| Under the dual influence of human act ivities and climate change,the non-point source pollution(NSP)situation in the river basin is severe,and it is also facing the problems of multiple factors coupled with the interaction of multiple temporal and spatial processes.In this paper,the Shaanxi Section of the Hanjiang River Basin is used as the research area,and the NSP study of the basin is carried out through a combination of monitoring and experiment.The variation characteristics of hydrological and meteorological elements in the basin are clarified.The characteristics,rules and mechanisms of NSP in the Hanjiang River Basin are investigated.A distributed NSP model is established.The impact of land use change and future climate change on NSP are discussed.The main research results and conclusions are as follows:(1)Through the analysis of the spatial and temporal variations of the meteorological,hydrological factors,it is found that the rainfall of the basin has a downward trend,the precipitation intensity has a slight upward trend,and the temperature has a significant upward trend.The average annual temperature of the basin in the past decade has increased by 1.0℃compared with that in the 1980s.Rainfall,precipitation intensity and temperature in the basin all have a main period of about 27 a.The rainfall and precipitation intensity increased from north to south,and the temperature gradually increased from northwest to southeast.The runoff of Wuhou Town,Ankang Station and Danfeng Station showed an insignificant decreasing trend at the significant level of 0.05,and the runoff of Majie Station showed an insignificant upward trend.There is no obvious change in the inter-annual runoff of each hydrological station.The runoff of the three hydrological stations has a main period of about 20 a,and the runoff of the Majie station has a period of about 7 a.The sediment of Wuhou and Ankang stations did not increase significantly with time,and the sediment of Majie and Danfeng stations did not decrease significantly.The periodicity of sediment at the four hydrological stations is not obvious.(2)The study of the NSP process under the influence of runoff plots,the Yangliu Small Watershed,and the basin above the Ankang Section of the Hanjiang River,shows that the rainfall-runoff in the three typical regions presents a significant non-linear relationship.There is a high positive correlation between runoff,sediment,and nitrogen and phosphorus output.The average intensity of nitrogen and phosphorus loss in each runoff plot are 0.12 kg/ha and 0.0137 kg/ha,respectively.The average intensity of nitrogen and phosphorus loss in small watershed are 0.16 kg/ha and 0.0165 kg/ha,respectively,it is found that the nitrogen and phosphorus loss in runoff plot is far less than that in small watershed scale.The sediment transport modulus of small watershed in flood season is 8.04 t/km2.Compared with the average soil loss of 1.31 t/km2 during the flood season of the plot,it is found that the soil loss in runoff plot is far less than that in small watershed scale.The main form of nitrogen loss and phosphorus loss in small watersheds and runoff plots are nitrate nitrogen and orthophosphorus,respectively.The average proportion of NSP loads of different monitoring indexes in the basin above Ankang section is over 60%for many years,and the contribution of individual years can reach over 80%.(3)The distributed NSP model is constructed from the rainfall runoff,soil erosion,and pollutant migration and transformation,and it is verified at different spatial scales.The distributed time-varying gain model(DTVGM)and the inverse Gaussian confluence model are selected for the rainfall-runoff module.The simulation results are as follows:The Nash coefficients(NSEs)of flood process simulation during the calibration(6 events)and verification(2 events)in the Yangliu small watershed of 2020 reach 0.68 and 0.73,respectively.The NSEs of the annual,monthly,and daily runoff processes during 2003-2018 in the Henghe River Basin reached 0.94,0.93,and 0.73,respectively.The NSEs of the annual,monthly,and daily scale discharge processes in the basin above the Ankang section from 2003 to 2018 are 0.95,0.91,and 0.68,respectively.The soil erosion module adopts the modified general soil loss equation(RUSLE),and the simulation results are as follows:the annual sediment transport ratios of the Yangliu small watershed and the basin above the Ankang section are 0.445 and 0.36,respectively.It is consistent with the research results of the Yangtze River Water Resources Commission(the sediment migration ratio in the Yangtze River Basin is about 0.1～0.4).Combined with soil erosion and sediment production process and runoff production and confluence process,the particulate NSP model and dissolved NSP model are established respectively.The simulation results are as follows:The loss of particulate nitrogen(PN)and particulate phosphorus(PP)in the small watershed is 31.36 kg/(hm2-a)and 14.66 kg/(hm2-a),respectively.The loss of PN and PP in the Ankang section is 957.84 kg/(km2-a)and 85.62 kg/(km2-a),respectively.By simulating the processes of pollutants such as TN,NH3-N,NO3-N,TP and SRP in different runoff stages of small watershed,the mean corresponding NSEs are 0.69,0.74,0.79,0.71 and 0.71,respectively.The NSEs of the NH3-N and TP pollution processes in the basin above the Ankang section are 0.78 and 0.83.This shows that the model is applicable in the study area and the simulation results are credible.(4)The land use change in the Shaanxi Section of the Hanjiang River Basin during 1995-2020 is relatively small,and the forest land has increased significantly in the past ten years.The dominant position of patch types is increased obviously,and the degree of fragmentation is alleviated,and the landscape types are more diverse than the original level.Through the analysis of the spatial distribution of NSP and the comparison with the results of the SWAT model,it is found that the simulation results are consistent.The more pollutants are produced in the southern part of basin with heavy rainfall.Grassland,as the largest land use type in the basin area,is also the most serious type of soil erosion.Both grassland and cultivated land contribute more to soil erosion and PN and PP.The 8-15°region brings the largest amount of soil erosion and the largest contribution of PP and PN load,and the contribution rate of the region of 5-8° is the second.The load of dissolved nitrogen and phosphorus decreases year by year,and grassland is the land use type that contributes the most to the load,followed by forest land and cultivated land.The region with a slope of 0～5° has the largest dissolved load,the slope 8～15° and 5～8° region is the next.The nitrogen and phosphorus load of NPS shows an obvious positive correlation with contagion index(CONTAG),the largest plaque index(LPI)and aggregation index(AI),the dissolved NH3-N and TP loads show a positive correlation with Landscape shape index(LSI),LPI and AI,indicating that the increase of landscape diversity,fragmentation degree and clustering pattern would increase the risk of nutrient output.(5)The weather generator NCC/GU-WG is used to generate climate change scenarios for the research area in the next 30 years(2021-2050),and the satisfactory result have been obtianed from historical meteorological observation data and the climate forecast period elements.The simulation effect of temperature is better than that of rainfall,and the simulation effect of daily minimum temperature is better than that of daily maximum temperature.Compared with the base period(1971～2000),the daily rainfall will not change much in the future scenarios.The rainfall of other stations except Shiquan Station will decrease,and the daily maximum/minimum temperature of each station will increase slightly.The response analysis of NSP load under climate change scenarios shows that the runoff,NH3-N,and TP changes of the basin in the next 30 years will all show a slight upward trend. |
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