| The Three Gorges Reservoir Area(TGRA)is one of the most ecologically sensitive areas in China.The construction of the Three Gorges Dam has led to a rise in water levels in TGRA and the inundation of a large amount of arable land.Migration has led to greater tension between people and land,seriously affecting the agricultural development and economic income of farmers in TGRA.The high income from citrus has led to the expansion of citrus orchards in the reservoir area.The excessive use of pesticides and fertilisers in agricultural production has led to the loss of large amounts of nitrogen and phosphorus,which has become the main source of nonpoint source pollution in the reservoir area and poses a serious threat to the water quality of the reservoir.The input,migration,and retention of nitrogen and phosphorus are usually influenced by specific environmental factors such as rainfall,land use,landscape pattern,and topography of a watershed,as well as socioeconomic factors such as population,economy,and agricultural production activities.The interaction between these determinants makes the relationship between determinants and nitrogen and phosphorus loss extremely complex.In addition,landscape characteristics are sensitive to changes in spatial patterns,resulting in variable effects of buffer landscape characteristics on nitrogen and phosphorus loss at different spatial scales.The relationship between buffer landscape characteristics on nitrogen and phosphorus loss is related to the distance of nitrogen and phosphorus along the hydrologic path into the receiving water body.The flow paths of nitrogen and phosphorus or the transport of nitrogen and phosphorus pollutants at multiple spatial and temporal scales are influenced by changes in hydrologic connectivity.The influence of hydrologic connectivity on nitrogen and phosphorus loss in watersheds remains unclear.The study of the determinants of nitrogen and phosphorus loss from typical small watersheds in TGRA and the influence of landscape characteristics and hydrological connectivity of buffer zones at different spatial scales on nitrogen and phosphorus loss can provide a theoretical basis for the management of nonpoint source pollution in the reservoir area and the development of stream water quality management programs.Therefore,the Wangjiaqiao watershed in TGRA was selected as the study area for this study.Through the results of field survey and land use delineation,13 water quality monitoring sites in the watershed were selected for long-term field fixed baseflow water quality monitoring and water quality monitoring based on rainfall events.Based on the data of land use,landscape pattern,topography,hydrological connectivity and socioeconomic in the study area,LOADEST model,two-parameter numerical filtering model,partial least squares regression model,redundancy analysis,hierarchical partitioning method and nonparametric mutation analysis were used in our study.We systematically studied the spatial and temporal distribution characteristics of nonpoint source pollutants in typical mountainous watersheds,clarified the contribution of baseflow and surface runoff to the nitrogen and phosphorus output of water bodies,revealed the nitrogen and phosphorus output characteristics of typical rainfall events in watersheds,analyzed the influence of environmental and socioeconomic factors on the nitrogen and phosphorus output of watersheds,and elucidated the influence of landscape characteristics and hydrological connectivity of buffer zones at different spatial scales on the nitrogen and phosphorus concentration of watersheds.The main findings of the study are as follows.(1)The nitrogen and phosphorus concentrations in Wangjiaqiao watershed showed obvious spatial and temporal distribution characteristics.The main performance of nitrogen and phosphorus pollution was high nitrogen and low phosphorus.Both TN and NO3~--N concentrations showed an increasing trend year by year.The NH4~+-N and TP concentrations showed a decreasing and then increasing trend.The water bodies in the small watershed were facing serious nitrogen pollution and mainly NO3~--N pollution,while phosphorus pollution was light.Human agricultural activities as well as rainfall caused the nitrogen and phosphorus concentrations in waters of the watershed to be greater during the wet season than during the dry season.Differences in land use also resulted in distinct spatial distribution characteristics of nitrogen and phosphorus concentrations.(2)Nitrogen and phosphorus export pathways in the Wangjiaqiao watershed were different.Model 3 of LOADEST model was found to be the optimal equation for the simulation of daily TN loading at the watershed outlet through calibration and validation,and model 9 was the optimal equation for the simulation of daily TP loading at the watershed outlet.The average annual TN output load was 60.78 t/year,of which the annual average contribution of baseflow and surface runoff to the total nitrogen load was 37.05t/year and 23.73 t/year,respectively.The TN output load was dominated by baseflow.The average annual TP load was 0.67 t/year,of which the average annual contributions of baseflow and surface runoff to total phosphorus load were 0.18 t/year and 0.49 t/year,respectively.The TP output load was dominated by surface runoff.The TN output load of the watershed was much higher than the TP load.(3)The nitrogen and phosphorus output patterns in different levels of rainfall events were different.There were obvious peaks of rainfall intensity and flow rate in the three levels of rainfall events,and the peak flow rate lagged behind the peak of rainfall intensity.The higher the rainfall intensity,the higher the peak flow rate.TN and NO3~--N concentrations showed a trend of decreasing and then increasing,and NH4~+-N and TP concentrations showed a trend of increasing and then decreasing during the rainfall runoff.TN and NO3~--N showed no significant first flush effect and were mainly transported in the middle and late stages of runoff through the loamy midstream.NH4~+-N and TP showed significant first flush effect and were mainly exported with runoff sediment in the early stage of rainfall runoff.(4)The determinants of variability in nitrogen and phosphorus concentrations in the Wangjiaqiao watershed were different.The key factors influencing TN concentration were topographic wetness index(TWI),planting structure(PS),interspersion and juxtaposition index(IJI),orchard proportion(OP),nitrogen fertilization(NF),per capita income(INCOME)and catchment area(AREA),while the key factors influencing TP concentration were slope gradient(SLOPE),topographic wetness index(TWI),hypsometric integral(HI),interspersion and juxtaposition index(IJI)and population density(POP).Environmental factors had a greater influence on the variability of nitrogen and phosphorus concentrations in Wangjiaqiao watershed.(5)The impact of watershed landscape characteristics and hydrological connectivity on stream nitrogen and phosphorus concentrations showed seasonal and scale differences.The explanation rates of landscape characteristics and hydrological connectivity of the watershed in the dry season were greater than those in the wet season.The explanation rates of landscape characteristics and hydrological connectivity in the buffer zone were larger than those in the sub-basin scale.30m buffer zone landscape characteristics and hydrological connectivity explained the largest variation in stream nitrogen and phosphorus concentrations(dry season:80.05%;wet season:72.87%).The individual effects of watershed landscape characteristics and hydrological connectivity on stream nitrogen and phosphorus concentrations were ranked in the following order:land use(37.2%-52.8%)>topography(19.5%-31.3%)>hydrological connectivity(12.3%-19.0%),and landscape pattern(1.5%-20.3%).The percentage of orchard area was the most important explanatory variable for the variation of nitrogen and phosphorus concentrations in water bodies in the buffer zone,which should be less than 0.60 in the 30 m buffer zone and less than 0.40 in the 60 m and 90 m buffer zones.TWI was the most important explanatory variable for the variation of nitrogen and phosphorus concentrations in water bodies in the sub-basin,with a threshold value of 7.30. |
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