Characteristics Of Water Infiltration And Spatial And Temporal Difference Of Soil Moisture In Permafrost Region

The Sanjiangyuan region is a vital ecological barrier for China and East Asia,but its ecological environment has become increasingly sensitive and fragile in recent years due to climate warming.Researchers have been concerned with the degradation of perennial permafrost areas caused by rising temperatures,which has resulted in significant changes to water circulation processes in the region.Soil hydrological processes in the permafrost zone differ from those in non-permafrost areas due to the impact of soil freeze-thaw cycles.To address this issue,it is crucial to gain a comprehensive understanding of the water infiltration characteristics of the active layer of permafrost and the spatial and temporal variability of soil water content in the context of climate change.Such knowledge would be invaluable in the study of hydrological processes in cold regions and their response to future climate change.In this study,we investigate the spatial and temporal variability of soil hydrological processes in the permafrost region by conducting water infiltration tests and analyzing the dynamic processes of temperature,precipitation,and soil water content in the wind volcanic sub-basin of the Yangtze River source area.The main research content and results are as follows:(1)To analyze the water infiltration characteristics in the active layer of permafrost,we conducted field tests in the wind volcanic study area during the complete thawing period and the early freezing period.Our aim was to investigate the spatial and temporal variability of water infiltration processes at different altitudes,slope directions,and periods,and to fit the data using an infiltration model.We found that the freeze-thaw cycle of the active layer has a significant impact on the water infiltration process in the permafrost.During the complete thawing period and the early freezing period,the water infiltration capacity of the upper and lower slopes of the sunny slope is higher than that of the shady slope,and the difference in water infiltration capacity between the upper and lower slopes of the sunny slope is larger during the complete thawing period than during the early freezing period,mainly due to the difference in vegetation and slope gradient.The infiltration capacity of the upper slope is generally higher than that of the lower slope,regardless of the slope direction,and the stable infiltration rate of the lower slope of the shaded slope is higher than that of the upper slope at the beginning of freezing.Among the infiltration models we tested,the Horton model fit the water infiltration process of the wind volcanic basin the best,with an R~2above 0.875 at the complete thawing period and above 0.975 at the beginning of freezing.Therefore,the Horton model can be used as the optimal fitting model for the water infiltration process in the active layer.(2)Analysis of temperature,precipitation,and soil water content changes:There are significant spatiotemporal differences in temperature,precipitation,and soil moisture content in the study area.The temperature in the research area is relatively low throughout the year,showing a single peak trend,with 131 days exceeding 0℃and 105 days of complete melting.The temperature changes were more significant during the early thawing and early freezing periods,while there were relatively small changes during the complete thawing and complete freezing periods.Precipitation events were concentrated mainly in the complete melting period,and they were mostly multi-frequency and small in amount.The precipitation and soil water content changes during the complete melting period were similar.The soil water content on the upper and lower slopes decreased with depth,and in the early melting and freezing periods,the soil water content at each depth on both slopes increased and decreased rapidly.During the complete thawing period,the correlation between air temperature and soil water content at each depth on the upper slope was higher,with an R value greater than 0.45.The correlation between air temperature and soil water content at each depth on the lower slope was higher during the complete freezing period and early freezing period,with an R value greater than 0.49.The correlation between precipitation and soil water content at each depth was higher during the complete freezing period and early freezing period.Moreover,the correlation between air temperature,precipitation,and soil water content decreased with depth.The contribution of precipitation to soil water content was mainly positive,whereas the contribution of temperature to soil water content was negative.Temperature and precipitation have a significant correlation with changes in soil moisture content,and are the main factors affecting changes in soil moisture content.(3)Simulation of dynamic change of soil moisture content:The Extream Learning Machine(ELM)model was employed to simulate the dynamics of soil water content at various slopes and depths in the active layer.Two models were developed using two and three input variables to represent the characteristics of soil water content changes.The accuracy and applicability of the models were evaluated.The two-input model demonstrated a high accuracy in simulating soil water content variations at different depths.By extending the simulation time,the model’s effectiveness significantly improved,achieving the best simulation effect after 14days,with an average Nash-Sutcliffe efficiency(NSE)of 0.86.On the other hand,the three-input model demonstrated higher simulation accuracy and achieved the best effect after seven days with an average NSE of 0.90.Compared to the physical process-based hydrological model and the BP neural network model,the ELM model outperformed in simulating soil water content in the active layer of multi-year permafrost.Thus,highlighting the ELM model’s advantages in simulating soil water content dynamics.Therefore,ELM model has better application advantages in simulating soil water content of permafrost active layer.(4)The response of soil water content to climate change was analyzed using different climate change scenarios and the ELM model.Results showed that the response of soil water content at different depths varied depending on the time of temperature increase.Specifically,the deep layer showed a greater rate of change in soil water content compared to the shallow layer under the same temperature increase.The rate of increase in soil water content at each depth during the early melting period ranged from 23.3%to 96.8%.On the other hand,the response of soil water content to an increase in precipitation was weaker.During the complete melting period,the upper slope exhibited the greatest rate of change in soil water content at each depth,with rates of change ranging from 56.6%to 96.8%.In contrast,the maximum rate of change for the lower slope was achieved at the beginning of melting,with rates of change ranging from 23.3%to 58.1%at each depth.The trend of increasing soil moisture content caused by rising temperature is more pronounced than that of increasing precipitation.