Comparison Of Different Permafrost Thermal Conductivity And Unfrozen Water Schemes Of VIC-CAS Model

Permafrost is an important part of the cryosphere,accurate simulation of the hydrothermal process in the active layer of permafrost is of great significance for understanding and predicting the impact of changes in the cryosphere on water resources and ecology.The land surface hydrological model is currently one of the important methods for simulating the hydrothermal process of permafrost.The thermal conductivity and unfrozen water content of the soil are two key parameters in the simulation of the hydrothermal process of permafrost,which have an important influence on the simulation results.In this study,on the basis of the VIC-CAS(Variable Infiltration Capacity-Chinese Academy Science)Model that has added the glacier module,the soil thermal conductivity scheme of the EBM(Enphy-Based Model)is used to replace the thermal conductivity of the VIC-CAS Model,the unfrozen water scheme of the CLM5.0(Community Land Model 5.0)to replace the unfrozen water scheme of the VIC-CAS Model,and use the continuous observation data of the Tuotuo River Station in the source area of the Yangtze River to carry out a numerical simulation comparison experiment.Analyzed the heat flux,temperature of different depths(20,40,60,80,100,130 and 160cm)and ice content,moisture of three-layer soil(0?10cm,10?60cm,60?160cm)of the active layer and other components with different thermal conductivity and unfrozen water schemes.The main conclusions are as follows:(1)By comparing the simulation results of each component of the VIC-CAS original thermal conductivity scheme and the EBM thermal conductivity scheme,it is found that replacing the EBM thermal conductivity scheme has a greater impact on the simulation results of soil heat flux,temperature,melting depth and rate,and has have little effect on the simulation results of soil ice content and moisture.In terms of the soil heat flux simulation,the EBM thermal conductivity scheme at 7 depths has a lower simulated value of the soil heat flux than the VIC-CAS thermal conductivity scheme.The difference between the two scheme is the largest at a depth of 20 cm,and it decreases with the increase of depth.In terms of the soil temperature simulation,according to the Nash coefficient(NSE)and root mean square error(RMSE)between the simulated value and the observed value of the two thermal conductivity schemes,the depth of 20?60cm,the simulation result is better than the original scheme;the depth of 80?160cm,the simulation result is worse than the original scheme.In terms of the simulation of soil ice content,after replacing the EBM thermal conductivity scheme,the difference between the simulated ice content of the surface and middle soil is not obvious,and the simulated ice content of the lower soil during the freezing period is lower than the original scheme.In terms of the soil moisture simulation,the EBM thermal conductivity scheme has little effect on the moisture simulation of the surface and middle soil.The moisture simulation value of the lower soil is higher than that of the VIC-CAS thermal conductivity scheme,and the NSE between the observed value and the simultated value is reduced,and the simulation effect becomes worse.In terms of melting depth and rate simulation,compared to the original scheme,the simulated melting depth rate and the maximum melting depth is reduced during the summer melting period,but it is closer to the observed value.(2)By comparing the simulation results of the water and heat components of the original VIC-CAS unfrozen water scheme and the CLM5.0 unfrozen water scheme,it is found that replacing the CLM5.0 unfrozen water scheme has a greater impact on the simulation results of soil ice content and moisture,and has have little effect on the simulation results of soil heat flux,temperature,melting depth and rate.In terms of the simulation of soil heat flux,the difference in heat flux between the two scheme at 7 depths is not obvious,only slightly different in the freezing period of the soil at the depth of 20?40cm.In terms of the soil temperature simulation,the two unfrozen water schemes have the same simulation trends for soil temperature and heat flux,which are close to the NSE and RMSE between the observations.In terms of the simulation of soil ice content,the simulated ice content of the CLM5.0 unfrozen water scheme in the surface and middle soil is higher than that of the original scheme,and the simulation results of the ice content of the lower soil have little change.In terms of the simulation of soil moisture,the simulated values of the three-layer soil moisture of the CLM5.0 unfrozen water scheme are all lower than the simulation results of the VIC-CAS unfrozen water scheme and the simulated difference of the lower soil moisture is smaller than that of the surface and middle soils.The simulation effect of surface and middle soil moisture has become worse,but the simulation effect of the lower soil is better than the original scheme.In terms of the melting depth and rate simulation,replacing the CLM5.0 unfrozen water scheme has less impact on the soil melting process,and the melting depth and rate are less different from the original scheme,and have a larger error with the observed value.Taken together,the simulation effect of the thermal conductivity and unfrozen water scheme in VIC-CAS Model is relatively robust.In this study,the comparative experiments of different schemes of thermal conductivity and unfrozen water provide a reference for further improving the relevant schemes of the permafrost hydrothermal simulation process in the VICCAS model.