Abstract:

Nonpoint source pollutants are mainly transported during flood events.The Fengle River,one of the main tributaries of the Chao Lake in the middleeast part of China,was chosen to study nonpoint source nutrient export characteristics in a largesized agricultural catchment.Two summer events (Event06,in the beginning of June and Event08,at the end of August,2010) were intensively sampled at Taoxi section of the river and analyzed for Total Nitrogen (TN),Ammonia,Nitrate,Total Phosphorus (TP) and Dissolved Phosphorus.Hourly discharge and rainfall data were also available at the same section.The drainage area of this section is 1500 km2 with elevatio between 6463m and main stream length is about 50km.Land use includes agriculture (about 45%),forest (39%),town and roads (10%) and water area (ponds and river,6%).No large cities or industry factories are located in this drainage catchment.Both events demonstrated single peak hydrograph.But Event06,which was caused by one storm,was relatively more intensive than Event08 which was caused by 3 separated storms.The base,peak and average discharge were 226,2163 and 1313m3/s for Event06 and 654,1564 and 1094m3/s for Event08,respectively.The minimum,maximum and average concentration values of each indicator of Event06 were much higher than those of Event08,which was mainly due to the fertilization at the end of May or in beginning of June just before the transplanting of rice seedling.Another reason might be that Event06 was more intensive and thus demonstrated stronger erosion and transportation capacity than Event08.The average concentrations of TN and TP were 405 and 011 mg/L for Event06 and 162 and 008 mg/L for Event08,respectively.Nitrogen was mainly exported in dissolved inorganic forms,ie.ammonium and nitrate,which were at similar level.Particulate phosphorus export was relatively more important than dissolved form.The transportation of particulate forms of both nitrogen and phosphorus was more important in the beginning and middle phase of rising limb than the rest flood phases.During the flood events,both concentrations and instantaneous loads of most of the determinants (with an exception of nitrate in one of the events) fluctuated but generally increased in the rising limb,attained the peak values before peak discharge and then declined.The relationships between concentrations of TN and TP and discharge over both events resulted in typical clockwise hysteresis,demonstrating the important washing off and transport capacity of flood water flow in the rising phase.Although obvious ‘first flush’ effect was not obtained through the cumulative mass and cumulative discharge analysis based on the two monitored events mainly due to the relatively large catchment area and gentle topography in most part of the catchment,the importance of nutrient export during flood event had been emphasized.The detailed pollutant accumulation processes in the river reaches and/or the area near the river reach and their transportation should be influenced by successive flood events,which need further study through detailed field and modeling work