Abstract:

Export Coefficient Model (ECM) is effectively used in generating reasonable estimates of nonpoint 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 CIRIECM in estimating annual clumped phosphorus load of Dianchi watershed in 2008 was -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 nonpoint sources in 2008 was 352.5 tons.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