Simulation of water and nitrogen movement mechanism in cold regions during freeze-thaw period based on a distributed nonpoint source pollution model closely coupled water, heat, and nitrogen processes at the watershed scale.

The nitrogen cycle in cold regions during the freeze-thaw period is complex. Although previous studies have investigated the phenomenon of nitrogen transport and transformation, the underlying mechanisms are vague. Existing models have limitations in terms of loose coupling or weak physical mechanisms. Therefore, a new distributed nonpoint source pollution model, the water and energy transfer processes and nitrogen cycle processes model in cold regions, was developed in this study, with closely coupled water, heat, and nitrogen processes at the watershed scale. The model considered the driving effects of pressure, gravity, solute, and temperature potentials on water and nitrogen movement in soil and the transformation relationship among nitrogen forms. Physical evaluation and simulations were conducted for the Heidingzi River Watershed during two freeze-thaw periods: 2017-2018 and 2018-2019. The soil temperature absolute error was < 0.82 â„ƒ. The relative errors in stratified liquid water, soil nitrogen content, river flow rate, and river nitrogen concentration were mostly < 10%. Nitrogen transport with water had an obvious "upward agglomeration effect" during the freezing period and a "concentrated release effect" during the thawing period, which was attributed to changes in soil water potential as the freezing front moved down. Disregarding the effects of solute potential and temperature potential will result in an underestimate of the outflow of pollutants during the thawing period. The model can be applied to reveal water quality deterioration in cold regions during thawing.