Characteristics of Non-point Source Pollution Loss under the Influence of Watershed Hydrology in the Southern Subtropical Zone of China

Under the global climate change, the watershed hydrological characteristics in the southern subtropical zone of China change sharply, meanwhile leading to the changes in the loss of non-point source (NPS) pollution. In this paper, Dongjiang River Basin was taken as the typical rice cultivation areas in southern China, with the distinctive watershed hydrological characteristics (the large rainfall, big river fall, relatively frequent extreme rainfall, and so on). The study area was divided into high, medium and low three groups according to the slope classification criteria, which was chosen by Digital Elevation Model (DEM). The results of NPS pollutant loss have been simulated by the hydrological model: Soil and Water Assessment Tool (SWAT), which mainly considering the extreme rainfall conditions and rainfall-runoff processes under the different slope conditions. The results were as follow: 1) Comparing with the different land use types, the agricultural land, which was the paddy land in the study watershed, was the one that most of NPS loss happened. The years of average TN and TP loss in the farmland reached to 29.9kg/ha and 6.5kg/ha, respectively; 2) More than 50% of annual TN and TP loss was exported between April to September when rains were frequent and intensive. 3) TN and TP loss during extreme rainfall events accounted for approximate 30% of annual loss. In years which had more extreme precipitation events, the nutrient losses were much higher than study years' average value. 4) With slope increased, soil erosion would aggravate, therefore the erosion-induced NPS increased. According the soil erosion amount, the main erosion slope ranged from 5° to 25°. Based on our results, we further provide the useful information for controlling the nonpoint source pollution and conserving the water quality of water source for such rice cultivation subtropical watershed.