Output Characteristics of Nitrogen and Phosphorus from Non-Point Source Pollution of Typical Land Use in A Micro-Watershed in Hilly Red Soil Region

Abstract

Scientific identification of runoff output characteristics of different land use patterns is the premise of controlling non-point source pollution in watersheds. The hydrology and output process of non-point source pollutants of forest, planting, and construction land with different rainfall characteristics were observed using a micro-watershed in a low mountain and hilly region of Southern China. The results showed that land use affected the hydrological characteristics and water quality processes of runoff. The characteristics of runoff generation time and cumulative rainfall under typical rainfall conditions were as follows:construction land(9 min, 2.0 mm), planting land(35 min, 11.4 mm), and forest land(108 min, 24 mm). There were significant differences in the three land use types in the pollution output process characteristics, such as pollutant concentration of total suspended matter(TSS), total nitrogen(TN) and phosphorus(TP), their components, N/P ratio change, and output intensity. Under typical rainfall, different land use types had similar pollution output stages, the mass concentrations of TSS, TN, and TP in the initial runoff were high, and then gradually stabilized. In addition, the first 30 min of the flow generation process contributed to the TSS, TN, and TP loads within the 23%-43% range. At the annual scale, there were significant differences between the contribution rate of each land use type to TN and TP load and ratio per unit area; the highest contribution to total pollution load was planting land(57% and 45%), while the highest in pollution load per unit area was construction land(9.50-12.50). The results also showed that the distribution of key non-point source areas had different spatial and temporal dynamics, which was comprehensively determined by the land use types in catchment units, the characteristics of annual rainfall, among other factors. With the increase in rainfall, the main contribution non-point source pollution in the micro-watershed changed from construction land to planting land. Targeted ecological interception strategies should be implemented based on the distribution characteristics of key source areas and characteristics of underlying surface runoff production process.