Emission Rule And Model Simulation Of Nitrogen And Phosphorus In Typical Test Area In Qingshitan Irrigation Area

Nowadays,agricultural nonpoint-source pollution is an issue worthy of our attention in any country and region.Along with the rapid development of agriculture in China,agricultural nonpoint-source pollution has become one of the important sources of water pollution.It is imperative to study its changing law,influencing factors and abating measures of agricultural nonpoint-source pollution in typical areas.Lijiang River Basin is the main water source for industrial and agricultural and residential purpose in Guilin City,and is the ultimate acceptor of pollutants in the river basin.The traditional agricultural management mode in the irrigation areas in the basin has led to severe nonpoint-source pollutions,and the water environment quality is not optimistic.In this thesis,field in-situ tests were carried out in Huixian trial area(377.83km~2)and Jingui River trial area(27.25km~2)of Qingshitan Irrigation District in Lijiang River Basin.And Xiangsi River,Mudong River and Huixian River,Antient Guiliu Canal(connecting the Lijiang River System and Liujiang River System)in Huixian trial area and Jingui River(branch of the West Main Canal)in Jingui River trial area were selected.Combined with field investigation and SWAT model simulation and analysis,Huixian trial area was divided into seven spatial scales(Scale 1 to Scale 7)and Jinguihe trial area was separated into five spatial scales(Scale a to Scale e),based on the river control area and hydraulic connection,and the characteristics and regularity of nitrogen and phosphorus emissions at different scales in two trial areas were analyzed.SWAT model was used to simulate runoff and nitrogen and phosphorus emission loads in Huixian trail area,and to calculate irrigation and fertilizer management,karst landform,pond-wetland and other underlying surface properties,as well as the impact of climate change on nitrogen and phosphorus emission loads.The main conclusions are as follows:(1)Between October,2016 and September,2018,the nitrogen and phosphorus emission concentrations in Huixian showed a trend of increasing first then decreasing,and that in Jingui River trail area displayed an upward tendency,which were both influenced by the land use patterns,river system connections and other factors;The nitrogen and phosphorus emission loads in both trial areas presented a upward trend followed by a downward trend with the expend of spatial scale.(2)Between October,2017 and September,2018,the concentrations and emission loads of nitrogen and phosphorus in rainy and irrigated season were higher than those in dry and non-irrigated season in two trial areas;While in Huixian,under the influences of rainfall reduction and abnormal emissions of N and P in the Scale 5 to Scale 7,the average TN concentration and emission load in dry and non-irrigated season were higher than those in rainy and irrigated season,which was contrary to that of Jingui River.(3)According to the simulation results of SWAT model,the annual average emissions of TN and TP in agricultural cultivated lands(paddy fields and dry lands)in Huixian trail area were 1068.36t/a and 55.71t/a respectively,accounting for 79.95%and 72.19%of the total annual average emissions of N and P,which indicated that the cultivated land contributes most to the total emission of the trial area among different land use types.(4)The SWAT simulation results showed that when the fertilizer application amount and irrigation water volume were reduced by 30%respectively,the annual average emissions of TN of the watershed outlet were decreased by 11.45%and 7.79%respectively,and the annual average emissions of TP were reduced by 8.98%and 5.81%respectively.The reduction of fertilizer application and irrigation water consumption led to a decrease in the export of nitrogen and phosphorus emission loads in the Huixian trail area,and with less fertilizer application and irrigation water consumption,the decrease of nitrogen and phosphorus discharge loads were more significant.(5)The SWAT simulation results illustrated that the change of wetland area or wetland storage capacity had an impact on nitrogen and phosphorus emissions in Huixian.Compared with changing single property of the wetland,change the wetland area and wetland storage capacity simultaneously had the most significant effect on nitrogen and phosphorus emission loads.If the wetland area and water storage capacity were both increased by 50%,the TN and TP discharge loads of the outlet would be reduced by 12.40%and 10.44%.Besides,the simulation results displayed that the change of karst development degree(changes of soil thickness,soil bulk density,soil available water content and saturated permeability coefficient)had different impact on N and P emissions from outlet of sub-basin.The variety of soil thickness(Z)or soil bulk density(POR)had the most significant influence on the emission of nitrogen and phosphorus.(6)With 19 climate change scenarios,the simulation results showed that the average annual runoff and nitrogen and phosphorus emissions were mainly affected by rainfall changes in different scenarios.With constant temperature,when the rainfall increased by 50%,the TN and TP emissions increased by 20.51%and 23.66%,respectively.