Changing runoff generation in the source area of the Yellow River: Mechanisms, seasonal patterns and trends

The hydrological regimes of the major rivers in cold regions of the world have changed remarkably since the 1960s, but the mechanisms underlying these changes have not yet been fully understood. Specifically, changes in freeze-thaw processes can affect the thicknesses of the permafrost layer and active layer, storage capacity for liquid water and subsequent surface runoff. In this study, we investigated the hydrological processes and runoff generation in the Yellow River source (YRS) region, which is located in a permafrost region, over the past four decades using ground observations. The impacts of runoff processes are assessed in terms of the spatiotemporal distributions of precipitation, air temperatures, and soil water dynamics in the active layer. The Mann-Kendall test and correlation analysis were employed to detect the variations in runoff and annual changes in influencing factors. Runoff generation in different periods was influenced by different main factors. In the spring flood and recession flow period (SP_FRP, which ranges from late March to mid-June), soil water storage played an important role in runoff generation. With the increased air temperature and enhanced soil ground thawing process, liquid water storage capacity of the soil increased, and constrained runoff. After the soil was saturated in the summer flood and recession flow period (SU_FRP, which ranges from late June to late October), runoff increased with precipitation. In the dry period (DP, which ranges from early November to Mid-March), little precipitation, high potential evaporation (308 mm) and increased air temperature (0.43 °C/decade) caused decreases in snow accumulation and water content. Without a sufficient water supply, river runoff decreased in DP. These results show that precipitation played an important role in runoff generation in SU_FRP. In permafrost regions, the freeze-thaw processes of the active layer of soils, heavy evaporation and the air temperature play primary roles in runoff generation during SP_FRP and DP.