Numerical Simulation Of Coupled Moisture And Heat Transfer In Soil Under Different Irrigation Conditions During Freezing And Thawing Period

As an important part of natural energy cycle,the hydrothermal system of freeze-thaw soil plays an extremely important role in resources,environment,energy and agricultural engineering.The coupled moisture and heat transfer process of seasonal freezing-thawing soil is very complicated.In order to reveal the influence of different winter irrigation volume on the coupled moisture and heat transfer process in the seasonal freeze-thaw period,two tillage treatments(uncultivated and cultivated plots)and six different irrigation volume(0,10,20,30,50 and 70mm)were set up for field experiment.Based on the monitoring data of field in-situ experiment,the effects of different winter irrigation volume on soil freezing-thawing process and soil coupled moisture and heat transfer characteristics were analyzed.Based on the measured meteorological data and soil boundary conditions,the SHAW model was established to simulate the soil freezing and thawing process and the coupled moisture and heat transfer process under different irrigation volume of uncultivated and cultivated plots.The results of model test showed that RMSE,NSE and NRMSE of simulated and measured soil temperature in the plots with different irrigation volume ranged from 0.005-1.965℃,-4.783-0.999℃and 0.009-0.308℃,respectively.The ranges of RMSE,NSE and NRMSE of simulated and measured soil water content were 0.003-0.0128 cm~3/cm~3,2.369-0.998 and 0.002-0.295 cm~3/cm~3,respectively.The results showed that the relationship between soil freezing depth(D)and the absolute of accumulated negative temperature(Ts)on the soil surface can be expressed by the formula:(?).The maximum frozen depth of soil decreased with the increase of irrigation volume,and the change of accumulated negative temperature on the soil surface could reflect the change process of soil freezing depth.The influence of air temperature and the irrigation volume on soil temperature and soil water content weakened along with the increase of soil depth.The shallow soil temperature changed in a V-shape with the air temperature.During the freezing period,the soil temperature of 0-20cm increased with the increase of irrigation volume,the irrigation water could reduce the variation of soil temperature and stable the soil temperature.During the thawing period,the soil temperature of 0-20cm increased with the increase of irrigation volume.The larger the amount of irrigation,the slower the soil temperature rise,irrigation was not conducive to soil melt.After irrigation,the soil water content of each soil layer increased significantly.At the end of freezing and thawing period,the soli water content in uncultivated plots with the irrigation volume of 10,30 and 50mm were0.225 cm~3/cm~3,0.245 cm~3/cm~3 and 0.264 cm~3/cm~3,respectively.The soli water content in cultivated plots with the irrigation volume of 10,30 and 50mm were0.140 cm~3/cm~3,0.169 cm~3/cm~3 and 0.182 cm~3/cm~3,respectively,which was about 9%lower than that of uncultivated plots.During the process of freezing and thawing,the soil water flux at 20cm was positively correlated with the irrigation volume,and the effect of irrigation volume on the accumulated soil evaporation was small.The maximum difference of the accumulated evaporation in different irrigated plots was 5.10 mm.This study can provide reference for the utilization of soil water and heat resources and the determination of winter irrigation parameters in permafrost area of Shanxi Province.