| Qinghai Province,as the water tower of Asia and the source of the Three Rivers,is very important to protect the regional water quality.The key to water quality protection is to clarify the influencing factors and their changing trends,and take scientific management and protection measures.Hehuang Valley and Qaidam Basin are two main agricultural areas in Qinghai Province.In recent years,with the warming of climate,changes in land use patterns and planting structures,local water quality has been affected to some extent,but the ways and extent of the changes in climate,land use patterns and planting structures on regional water quality were still unclear.Therefore,in this study,the surface water bodies(rivers,reservoirs and farmland irrigation water)in Hehuang Valley and Qaidam Basin were taken as the research objects,and the water quality was sampled and monitored six times in the typical monitoring sections such as the upper and lower reaches of rivers and the entrances and exits of irrigation canals in the typical agricultural areas from 2020 to 2023,and 630 measured water quality data are obtained.And collected a total of 9840 water quality monitoring data from national control monitoring sections from 2020 to 2022;At the same time,in 2021,more than 2,000 questionnaires were distributed in more than 300 villages,and the local agricultural production situation(cultivated land type,crop planting structure,use of chemical fertilizers and pesticides,breeding situation,etc.)was comprehensively investigated;The land use structure of the study area was determined by using remote sensing images.On this basis,more than 10 kinds of machine learning models(random forest,particle swarm optimization BP neural network,support vector machine,naive Bayes,etc.)were used for analysis and evaluation,aiming at clarifying the temporal and spatial distribution characteristics of water quality,main pollutants and their influencing factors in typical agricultural areas of Qinghai Province;To explore the ways and extent of the influence of agricultural production,land use structure and climate on water quality;Clarify the changing trends of total nitrogen and total phosphorus in different rivers during wet,flat,and dry seasons under changing climate and planting structure conditions,and put forward water quality protection countermeasures by changing the land use mode in the 200 m buffer zone of rivers.The main results are as follows:(1)The water quality of Hehuang Valley and the agricultural area of Qaidam Basin was generally good,and most of the monitoring points meet the national Class III water standard for surface water(GB 3838-2002),but 14 points in Hehuang Valley located in the lower reaches of rivers and reservoirs exceed the standard;In Qaidam Basin,there were 18 sampling points distributed at the outlet of irrigation canal and downstream of the main stream,all of which were in high water period and normal water period.The contents of p H,dissolved oxygen,permanganate index and ammonia nitrogen in the water were low,which meets the Class III water standard on the whole,but the concentrations of total nitrogen and total phosphorus were high: 44.83% and 8.36% of the sampling points in Hehuang Valley respectively exceed the Class V water standard of surface water;The concentrations of total nitrogen and total phosphorus in 22.83% and 23.79% of the sampling points in Qaidam Basin respectively exceeded the V water standard.The sampling points of total nitrogen exceeding the standard in Hehuang Valley were mainly distributed in Huzhu County and Minhe County,and the sampling points of total phosphorus exceeding the standard were mainly distributed in Huangyuan County and Minhe County.The samples of total nitrogen exceeding the standard in Qaidam Basin were mainly distributed in dulan county and Delingha,and the samples of total phosphorus exceeding the standard are mainly distributed in dulan county.The period of exceeding the standard varies with the location and index.(2)The effects of agricultural production,land use type and climate on the total nitrogen concentration in Hehuang Valley were 49.8%,28.94% and 17.35%,respectively,and the effects on ammonia nitrogen were 32.18%,46.89% and 16.29%,and the effects on total phosphorus were 28.66%,19.16% and 51.45,respectively.In the Qaidam Basin,the effects on total nitrogen were 6.79%,32.89% and 51.93%,ammonia nitrogen was 16.66%,38.23%and 34.93%,total phosphorus was 5.26%,38.90% and 51.39%,and permanganate index was10.29%.In agricultural production,the planting area of different crops,the amount and frequency of chemical fertilizers and the number of pesticide applications have great influence on water quality parameters.Among all climatic factors,air temperature,wind speed,air pressure and evaporation have great influence on water quality,and air temperature is positively correlated with total phosphorus in Hehuang Valley,total nitrogen and total phosphorus in Qaidam Basin.The zonal wind speed was positively correlated with the total nitrogen in Hehuang Valley,while the sea level pressure and potential evaporation are negatively correlated with the total phosphorus in Hehuang and various water quality parameters in Qaidam.There were regional and scale differences in the impact of land use on water quality: the proportion of 2000 m and 200 m forest land in Hehuang Valley was negatively correlated with total nitrogen,500 m forest land was positively correlated with total phosphorus,and 200 m,500 m and 1000 m forest land wass positively correlated with ammonia nitrogen.There was a significant positive correlation between 2000 m woodland,200 m,500 m,1000 m and 2000 m grassland and permanganate index in Qaidam Basin,and a significant negative correlation between 200 m,500 m and 2000 m bare land and permanganate index.(3)The contents of total nitrogen and total phosphorus in Hehuang Valley and Qaidam Basin will be affected by climate change.In the three future scenarios of SSP119,SSP245 and SSP585,the average content of total nitrogen in Hehuang Valley in 2100 increased by 0.83mg/L,1.20 mg/L and 1.22 mg/L,and total phosphorus increased by 0.01 mg/L,0.07 mg/L and0.08 mg/L,respectively.In addition,nitrogen and phosphorus increase greatly in wet season.Based on the changes of total nitrogen and total phosphorus,13 and 11 rivers with a large increase in pollutant concentration were selected respectively.The mean total nitrogen content in Qaidam Basin in 2100 increased by 0.11 mg/L,0.08 mg/L and 0.14 mg/L,respectively,in the three scenarios.Total phosphorus increased by 1.12 mg/L,0.92 mg/L and 0.79 mg/L respectively,and nitrogen and phosphorus increased greatly in dry season.Based on the changes of total nitrogen and total phosphorus,4 or 5 rivers with a large increase in pollutant concentration were selected respectively.(4)Affected by climate,economy and population changes,farmers’ planting intention in2100 will shift from cash crops and migrant workers to food crops.In SSP119,SSP245 and SSP585,the planting area of cash crops will decrease by 6.52%,7.22% and 9.01%,respectively.The planted area of grain crops increased by 9.78%,9.99% and 12.98%respectively.The concentrations of total nitrogen and phosphorus in water bodies of Hehuang Valley showed an increasing trend due to the change of planting structure.Total nitrogen increased by 0.25 mg/L,0.28 mg/L and 0.29 mg/L in three scenarios,and total phosphorus increased by 0.01 mg/L,0.07 mg/L and 0.08 mg/L in three scenarios,respectively.Moreover,nitrogen and phosphorus increased greatly in wet season.Based on the increase of total nitrogen and total phosphorus,10 and 6 rivers were selected which were affected by the change of planting structure.In the river of Hehuang Valley,the change of 2% and 10% of cultivated land into grassland in the buffer zone of 200 m can effectively reduce the content of nitrogen and phosphorus in water.Based on the reduction of total nitrogen and total phosphorus,6 and10 rivers suitable for the change of land use mode were selected respectively in the river of Hehuang Valley.The above results clarified the spatial and temporal distribution characteristics of surface water quality in typical agricultural areas of Qinghai Province,explore the influence of agricultural production,climate and land use types on water quality,and quantify their influence ratio,simulate the change trend of major pollutants in water under the conditions of planting structure,climate and land use changes,the research methods can provide ideas for exploring the effects of climate,agricultural production and land use on water quality.The research results can provide theoretical basis and data support for the protection of surface water quality in typical agricultural areas in Qinghai Province. |
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