Simulation Of Nitrogen Transport And Spatiotemporal Variations In The River Network Of An Agricultural Watershed

Hilly areas contribute predominantly to water flow and pollutants loads of Poyang Lake.Study on transport and variations of inorganic nitrogen（IN）in hilly catchments can be important for water environmental protection and management of Poyang Lake.To this end,Yifeng River catchment was selected as study site,based on air temperature,precipitation,runoff and water quality data during 1999-2019,spatial and temporal variations of IN concentration were analyzed.A coupled hydrological-hydrodynamic-water quality model was used to simulate the discharge and IN concentration.From the aspects of fertilizer application and future climate change,9 scenarios were set up,the spatial and temporal variations of IN output load were analyzed under different scenarios to explore the effects of anthropogenic and natural factors on IN export process.The main conclusions were summarized as follows:（1）From 1999 to 2019,stream water NO₃-N and NH₄-N concentrations showed an increasing trend.Seasonally,the NO₃-N and NH₄-N concentrations were higher in summer and lower in winter due to agricultural activities and rainfall-runoff processes.Spatially,the NO₃-N concentration of downstream（Yifeng station）were significantly higher than upstream（Tanshan station）.The NO₃-N and NH₄-N concentrations were positively correlated with temperature,rainfall and runoff on both monthly and annual scales.During the study period,the area of urban land and the usage of fertilizers had increased which may be the reasons for the increase in IN concentration.（2）The HYPE（HYdrological Predictions for the Environment）model was constructed to simulate the discharge and IN concentration of the Yifeng River catchment.The HYPE model was proved to be capable of capturing dynamics and balances of water with the Nash-Sutcliffe coefficient（NSE）and correlation coefficient（R²）both above 0.75 and relative error（RE）below 10%.The NSE and R² of IN concentration simulation were greater than 0.35 at Tanshan station which were 0.09 and0.33 at Yifeng station,RE were both less than 5%,the HYPE model could be considered to have achieved acceptable simulation accuracy.The simulation results showed that the IN load mostly occurred in March-August when agricultural activities are intensive and precipitation is high.Due to the spatial differences in topography,land use and climate,IN load showed the spatial distribution of lower in upper reaches and higher in lower reaches,indicating that IN load is influenced by hydrological processes,land use distribution and agricultural activities.（3）The input-output external coupling technique was used to couple the hydrological model HYPE and the hydrodynamic model MIKE 11.The linkage of two model components was aimed to perform complete simulation of the Yifeng River catchment system.The results indicated that the MIKE11 model gained satisfactory simulation effects in terms of the streamflow with the NSE and R² both above 0.70 and RE below 10%during calibration and verification.The model could reproduce the trend of IN concentration well showing that RE was-3.12%.The simulation results showed that the water level and discharge both had obvious spatial gradients in different hydrological periods.In dry season,some upper reaches were exposed;in wet season,the submerged area of downstream expanded.Spatially,the instream IN concentrations were higher in upper reaches and lower in lower reaches;temporally,IN concentrations were higher in wet season and lower in dry season.Due to the changes in hydrodynamic conditions of Yifeng River,the decrease of IN concentration in upper reaches is larger than that in lower reaches.（4）Under the combined scenario of SSP1-2.6 climate scenario and reduction of fertilizer application by 40%,the reduction of IN load at upstream,midstream and downstream were largest and mostly occurred in wet season,indicating that the rainfall-runoff process was an important influencing factor of agricultural non-point nitrogen pollution.In the future,the concept of green and sustainable development should be adhered to,and the fertilizer reduction and control technologies should be promoted.