Considering atmospheric NO dynamic in SWAT model avoids the overestimation of NO emissions in river networks.

Modeling studies have focused on NO emissions in temperate rivers under static atmospheric NO (NO), with cold temperate river networks under dynamic NO receiving less attention. To address this knowledge and methodological gap, the dissolved NO concentration (NO) and NO algorithms were integrated with an air-water gas exchange model (F) into the SWAT (Soil and Water Assessment Tool). This new model (SWAT-F) allows users to simulate daily riverine NO emissions under dynamic atmospheric NO. The spatiotemporal fluctuations in the riverine NO emissions was simulated and its response to the static and dynamic atmospheric NO were analyzed in a middle-high latitude agricultural watershed in northeastern China. The results show that the SWAT-F model is a useful method for capturing the hotspots in riverine NO emissions. The model showed strong riverine NO absorption and weak NO emissions from September to February, which acted as a sink for atmospheric NO in this cold temperate area. High NO emissions occurred from April to July, which accounted for 83.34% of the yearly emissions. Spatial analysis indicated that the main stream and its tributary could contribute 302.3-1043.7 and 41.5-163.4 μg NO/(m·d) to the total riverine NO emissions (15.02 t/a), respectively. The riverine NO emissions rates in the subbasins dominated by forests and paddy fields were lower than those in the subbasins dominated by arable and residential land. Riverine NO emissions can be overestimated under the static atmospheric NO rather than under the increasing atmospheric NO. This overestimation has increased from 1.52% to 23.97% from 1990 to 2016 under the static atmospheric NO. The results of this study are valuable for water quality and future climate change assessments that aim to protect aquatic and atmospheric environments.