Yield calculation of agricultural non-point source pollutants in Huntai River Basin based on SWAT model

Abstract: The establishment of non-point source pollutants output load model under the mode of rainfall-runoff and land use, the analog calculation of agricultural non-point source pollutants in the process of migration and transformation, and the systematic analysis of non-point source pollutants discharge quantity, distribution and composition characteristics are based on actual monitoring data, calibration and validation model, in consideration of underlying surface, hydrology and meteorology, and physical features of Huntai River basin. The areas 1 km away from each side of master stream Huntai River, Taizihe River and Daliaohe River and 5 km away from reservoir were defined as buffer zone, where the mode of land use was transformed so as to restore the natural ecosystem. The process of pollutant migration and conversion was simulated based on the calibration of key hydrological parameters, and the causes as well as the migratory features of non-point source pollution were investigated. The primary area of water environment pollution was mainly distributed along both sides of the water channel of the mainstreams of Huntai River. The point-source pollutant was mainly related to the distribution of industry and the amount of discharged wastewater. The risk of non-point pollution was mainly related to the pattern of agricultural plantation and farmland utilization. The secondary area of water environment pollution was mainly distributed along both sides of the water channel of tributaries. Therefore, the situation of pollutant production corresponding to the intra-regional regulation of industrial structure, land utilization pattern surrounding the water channel should be highlighted. The non-point pollution in Huntai watershed was dominated by farmland pollution, and the main indices of pollutants were total nitrogen (N) and total phosphorus (P). The contribution rate of pollutants was farmland runoff > livestock and poultry breeding > urban runoff > water and soil erosion > rural household wastes. For the chemical oxygen demand (COD), the primary source of pollution was the livestock and poultry breeding, followed by urban runoff and rural household wastes. Seen from unit area, the high loading intensity for total N was mainly distributed in Tai’an County and Haicheng City that locate at the lower reaches of Dahuofang Reservoir, and in Fushun County and Haicheng City that locate at the lower reaches of Taizi River. The high loading intensity for total P was mainly distributed in Tai’an County and Haicheng City, and in Fushun County. The loading intensity of total P in Dahuofang Reservoir and surrounding area, Tanghe Reservoir, and Shenwo Reservoir was not high. Considering the topography and the distribution of soil type, soil type at the upper reaches with large slope was dominated by brown soil and salinized paddy soil, which were easy to be eroded. The topography in Anshan, Haicheng, Yingkou and Panjin was even and had lower altitude. The soil type in these areas was dominated by meadow soil and brown soil. The amount of soil erosion and loss was not high and the loading capacity of pollutant was relatively small. The loading intensity of total N and total P in the middle and lower reaches of this area, such as Xinmin County, Shenyang City and Lianzhong County, was high. Seen from the distribution of land types in Taizi River watershed, the dry and wet farmlands in plain area were the key source areas of pollution load. In view of the topography, landform, land use type and condition, the pollution load in the upper reaches of this watershed area was relatively low because of higher vegetation density and less plow land, while it was high in the lower reaches because of more farmland and agricultural fertilization, and severer water erosion and soil loss. Under the pattern of conventional development, average annual soil erosion modulus of Huntai River basin was 400 kg/hm2, and total P and N output intensity was 7 and 19 kg/hm2, respectively. From the aspect of unit area, the maximum load intensity for total N and P was 317 and 260 kg/hm2, respectively. Owing to spatial variation in load intensity, average load intensity for total N and P was 29 and 10 kg/hm2 respectively based on the calculation of area weighting. Under prevailing pattern of ecological protection, the loss amount of N and P was decreased. Annual reduction of total N and P was 9500 and 900 kg/hm2, respectively. The reduction amount of COD and ammonia nitrogen pollutants, caused by the conversion of arable land to forestry, was 14 and 5 kg/hm2, respectively. Through the comparison, the amount of pollutants under prevailing pattern of ecological protection was 9% less than that under the pattern of conventional development, and the reductions in COD, ammonia nitrogen, total N and total P were 5.12%, 31.67%, 10.40% and 25.95%, respectively.