Effect of snow cover on water and heat transfer in alpine meadows in the source region of Yellow River.

Extreme snowfall events have been increasing in the Tibetan Plateau, causing greater variations in the snow cover conditions. However, the soil water-heat transfer under different snow conditions has rarely been characterized in detail. Here, by using the multi-source observation data of five years, we analyzed the influences of snow cover on water-heat transfer in alpine meadows of the source region of the Yellow River. The main findings are as follows: In the deep soil, the yearly warming rate from spring to summer was much faster than the cooling rate from autumn to winter, while in the shallow soil, conversely, the former was slower than the latter. Snow cover not only decreased the average soil temperature but also inhibited the occurrence of extremely low temperatures in the soil. The insulation effect of snow was mainly in the mid-frozen period. It was insufficient to balance out the heat lost by the high albedo during early and late frozen periods. In years with more snow, different depths of the soil featured similar thawing dates and plenty of soil voids due to small solid water content and high gravel content, together creating favorable conditions for the snowmelt infiltration, which passed through the frozen layer and infiltrated into the soil of 3.20 m or deeper. In years with less snow, the long-term freezing-thawing cycles aggravated the evaporation and loss of surface soil water in spring. Under different snow cover conditions, the difference in the sensible heat flux was much larger than the latent heat flux in winter and early spring. This study provides a refined physical image of soil water-heat transfer under extreme snow cover conditions in the Tibetan Plateau, which is expected to light the snow cover-frozen soil interaction in the mid-latitude and high-elevation areas.

Changes in climate and snow cover and their synergistic influence on spring runoff in the source region of the Yellow River.

As an important runoff-producing area, the runoff variation in the source region of the Yellow River (SRYR) has critical importance for the whole basin in broad aspects. In recent decades, the climate in the SRYR has undergone drastic changes, which affected runoff across different time scales. Many studies have focused on runoff in the SRYR with a long-time series, and presented a discordant relationship between precipitation and runoff. However, differences in this relationship over different time scales are ignored. Here, by using multi-source observation data and correlation analysis, climate elasticity, and principal component analysis methods, we document the changes in climate and snow cover and their synergistic influence on spring runoff. When the 20-year period was innovatively adopted, the runoff and precipitation coincided well during last three periods (1960-2019). The yearly precipitation presented a bimodal pattern, with the most significant increase in late spring and early summer. A bimodal pattern also appeared in annual runoff, and the rate of increase was much greater than that of precipitation (2.51%/year vs 1.01%/year). The runoff during main increase period (particularly in April) showed a high correlation with the remote sensing snow depth from November to March, but a poor correlation with snow depth from meteorological stations. Climate warming in the SRYR was much more reflected in minimum surface temperature (0.235 °C/year) than in air temperature minimum (0.081 °C/year) in last 20 years. However, the principal component analysis shows that the effect of temperature on spring runoff was obviously less than that of snow cover. A 1% variation in snow depth in the SRYR from November to March caused a 0.43% variation in runoff in April, and a 1% variation in snow days caused a 0.82% variation in runoff. This study will bring to light for understanding the evolution mechanism of spring runoff in the SRYR.

Local Circulation Maintains the Coexistence of Lake-dune Pattern in the Badain Jaran Desert.

Previous studies proposed various hypotheses to the formation of the mega-dunes and water recharge of the lakes in the Badain Jaran Desert but left the coexistence of lake-dune pattern unsolved. This research found that the local circulation, generated from the differences of thermodynamic properties and the unique landscape settings between lakes and mega-dunes, can be applied to interpret the pattern.