Implementation of salt-induced freezing point depression function into CoupModel_v5 for improvement of modelling seasonally frozen soils

Abstract. Soil freezing/thawing is important for soil hydrology and water management in cold regions. Salt in agricultural field impacts soil freezing/thawing characteristics and therefore soil hydrologic process. In this context, we conducted field experiments on soil water, heat and salt dynamics in two seasonally frozen agricultural regions of northern China to understand influences of salt on cold regions hydrology. We developed CoupModel by implementing impacts of salt on freezing point depression. We employed a Monte-Carlo sampling method to calibrate the new model with field observations. The new model improved soil temperature mean error (ME) by 16 % to 77 % when new freezing point equations were implemented into CoupModel. Nevertheless, we found that parameters related to energy balance and soil freezing characteristics in the new model were sensitive to soil heat and water transport at both sites. However, a systematic model sensitivity and calibration has shown to be able to improve model performance, with mean values of R² from behavioral simulations for soil temperature at 5 cm depth as high as 0.87 and 0.90, and mean value of R² for simulated soil water (liquid or total water contents at 5 cm depth) of 0.31 and 0.80 at site Qianguo and site Yonglian, respectively. This study provided a new approach considering influences of salt on soil freezing/thawing in numerical models and highlighted the importance of salt in soil hydrology of seasonally frozen agricultural soils.