| Water resources are very important to ensure the sustainable development of society.Land use,as an important factor affecting the hydrological process and water quality,not only determines the regional vegetation type and landscape pattern,but also further affects the surface water cycle process,which in turn has a certain impact on water quality.However,the mechanism by which land use affects regional water quality through vegetation type and landscape pattern needs to be studied in depth.Therefore,this paper took the Hehuang Valley of Qinghai Province as the study area,and investigated the water quality of the main streams and tributaries in Huangyuan County,Mutual Tujia Autonomous County and Minhe Hui Tujia Autonomous County during the wet and normal season.Based on the measured water quality data,the seasonal influence of land use structure and its spatial pattern on regional water quality at each spatial and temporal scale was analyzed by combining mathematical and statistical methods such as random forest model,remote sensing technology and redundancy analysis.The SWAT model was constructed to simulate the changes of nitrogen and phosphorus output in the watershed under different periods and fertilization conditions in 2020.The following main insights were obtained from the study.(1)Distribution characteristics of regional water quality in Huangshan Valley from summer 2020 to spring 2021: p H,dissolved oxygen concentration and permanganate index all met the Class III standard in the Surface Water Environmental Quality Standard(GB3838-2002)set by the state;the nitrate nitrogen concentration at each point was within the limit of the quality standard,and the proportion of sample points where sulfate concentration and chloride concentration meet the limits of environmental quality standards are 96% and 99%,respectively.However,the average value of total nitrogen concentration exceeded V standard,10.2% of sample points exceeded III standard for ammonia nitrogen concentration,and 19.7%of sample points exceeded III standard for total phosphorus concentration.The groundwater sample sites as a whole did not exceed the standard limits set by the State Ministry of Environment for water quality at water sources.Random forest model on the overall evaluation of water quality showed that a total of 32 % and 21 % of the sample points exceeded the class Ⅲ water standards in wet season and normal season respectively,Ⅳ and V water quality more concentrated in the Huangshui River main stream and the confluence of the tributaries and downstream.(2)Seasonal influence of different scales of land use structure and pattern on regional water quality in Hehuang Valley: Through RDA analysis,the explanation rate of land use on water quality in the normal season was higher than that in the wet season,the optimal scale in the normal season was 200 m buffer zone,and the farmland and towns were the main influencing factors.The optimal scale in wet season was 5km buffer zone scale,and the main influencing factors were forest land.Farmland,grassland and towns were the"source"landscape of pollutants,but farmland also played a role in intercepting pollutants to a certain extent;forest land was the"sink"landscape of pollutants.The spatial pattern of forest land at the scale of 200m buffer zone in normal season had a high explanation rate on water quality,in which indicators such as LPI(maximum patch index)and PD(patch density)were the main influencing factors,which were negatively correlated with pollutant concentration.The study showed that reasonable planning of residential land and farmland area ratio,improve the forest land coverage and aggregation around the riparian zone,is an important measure to purify the surface water quality in Hehuang Valley.(3)The influence mechanism of land use structure and spatial pattern of Huangshui River basin on the nitrogen and phosphorus load in regional water bodies: The hydrological cycle and nutrient cycling process of Huangshui River basin were simulated by establishing SWAT model.Compared with not adding fertilization data to the model,total nitrogen and total phosphorus load in Huangshui River basin increased after adding fertilization data,and it was mainly due to the increase of nitrate nitrogen and organic phosphorus.The periods with larger increases in nitrogen and phosphorus loads mainly occurred from March to April and July to September,when agricultural fertilization led to the accumulation of nitrogen and phosphorus nutrients in the soil and migrated with the runoff in the watershed during the wet season,thus intensifying the output of nitrogen and phosphorus nutrient salts in the watershed.On the spatial scale,the areas with greater increase in nitrogen and phosphorus load were mainly in the important tributaries of Datong River and the downstream watershed of Huangshui River.More concentrated farmland and towns increased the output of nitrogen and phosphorus elements in the watershed.The explanation rate of the increase in nitrogen and phosphorus load was higher for the proportion of farmland area in normal season and for the proportion of town area in wet season.Theoretically,grassland can purify water quality to some extent.The higher the grassland fragmentation,the greater the nitrogen and phosphorus load into the water generated by fertilization.The above research results clarify the water pollution characteristics of the study area,reveal the influence mechanism of local land use structure and pattern on hydrological and water quality,which can provide theoretical basis and technical reference for reducing pollutant loss,optimizing land use structure and water resources protection,and improving water ecological environment quality in Hehuang valley... |
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