Baseflow: Potential Pathway Underlying a High Nitrogen Concentration in a Less-rainy Watershed—Taking Chaohe River Basin of China as an Example

Baseflow is one of the major pathways of runoff in hilly areas, and its contributions to surface water resources and pollutant loads cannot be ignored. In this study, based on water quantity and quality data from 1988 to 2019 in hilly and low rainfall watersheds, we focused on the impact of long-term baseflow on nitrogen load using the load allocation based on the baseflow separation method. We also constructed a nitrogen balance model for the Chaohe River Basin of China from 2012 to 2021 to analyze the nitrogen accumulation in the basin. We used the baseflow nitrogen load lag analysis method to study the lag characteristics of the baseflow discharge process and analyzed the response and periodicity of baseflow nitrogen to precipitation and soil accumulation using time delay analysis. The results showed that the contribution rate of baseflow nitrogen reached 69% and showed a slight increasing trend from 1988 to 2019. The effects of changes in precipitation and nitrogen accumulation on the baseflow contribution was observed after 1−2 and 2 yr, respectively. After nitrogen accumulation, it entered the river channel through baseflow, which was already the main and continuous source of nitrogen in rivers in hilly areas.