Spatial Distribution of Nitrogen and Phosphorus in Haihe Basin and Their Determinants

Abstract

【Background and objective】 Haihe basin is one of the most polluted regions in northern China with most of its pollutants originating from agricultural production. The purpose of this paper is to analyze spatial distribution of nitrogen (N) and phosphorus (P) in the basin, as well as their effect on water quality and origins. 【Method】 Using the InVEST model, we analyzed N and P load from farmlands, as well as their discharges to the river, from which we estimated their concentration in the runoff. The sources of N and P were analyzed using GIS based on hydrological network in the basin. 【Result】 ①The N and P released from farmland to the river was 2.41 kg/hm2, 0.56 kg/hm2 respectively in 2015, with total N and P discharged to the river being 3.493 4 million tons and 0.807 7 million tons respectively. Average N and P concentration in runoff in the river was 5.97 mg/L and 1.47 mg/L respectively, with N and P concentrations in 55% of the river exceeded the Surface Water Class V Standard. ②N and P released from farmlands in the mountain areas was lower than that from the middle plain regions. N and P concentration and N and P discharged to the river vary spatially, with the latter concentrated in Tuhaimajia, Zhangwei and Ziya sub-basins, while the former located in Tuhaimajia river, middle and low reaches of Heilonggang river, the tributaries of Daqing river and the upstream of Ziya river. N and P concentrations in the upstream and the tributaries of the basin were higher than that in the mainstream. ③The hotspot areas of N and P account for 14.45% and 16.02% of the total areas, which contributed 23.39% and 27.71% of N and P discharged to the river. 【Conclusion】 N and P pollution from farmland in Haihe basin is severe, with the mountainous areas in the northwest more polluted than the central and southern plains. N and P pollution in the central and southern basin, as well as the areas in the proximity of Shijiazhuang and Qinhuangdao are most concerning.

Keywords

• hydrological network
• non-point source pollution
• farmland
• n and p load
• critical source areas