Estimating nonpoint source pollution load using four modified export coefficient models in a large easily eroded watershed of the loess hilly–gully region, China

Abstract

The quantitative estimation of nonpoint source (NPS) pollution load in large river basins is one of the important issues in watershed modeling. This study focuses on Beiluohe River watershed and takes Johnes export coefficient model as a starting point to establish three kinds of modified export coefficient model and estimate NPS total nitrogen (TN) and total phosphorus (TP) load of the watershed from 1995 to 2010. Results indicate that: (1) The first proposed export coefficient model considering effects of soil loss has characteristics of high precision, simple algorithm and less parameters; it is more suitable for loess hilly and gully region with a sparse vegetation cover and severe soil erosion. (2) NPS TN load in Beiluohe River watershed was 12,778.17 tons in 1995; in 2010, it fell to 4673.37 tons, a decrease of 63.42 %. NPS TP load in Beiluohe River watershed was 643.52 tons in 1995; by the year of 2010, it fell to 254.93 tons, a decrease of 60.38 %. (3) The highest contribution of NPS TN load is from farmland (42.16 %); the contribution rates of agricultural population, big livestock, pig, forestland and grassland are 18.14, 14.05, 8.03, 8.02 and 6.17 %, respectively; The highest contribution of NPS TP load is also from farmland (31.41 %); grassland, agricultural population and pig also account for large proportions, which are 21.80, 19.58 and 16.72 %, respectively. (4) The top priority for improving water environment quality is to further strengthen the source control of NPS pollution. The following is the migration control and then the end control of waste and wastewater treatment. Results may provide a theoretical basis and data reference for quantitative estimation and source control of NPS pollution at watershed scales.