and Retention Index into classical Export Coefficient Model,the CIRI-ECM was demonstrated to perform better both in watershed phosphorus load estimation and critical source area identification.CIRI-ECM can be used as an assistant tool for non-point phosphorus pollution control and watershed management,especially in middle-scaled and large-scaled watersheds with great spatial heterogeneity of precipitation,terrain and vegetated buffer strips.