Research On Non-point Source Pollution Export Characteristics And Nutrient Import/Export In Danjiangkou Reservoir Area

Non-point source pollution has been identified as a major cause of water quality deterioration and a crucial trigger for the safety of human life.The generation and export of non-piont source pollution is related with multiple factors,including hydrological condition,physiographic charcteristics,landscape composition and pattern,or the population distribution,human pollution inputs.Previous non-point source pollution study have been carried out in watershed scale.Understanding the factors that affect seasonal non-point source pollution characteristics is critical for developing water quality models.Estimating the human pollution inputs and the contribution to riverine pollution outputs can provide effective and useful information for water quality management and pollution prevention.The Danjiangkou Reservoir as a source of drinking water,its water quality problem is concerned.Based on the collection of topography,soil,climate,land use and socio-economic data in the study area,we conducted field investigation and routine monitoring of the watershed.Our study used remote sensing,geography information systems,landscape pattern analysis,partial least-squares regression model,nutrient balance method,the human input of net nitrogen model,path analysis and other methods to research for non-point source pollution from multiple perspectives.We constructed the model of non-point source pollution output responsed to the hydrological and environmental background characteristics of the Hujiashan watershed and found the main controls that affect seasonal output of pollutants.Meanwhile,we evaluated the situation of nutrient import and export in a typical small watershed.We found out each kind of pollutant source input and its contribution to the non-point source output in 9 study watersheds.The main results and conclusions of the study are as follows:(1)In order to understand the non-point source pollution characteristics of Danjiangkou reservoir,we set water quality monitoring points at the tributaries outlets in the Hujiashan watershed.Long-term monitoring of water quality indicators such as nitrogen and phosphorus were carried out.The total nitrogen and total phosphorus concentrations showed a downward trend from 2008 to 2015.The total nitrogen concentration ranged from 0.38 mg/L to 7.18 mg/L,the highest average total nitrogen concentration was 5.06 mg/L in 2008.The total phosphorus concentration ranged from 0.02 mg/L to 0.53 mg/L,and peak of 0.27 mg/L was in 2008.The total phosphorus concentration showed a downward trend in the followed 5 years and reached the lowest level of 0.03 mg/L in 2010.The main water quality characteristic of the Hujiashan watershed was high nitrogen concentration and low phosphorus concentration,and water quality has been gradually optimized since the monitoring.There was a significant difference of nitrogen concentration during dry season and wet season,but no significant seasonal difference in total phosphorus concentration.(2)Nitrate is the main nitrogen export form in the Hujiashan watershed.The nitrate concentration in wet season was higher than that in dry season from 2008 to 2012,with an average of 2.07 mg / L in wet season and 1.79 mg / L in dry season.The nitrate flux in different seasons was estimated by hydrological simulation and monitoring data.Nitrate flux showed significant seasonal difference(p <0.001),the nitrate flux in dry season was from 0.05 kg/ha to 3.75 kg/ha.The average nitrate flux was 4.23 kg/ha during the wet season corresponding higher runoff.(3)The main controlling factors of nitrate expot in different seasons in the Hujiashan watershed were different.We extracted the hydrological and landscape characteristics of 15 nested sub-watersheds in the Hujiashan watershed.The partial least-squares regression models linking the watershed characteristic factors with nitrate concentrations and fluxes in different seasons were useful to find the main control factors.The dominate controls of annual nitrate concentration and flux were the area percentage of agriculture,forest and residential areas,followed by runoff index(RC),slope(SLOPE),slope variance(SGV),the flow path gradient(FPG),and largest patch index(LPI)and split index(SPLIT),but the contribution order of each factor to nitrate concentration or flux was not the same.Rainfall(RAIN),flashiness index(FI)and baseflow index(BFI)only played a significant role in the change of nitrate flux,and nitrate flux in dry season was more sentive to hydrological indexs.(4)The type of planting crops and the way of fertilizer application affect the nutrient import and export and determine the extent of non-point source pollution.Based on field surveys of farmers,we determined multiple paths of nitrogen input and output in a typical small watershed,and nutrient balance model was used to estimate nitrogen balance in the Wulongchi catchment.The types of crops planted in the Wulongchi catchment in the first half and second half of the year were different.The nutrient balance was closely related to the seasonal changes and crop types.Tobacco and rape planting areas were the largest areas of nitrogen surplus,and the corn planting area was the largest area of nitrogen loss.Tillage methods,fertilization,and planting structure affected the status of watershed nutrients in a certain extent.(5)Pollution inputs from human have a decisive effect on the production of non-point source pollution.Based on the socio-economic statistics of 9 watersheds in the Danjiangkou Reservor Area,various types of nitrogen sources,including atmospheric deposition,chemical fertilizers application,nitrogen fixation and net import in food and feed,were estimated using the Net Anthropogenic Nitrogen Input(NANI)model.The results showed that the nitrogen input of each basin little changed in five years,but the main input sources of each basin were obviously different.As a whole,the main anthropogenic sources of pollutions were fertilizer application and net import in food and feed,accounting for 42.0% and 41.8% of NANI,respectively,followed by atmospheric deposition and biological nitrogen fixation,accounting for 11.7% and 4.5% of NANI.About 14% of NANI exported to the river,and the rest nitrogen was stored in the watershed or returned to the atmosphere as gas in our study.The direct effects(direct path coefficients)of NANI components on nitrogen output were 0.148,0.593,0.358 and 0.357,respectively.In addition,NANI components expressed indirect effects on nitrogen export via other variables,and the indirect effect could be characterized by indirect path coefficients.