[Spatial-temporal Variations and Driving Factors Analysis of Total Nitrogen Concentration in the Haihe River Basin...

The Beijing-Tianjin-Hebei （Jing-Jin-Ji） Region is home to the most acute economic, resource, and environmental conflicts in the Bohai Sea region, and the rivers entering the sea carry abundant total nitrogen （TN） input into the Bohai Bay, which is the main land-based input causing eutrophication of the bay. The Haihe River Basin in the Jing-Jin-Ji Region was divided into 112 （2018-2019） and 187 （2020-2022） control units, and the spatial and temporal variations in TN concentration in the surface water of the Haihe River Basin in the Jing-Jin-Ji Region were systematically analyzed from 2018 to 2022 by combining the Euclidean distance analysis method and the K-means clustering analysis method. The results showed that the annual average concentration of TN in the region showed a trend of decreasing （2018-2020） and then increasing （2021-2022）, in which the concentration of TN increased significantly from June 2021 to June 2022. The concentration of TN throughout the year showed a "U"-shaped distribution, with a lower concentration in the abundant water period and a higher concentration in the dry water period. Compared with that in the dry water period, the number of control units in the high-concentration interval decreased, and the number of control units in the low-concentration interval increased in the abundant water period. TN concentrations were higher in control units during the flat-water period （October to December） compared to those in the flat-water period （March to May）. Control units with concentration gradients exceeding 10 mg·L^-1 and 0-1 mg·L^-1 were characterized by significant spatial migration, with the center of mass of control units exceeding 10 mg·L^-1 shifting to the northeast, and the center of mass of control units with 0-1 mg·L^-1 shifting from the center to the south-central part. Three patterns of TN concentration changes in the control unit were identified, and one "U"-shaped pattern and one "W"-shaped pattern characterized the increase in TN concentration in the flood season. The spatial and temporal distribution of TN concentration in the watershed was characterized by the combined effects of soil nitrogen storage, rainfall, and temperature. The significant increase in TN concentration after 2021 was caused by the short-term flushing of the extreme rainfall event in the summer of that year, and by the effect of groundwater uplift after heavy rainfall.