Soil temperature-threshold based runoff generation processes in a permafrost catchment

Abstract. The contributing-area concept was the universal approach in rainfall–runoff processes modelling. However, it is unclear of the role of permafrost in controlling runoff generation processes. The areas that contribute to runoff generation are complex, variable and difficult to determine in permafrost catchments, and thus, there is no suitable quantitative approach for the simulation of runoff generating dynamics. To understand how thaw-freezing cycle in permafrost catchment effect the runoff generation processes, a typical catchment of continuous permafrost on the Tibetan Plateau was measured, and the spring and autumn season when runoff generation obviously differs from non-permafrost regions were focused on in this study. By introducing soil temperature threshold functions for surface saturation excess runoff generation and subsurface groundwater discharge, two dominant runoff generation types for permafrost catchments in different seasons are analysed, and corresponding simple quantitative approach related to the thawing and freezing periods are presented. The results show that the new approach can exactly identify the runoff generation dynamics of spring thawing and autumn freezing processes. In the permafrost headwater catchments of alpine meadows, the surface soil temperature or thawed depth threshold for variable runoff generation area depend on the zero thawing isotherms, which reach a depth of 40 cm. The subsurface groundwater discharge, which is controlled by soil temperature, contributes more than 85 % of the total river discharge in the autumn freezing period. The crucial variable for the spatial–temporal variation of runoff contributing area in the permafrost catchment is the soil temperature rather than soil moisture.