Hydrothermal Processes Of The Active Layer And Its Impacts On Surface Hydrologic Process In Heihe River Basin

Global warming increases thickness of the active layer in permafrost regions, so that the active layer hydrothermal processes are significantly affected. Investigation on the hydrothermal processes of the active layer, can help us to better understand the hydrological processes, surface energy balance, ecological processes and carbon cycle, and the impacts of permafrost engineering. Further understanding the impact of the active layer hydrothermal processes on surface hydrology mechanism will help for the rational development and utilization of water resources and the effective protection of the eco-hydrological environment. The overall objective of this study is to improve our understanding on the hydrothermal dynamic process and its impacts on surface hydrologic mechanism of the active layer in the upper reaches of Heihe River basin. The in- situ data sets were obtained from PT1, PT3, PT4, PT5 and EboTA sites in permafrost regions over the upper reaches of Heihe River, including the soil temperatures at different depths, soil volumetric water content, air temperature, ground surface temperature and precipitation data, runoff data that observed by hydrological section and thaw depth data of the active layer that measured by iron pole. Based on these data, we used the watershed analysis, soil temperature and moisture spatial interpolation and thaw depth simulation of active layer methods to conduct the analysis. The results show that:(1) In the upper reaches of Heihe River, thawing of the active layer began in the late April and lasted about 160 days, the freezing process lasted only 40 days which ended in early November., and the two-way freezing process occurred. It means that the thawing rate is smaller than freezing rate, and compared with the two-way freezing process, the unidirectional freezing process had shorter duration and smaller change rate.(2) In the upper reaches of Heihe River, the changes of temperature and moisture in the shallow soil layer of the active layer were relatively intense, and the amplitude of variation with increasing depth was reduced, which indicated a significant lag. In the active layer of west branches, soil temperature increased and moisture decreased with decreasing altitude, meanwhile the amplitude of fluctuation increased, and the duration of the freezing process was extended. Compared with west branches, Eboling the change of temperature was smaller, the freezing duration was longer, the soil moisture content was higher, and the change rate of water phase was faster in the freezing and thawing processes in Eboling.(3) The hydrothermal collaborative change showed a better performance between-2℃ and 2℃ in the upper reaches of Heihe River, especially in Eboling. The soil temperature of freezing and thawing processes in west branches increased with decreasing altitude, which made the water content change rate decreased with the temperature change.(4) Under the influence of soil temperature and moisture, the thawing depth of active layer changed with altitude in the west branches of the upper reaches of Heihe River. From April to mid-May, the thawing process was slower, and thawing depth had increasing trend with the decreasing altitude. From June to August, thawing depth increased rapidly, and its increasing rate increased with the decreasing altitude. In early October, thawing depth reached the highest value, and the maximum thawing depth had increasing trend with decreasing altitude. Under the influence of the thick layer of peat, the maximum thawing depth of Eboling was relatively small.(5) Because of the increasing soil temperature in the west branches of the upper reaches of Heihe River, snow sublimation was intensified in winter, the supply of snowmelt water was reduced, and with increased maximum thawing depth of active layer thickness in summer, the storage space of soil layer was increased, so that the groundwater recharge was reduced, which made the winter runoff reduced. In the freezing and thawing processes of active layer in the Eboling basin, the distribution of soil moisture content and the change of thawing depth influenced the amount of water infiltration and the downward migration rate, resulted in different extent of impacts on the runoff processes and the runoff had a decreasing trend.