Preliminary Studies On Nitrogen Migration Characteristics And Its Hydrologic Driving Mechanisms In A Typical Agricultural Watershed

Agricultural watershed is widely distributed in hilly areas of China.Due to factors as step slope,concentrated rainfall,erosion of soil and human activities,Agricultural watershed area is a frequency soil erosion and further causing environmental problems such as non-point source pollution.This paper is fully considered the on-point nitrogen variation characters and its mechanisms of yield and aimed to explore the nitrogen migration and nitrogen flux features.The goal of this paper are providing abound basic data for no-point nitrogen pollution prevention.To achieve the goal,3 different scale typical agricultural watershed were selected as study object.Meanwhile,the rainfall,runoff,sediment yield,nitrogen migration and fluxes were collected in 2018-2019,and the human activities such as land using in the basins were investigated and analyzed by questionnaire.The runoff and sediment yielding and nitrogen migration in 8 key-point typical rainfall cases are analyzed in detail.Based on the analysis of the response of runoff and sediment yielding and nitrogen migration with rainfall processes in different watershed,as well as the analysis of the nitrogens format and their concentration and flux varied in the water body of watershed,the characteristics of rainfall-runoff-sediment processes in a typical agricultural watershed was studied.In this paper,the characteristics of nitrogen migration processes and flux in small watershed are defined and the influencing factors are analyzed,and the hydrologic driving mechanism of nitrogen migration was also preliminarily identified.The main conclusions and results are summarized as follows:（1）The analysis of the influence of the rainfall characteristics on sediment and runoff production shows that the peak discharge mainly presents hysteresis characteristics,and the hysteresis duration increases with the increase of area.Besides,the sediment concentration related to erosive rainfall which leads the average sediment concentration positively correlated with the basin area and the sediment transport rate positively correlated with the rainfall intensity.（2）The analysis of the influence of runoff on nitrogen in different river basins shows that in the small watershed of Jieliu,the TN migration shows the"initial scour effect"due to the Leaching effect of initial runoff precipitation.While,in the wan’an small watershed,the“end flush”occurred due to the increased the erosion of desorption capacity of sub-surface runoff.In addition,DN（Dissloved Nitrogen）dominated nitrogen migration in runoff,while,the loss ratio NO₂^--N（Semi Nitrate Nitrogen）is contrary to NN.The sediment is disturbed by rainfall runoff,resulting in the release of DON（Dissloved Organic Nitrogen）and the increase in sediment concentration.In the process of nitrogen loss,the drainage area increased significantly from 34.64 to 522 to 1236hm²,indicating that the nitrogen output increased with the increase of the area.（3）The analysis of relationship between sediment production and nitrogen migration shows that sediment concentration is positively correlated with TN（Total Nitrogen）,PN（Particulate Nitrogen）and AN（Ammonia Nitrogen）under the influence of sediment and water influence.While,the peak time of sediment concentration and TN,PN and AN concentration under different rainfall is basically similar that presenting the initial scour effect".Meanwhile,there is a concentration gradient along the flow direction which means the nitrogen in sediment and runoff in the downstream water body is higher than that in the upstream.（4）The analysis of nitrogen concentration and the flux characteristic in rainy season and the un-rainy season in Jiuliu small watershed showed that nitrogen change with season obviously.The concentration of the TN,the DN,the PN and the NN（Nitrate Nitrogen）reached its highest in winter.While,the AN concentration reaches its highest in autumn and lowest in summer.In the spring and winter,the variation of AN concentration is normal.In contrast,the concentrations of NO₂^--N and DON reached its highest value during the summer.（5）The analysis of nitrogen loss factors show that the runoff is an important factor affecting nitrogen loss due to the artificial water storage and discharge causing a large amount of nitrogen loss.