| Under the background of climate warming,the permafrost in Daxing ’anling presents a broad and rapid degradation trend,and the change of permafrost affects the regional soil water and heat migration,further affecting the regional hydrological process.As an important ecological barrier in North China Plain,hydrological changes in Daxing ’anling region have an important impact on regional water resources security and sustainable utilization.It is of great significance to understand the soil moisture and heat characteristics in the permafrost region of Daxing ’anling and master the characteristics of runoff change in Daxing ’anling area for formulating the sustainable utilization plan of regional water resources.In this paper,the Huma River basin in the hinterland of Daxing’anling region is selected as the research object to explore the soil hydrothermal characteristics and the reasons for the change of surface runoff in the permafrost region of Daxing ’anling region.Firstly,the monitoring network of soil profile temperature and moisture is established at the basin scale,and the dynamics of soil water and heat and its influencing factors are analyzed.Secondly,according to the characteristics of high development of surface humus layer in Daxing ’anling area and its significant influence on soil water and heat transport,the spatial distribution map of huma river basin huma layer thickness is drawn,and the influence of humus layer thickness on soil water and heat transport process is quantified.Thirdly,the humus layer is taken into account in the distributed hydrological model GBEHM,which makes it suitable for Daxinganling area;Finally,the localized GBEHM model is applied to explore the causes of runoff changes in Huma River Basin from 1985 to 2019.The paper also simulates the changes of permafrost and surface runoff in Tahe basin,the main tributary of Huma River(where permafrost is more widely distributed)during the study period,in order to explore the differences of permafrost and surface runoff under different permafrost area ratios.The main conclusions are as follows:(1)The annual amplitude of soil temperature in Huma River basin decreases with the increase of soil depth,but the difference of annual average soil temperature among different soil layers is small.The influence of environmental factors on soil temperature is humus layer thickness > soil granularity > vegetation coverage > soil organic carbon,in which the annual amplitude of soil temperature decreases significantly with the increase of humus layer thickness,and the annual amplitude of soil temperature decreases by about 0.53℃ for every 1cm increase of average humus layer thickness.The change of soil moisture content in the basin has obvious seasonal characteristics;In the stable period of melting,the soil moisture content decreases with the increase of soil depth;During the frozen stable period,the soil moisture increased with the increase of soil depth.Humic layer thickness and soil organic carbon content are important factors affecting soil saturated moisture content.Generally speaking,the soil temperature in the permafrost region of Huma River is higher than that in other permafrost regions.Humid soil is conducive to the growth and development of vegetation,and the thickness of humus layer is the most important factor affecting the soil water and heat dynamics.(2)The spatial heterogeneity of humus layer thickness distribution in Huma River Basin is high.Elevation,slope,aspect,average annual surface temperature and soil water content are important factors affecting humus layer thickness,among which topographic factors play a leading role in the spatial distribution of humus layer thickness.The spatial mapping of the humus layer thickness shows that the humus layer thickness in Huma River basin is between 1.1 cm and 26.7 cm,with an average of 7.1cm.The humus layer thickness in the west of the basin is higher than that in the east,and the humus layer thickness in the shady slope with high altitude and flat terrain is even greater.The thickness of humus layer is significantly related to the distribution of permafrost,and the overall thickness increases with the increase of permafrost continuity.Under the background of climate warming,the degradation of permafrost may lead to the thinning of humus layer.(3)Considering that the thickness of humus layer can significantly improve the simulation results of soil water and heat transport,the fitting R2 of water and heat characteristic parameters of humus layer is 0.81 and 0.82 based on Hydrus-1D,and the fitting effect is good.The humus layer is considered in the GBEHM model to optimize the model locally.The results of daily runoff simulation at Huzhong,Tahe and Huma hydrological stations verify that the NSE is higher than 0.88,and the R2 is higher than0.9.The thawing depth of permafrost simulated by the model is in good agreement with the measured results.The results show that it is necessary to consider the influence of humus layer in simulating the process of soil water and heat migration in Daxinganling area.The locally optimized GBEHM model can accurately simulate the frozen soil change and surface runoff process in Huma River Basin.(4)From 1985 to 2019,the average temperature in Huma River Basin was-1.79℃,showing an increasing trend,and the rising rate was about 0.12℃/10 a.The average annual precipitation is 482 mm,showing a downward trend,and the decline rate is about6mm/10 a.During the study period,the average annual temperature in Tahe Basin was-2.01℃,rising at a rate of about 0.08℃/10 a.The average annual precipitation is 502 mm,increasing at a rate of about 7mm/10 a.During the study period,the permafrost in Huma River Basin deteriorated obviously,the proportion of permafrost decreased by about 8%,and the average thickness of permafrost active layer increased at an average rate of8.3cm/10 a.Among them,the permafrost in the upper reaches is relatively stable and degraded slightly,and the reduction rate of permafrost area in the middle reaches and lower reaches exceeds 50% of the original permafrost area.During the study period,the area of permafrost in Tahe River Basin decreased by about 6%,and the thickness of active layer increased at a rate of 7cm/10 a,and the degradation rate of permafrost was less than the average level in Huma River Basin.(5)From 1985 to 2019,the surface runoff in Huma River Basin showed a downward trend,and the rate of decline was about 4mm/10 a.Attribution analysis showed that the decrease of precipitation was the main reason for the decrease of runoff in Huma River Basin.The decrease of precipitation by 1mm led to the decrease of runoff depth by0.62 mm,and the change of precipitation led to the decrease of runoff by 4.3mm/10 a.During the study period,the runoff of Tahe River Basin also showed a downward trend,and the rate of decline was about 5mm/10 a.Attribution analysis shows that permafrost degradation is the main reason for the decrease of runoff in Tahe River Basin,and its influencing mechanism is as follows: permafrost degradation with the increase of active layer thickness leads to the increase of water storage capacity and the decrease of runoff yield after full storage.In addition,the degradation of permafrost increases the connection between surface water and groundwater,and part of groundwater is converted into groundwater to further reduce surface runoff.The results show that the influence of permafrost degradation on surface runoff is more significant in areas with high proportion of permafrost. |
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