Analysis on the influence of underlying surface conditions in permafrost regions on the key water cycle processes at slope scale

Herein, the slope-scale dynamic changes in water cycle processes such as infiltration, evapotranspiration, thermal-moisture conditions in the active layer, and suprapermafrost groundwater in response to changes in the underlying surface conditions are analyzed based on the complete hydrometeorological observation network in the small Fenghuoshan watershed (FHS)-a typical permafrost basin-in the source region of the Yangtze River. The following four key results were obtained: ① The multi-year average of actual evapotranspiration (E_T) of the FHS during the growing season was 472.1±42.9 mm, and the order of meteorological factors affecting the E_T was as follows: net radiation (sensitivity coefficient S_Rn=1.22, correlation coefficient R=0.93) > air temperature (S_Ta=0.33, R=0.84) > relative humidity (S_HR=0.32, R=0.46) > wind speed (S_U=-0.25, R=- 0.27). The E_T was positively correlated with vegetation cover and altitude on the slope scale. ② The initial and stable infiltration rates increased with altitude on the slope scale. During the initial thawing, completely thawed, and initial freezing periods, the respective stable infiltration rates at various slope positions were as follows: top of slope (1.07 mm/min, 0.63 mm/min, 0.88 mm/min) > middle of slope (0.29 mm/min, 0.45 mm/min, 0.21 mm/min) > base of slope (0.11 mm/min, 0.30 mm/min, 0.10 mm/min), while the influence of vegetation coverage on the infiltration rate varied with the freezing-thawing period. ③ The differences in the freezing and thawing initiation times on the sunny and shady slopes, and at different slope positions, were reflected in the changes in soil moisture and groundwater level, thus explaining the spatial-temporal differences in the runoff-generation areas of the permafrost basin in going from the sunny slope to the shady slope, and from low altitude to high altitude.④ The freezing-thawing regime of the active layer below 50 cm depth was found to significantly affect the groundwater dynamics, and the influence of topography, vegetation, and soil texture differences on heat transfer jointly led to the spatial variations in the suprapermafrost groundwater dynamics on the slope scale.