Hydrological impacts of interannual variations in surface soil freezing processes in the upper Nu–Salween River basin

ABSTRACT The upper Nu–Salween River basin in the Tibetan Plateau is mainly covered with seasonal frozen soils. We used daily surface freeze–thaw states, detected from Special Sensor Microwave/Imager (SSM/I) daily brightness temperature data, to analyze the variations in surface freeze–thaw states and the relationship with air temperature. We also examined baseflow to explore the influences of interannual variations in the start time of soil freezing on hydrological processes. The results showed that (1) interannual air temperature fluctuations led to differences in the area and start time of surface freezing. When surface soil froze, flow was mainly dependent on existing groundwater storage. (2) The interannual variation in the surface freezing time directly affected the flow generation processes. When soil water froze and remained in the frozen layer, it was hard to generate surface flow, so flow mainly consisted of baseflow, causing the proportion of the baseflow in the total flow to gradually increase. (3) The surface freeze–thaw states obtained from the passive microwave remote sensing data may be applied to support further research on the hydrological impacts of freeze–thaw cycle variations in plateau mountain basins