Simulation Of Freezing-thawing Soil Water-heat Process In Black Soil Area Of Songnen Plain

The black soil area of the Songnen Plain is a typical seasonal frozen soil area in Northeast China.During the freezing period,the soil surface is generally covered with stable snow.The freezing and thawing of snow directly cause the redistribution of soil water and heat characteristics,which affects the soil spring moisture,crop growth and development.Due to the complexity of the water-heat coupling process of freezing and thawing soil under snow cover,the mechanism and model simulation of this process need to be further improved.In this study,the soil of black soil area of Songnen Plain was taken as the research object.Through laboratory and field experiments,the water and heat movement parameters and the process of water and heat movement of freezing and thawing soil under different snow cover conditions were measured,and the regulars of water and heat movement were analyzed.By using the COUPMODEL model of water and heat transport based on physical processes and introducing the universal likelihood uncertainty estimation method,the uncertainty of the simulation of water and heat transport in freezing and thawing soil under different snow cover conditions were deeply analyzed.The difference between the SHAW model and COUPMODEL model were compared,and then the differences of the simulation results were analyzed.The main results were as follows:(1)The coupled water-heat COUPMODEL model of freezing and thawing soil under bare band,natural snow,compacted snow and thickened snow conditions were established.The snow,soil water and heat transport parameters were estimated by combining with the GLUE method.Uncertainty analysis showed that there were different sensitivity parameters under four different snow cover conditons,and the number of sensitive parameters were different.The number of sensitivity parameters from more to less were: compacted snow,thickened snow,natural snow,bare land.After the sensitive parameters were calibrated by GLUE method,the uncertainty of the simulation results about soil temperature and soil liquid water content under different snow cover conditions have been reduced to some extent,but there were still some uncertainty in modeling results.It was necessary to increase the observation of the required parameters.Meanwhile,the observation accuracy needed to be improved for datas such as driving and verification.The qualitative analysis of the good or bad about the uncertainty of the simulation results,that showed the width of the uncertainty interval about the soil temperature under different snow cover conditions were relatively narrower than the soil liquid water content,and the number of measured values falling into the uncertainty interval were more.(2)Based on the uncertainty analysis,the COUPMODEL model selected the average value of each parameter under different snow cover conditions to simulate and analyze the dynamic change process of soil temperature and soil liquid water content.The COUPMODEL model could generally simulate the soil temperature of each layer under different snow cover conditions.However as the depth increases,the simulated values were less than the measured values,mainly related to the model's setting of the freezing temperature threshold.Relative to the soil temperature of each layer under different snow conditions,the simulation effect of the soil liquid water content of each layer was slightly worse,but the model could basically simulate the change process of the liquid water content of each soil layer under different snow conditions.Some deviations in the simulation results may related to the snow and soil water and heat parameters still needed to be calibrated.(3)Based on the differences between the SHAW model and the COUPMODEL model in terms of model principle and operating structure,the soil temperature and soil total water content under different snow cover conditions were simulated.The results showed that the COUPMODEL model and the SHAW model both could greatly simulate the dynamic change process of soil temperature.But simulation results of the COUPMODEL model was better than the SHAW model in terms of consistency,and the SHAW model was slightly better than the COUPMODEL model in terms of deviation and agreement.Compared with the measured value,the simulated value of the COUPMODEL model was low,and the simulated value of the SHAW model was high.Both the COUPMODEL model and the SHAW model could basically simulate the dynamic change process of the soil total water content.The comparison showed that the COUPMODEL model was better than the SHAW model.