| The problem of water eutrophication caused by the overexploitation and utilization of land in the river basin has been attached great importance by the government.Tianmu Lake basin was chosen as the study area because it can represent the vast majority of basins in southern China where water bodies are eutrophicated due to overreclamation.The water ecosystem of the Daxi and Shahe reservoirs in the basin has been seriously threatened by a variety of pollution sources,which are associated with many complex human activities,especially agricultural production.In this study,on the premise of analyzing the water quality of the basin,the key pollution sources of nitrogen and phosphorus in the basin were determined according to the nutrient load and nutrient load intensity by using the optimized SWAT hydrological model,and then the main pollution causes were discussed,and the corresponding treatment measures were proposed.The main research results are as follows:（1）According to the measured data of total nitrogen（TN）and total phosphorus（TP）in Shahe and Daxi reservoirs from 2000 to 2019 and the permanganate index（CODMn）from 2008 to 2018 and the"Surface Water Environmental Quality Standard"（GB3838）-2002)A comparative analysis of the water quality standards in the middle of the year resulted in the water quality status of Tianmu Lake Reservoir.In terms of average concentration,TN of Shahe and Daxi reservoirs are mainly classified as Class III-IV,TP is classified as Class II-III,and CODMn is stable at Class II water quality standard.From the perspective of the storage volume,the TN storage volume of the Shahe Reservoir exceeded the Class II standard capacity in different periods.The Daxi Reservoir TN only reached the Class II standard capacity during the dry season from 2000 to 2008,and failed to meet the standard in the rest of the time.Both TP and CODMn of the two major reservoirs exceed the standard capacity of Type II during the wet season,and can reach the standard capacity of Type II after 2017 in the dry season and normal water period.Excessive discharges in Shahe Reservoir are more serious than Daxi Reservoir.（2）Analyze the pollution status of slope runoff from different land use types through five sampling results.TN,TP,ammonia nitrogen（NH3）and CODMn of 14types of land use were analyzed separately.Among them,the pollution of tea gardens,rural residential land,cultivated land,and woodland is more serious,specifically TN pollution of tea gardens,TP and NH3pollution of rural residential land,CODMn pollution of cultivated land,and TN and CODMn pollution of forest land.Through the comparison of actual data,it is found that the work of purchasing and storing tea gardens and returning farmland to forests adopted by local governments has a certain effect on the water quality of the river basin.The concentration is reduced by about50%.（3）A hydrological model of the Tianmu Lake basin was established based on Arc SWAT,and the parameters of the model were calibrated and verified using the data from 2013 to 2017.The applicability evaluation results of the model show that the accuracy of the model meets the simulation requirements of runoff and nitrogen and phosphorus in the study area,and can be successfully applied to the Tianmu Lake basin.Therefore,based on the SWAT model,the nitrogen and phosphorus load output of the two major reservoirs from 2014 to 2018 was calculated.Three methods of nutrient load,nutrient load intensity,nitrogen and phosphorus comprehensive index were selected to identify key pollution source areas.Among them,18 key pollution source areas were identified based on nutrient load;25 key pollution source areas were identified based on nutrient load intensity;29 key pollution source areas were identified based on comprehensive nitrogen and phosphorus indicators.When analyzing the contribution of different sources to the nutrient load,first of all,from the perspective of the overall Tianmu Lake basin,tea gardens,woodlands,arable land,and garden land（except tea gardens）contribute more to the nutrient load,and tea gardens have the highest contribution rate.From the perspective of the main polluted rivers,the main sources of TN and TP nutrient loads in Daxi Reservoir are tea gardens and cultivated land.The TN load of Shahe Reservoir mainly comes from woodland,garden land and cultivated land,and the TP load mainly comes from tea garden,woodland and point source.Finally,from the perspective of key pollution source areas,tea gardens and cultivated land are the main nutrient contributors to the Daxi Reservoir.The main nutrient sources of Shahe Reservoir are tea gardens,garden plots,arable land,woodland and point sources.（4）To evaluate the reduction effect of the best management measures in the key pollution source areas identified by the two major reservoirs,reduce the amount of fertilization on cultivated land by 20%,reduce the amount of fertilization in tea gardens by 20%,set up 10M vegetation filter belts,return farmland to forests,purchase and store tea gardens,and combine them.Type（reduction of fertilization by20%+vegetation filter belt and returning farmland to forest+tea garden purchase and storage）reduction measures.The average reduction rate of measures to reduce the amount of fertilizer is below 10%,and the effect is limited.The average reduction rate of total nitrogen is about 35%by setting up a 10m vegetation filter belt,and the total phosphorus is as high as 70%or more.The average reduction rate of total nitrogen and total phosphorus in returning farmland to forests is 15%and 20%or more.The average reduction rate of total nitrogen and total phosphorus in tea garden purchases and storage is above 50%.The reduction effect of a single measure is that the amount of fertilization on arable land is reduced by 20%<the amount of fertilization in tea gardens is reduced by 20%<returning farmland to forest<10M vegetation filter belt<tea garden purchase and storage.The research results also show that combined reduction measures have better governance effects than single reduction measures.Among them,the combined measures of returning farmland to forests and tea garden procurement and storage have the best pollution reduction effect.According to the treatment target of a non-point source pollution treatment rate of over 85%in the"Tianmu Lake Protection Plan（2018-2035）",the combined reduction measures are compared with the reductions of the two major reservoirs meeting the Class II water quality standards.The total phosphorus of the reservoir and the total nitrogen of the Daxi Reservoir have reached the governance target. |
