Identifying controlling climate factors conducive to water and nitrogen export from agricultural watershed during snowmelt runoff period by using the SWAT model

Abstract. Temperature and precipitation variations during the freezing-thawing period affect snowmelt and accompanying nitrogen export in a complex manner. These influences can be long-lasting, superimposed, and strengthened. Daily discharge and nitrate nitrogen NO₃^--N concentrations were monitored during the snowmelt periods of 2015 and 2016 in an agricultural watershed in northeastern China. The SWAT model was used to simulate the water and NO₃^--N export during the snowmelt period of 1951–2014 to identify the controlling climate factors and confirm their suitable combination that facilitates snowmelt water and NO₃^--N export. Our results show that the SWAT model performs well for Re values in simulating the daily snowmelt runoff and NO₃^--N export, but poorly for NSE and R² values in simulating NO₃^--N export. This is attributed to the absence of snowmelt water refrozen and hysteresis modules. The number of days and precipitation of the stable freezing period and the stating day of snowmelt period are controlling factors of daily snowmelt runoff, while daily NO₃^--N export are mostly affected by precipitation during the snowmelt period. The combinations of climatic factors favored by snowmelt runoff and NO₃^--N export were different. Years with longer stable freezing periods, later snowmelt period starting days, and higher rainfall during snowmelt, more readily generated high snowmelt runoff. Correspondingly, later appeared, higher and concentrated rainfall events, and higher temperatures between these rainfall and snowmelt events favored the NO₃^--N export. Finally, this study is of great importance for the prevention of spring floods and water pollution during snowmelt periods.