The hydrothermal changes of permafrost active layer and their impact on summer rainfall-runoff processes in an alpine meadow watershed, northwest China

Abstract

The freezing-thawing variation of permafrost active layer increases the complexity of rainfall-runoff processes in alpine river basins, Northwest China. And alpine meadow is the prominent ecosystem in these basins. This study selected a small alpine meadow watershed in the upper reaches of the Shule River Basin, China. We investigated alpine rainfall-runoff processes, as well as impacts of summer thaw depth of active layer, soil temperature and moisture variation on streamflow based on in-situ observations from July 2015 to December 2020. Some hydrologic parameters or indices were calculated using statistical methods, and impacts of permafrost change on river runoff were assessed using the variable infiltration capacity model (VIC). In the alpine meadow, surface soil (0–10 cm depth) of the active layer starts to freeze in mid-October each year, and begins to thaw in early April. Also, the deeper soil (70–80 cm depth) of the active layer starts to freeze in late October, and begins to thaw in late June. Moisture content in shallow soils fluctuates regularly, whereas deeper soils are more stable, and their response to rainstorms is negligible. During active layer thawing, the moisture content increases with soil depth. In the alpine meadow, vertical infiltration only occurred in soils up to 40 cm deep, and lateral flow occurred in 0–20 and 60–80 cm deep soils at current rainfall intensity. Summer runoff ratios were 0.06–0.31, and runoff floods show lags of 9.5–23.0 h following the rainfall event in the study area. The freeze–thaw process also significantly impacts runoff regression coefficients, which were 0.0088–0.0654 per hour. Recession coefficient decrease negatively correlates with active layer thawing depth in summer and autumn. Alpine river basin permafrost can effectively increase peak discharge and reduce low flow. These findings are highly significant for rainfall–runoff conversion research in alpine areas of inland rivers.