Ruixia He

Thermal regime of warm-dry permafrost in relation to ground surface temperature in the Source Areas of the Yangtze and Yellow rivers on the Qinghai-Tibet Plateau, SW China

Ecology, hydrology, and natural resources in the source areas of the Yangtze and Yellow rivers (SAYYR) are closely linked to interactions between climate and permafrost. However, a comprehensive study of the interactions is currently hampered by sparsely- and unevenly-distributed monitoring sites and limited field investigations. In this study, the thermal regime of warm-dry permafrost in the SAYYR was systematically analyzed based on extensive data collected during 2010-2016 of air temperature (Ta), ground surface temperature (GST) and ground temperature across a range of areas with contrasting land-surface characteristics. Mean annual Ta (MAAT) and mean annual GST (MAGST) were regionally averaged at -3.19±0.71°C and -0.40±1.26°C. There is a close relationship between GST and Ta (R2=0.8477) as obtained by a linear regression analysis with all available daily averages. The mean annual temperature at the bottom of the active layer (TTOP) was regionally averaged at -0.72±1.01°C and mostly in the range of -1.0°C and 0°C except at Chalaping (~-2.0°C). Surface offset (MAGST-MAAT) was regionally averaged at 2.54±0.71°C. Thermal offset (TTOP-MAGST) was regionally averaged at -0.17±0.84°C, which was generally within -0.5°C and 0.5°C. Relatively consistent thermal conductivity between the thawed and frozen states of the soils may be responsible for the small thermal offset. Active layer thickness was generally smaller at Chalaping than that on other parts of the QTP, presumably due to smaller climatic continentality index and the thermal dampening of surface temperature variability under the presence of dense vegetation and thick peaty substrates. We conclude that the accurate mapping of permafrost on the rugged elevational QTP could be potentially obtained by correlating the parameters of GST, thermal offset, and temperature gradient in the shallow permafrost.

Freeze-Thaw Processes of Soils in Active Layers in Northeast China

The processes of thawing and freezing and their associated complex hydrothermal coupling can significantly affect variations in mean annual temperatures and the formation of ground ice in permafrost regions. In this article, using soil temperature and moisture data in the permafrost region of the Nanweng’he River in the Da xing’anling Mountains, the freeze-thaw characteristics of the permafrost were studied. Variations in the soil temperature and the moisture were analyzed during each stage of the freeze-thaw process, and the effects of the soil moisture and ground vegetation on the freezing-thaw were discussed in this paper. The study results show that the thawing in the active layer is unidirectional, while the ground freezing is bidirectional (upward from the bottom of the active layer and downward from the ground surface). During the annual freeze-thaw cycle, the migration of soil moisture had different characteristics at different stages. In general, during of a freezing-thawing cycle, the soil water molecules will migrate downwards, i.e., soil moisture will transport from the entire active layer to the upper limit of permafrost. In the meantime, freeze-thaw in the active layer can be significantly affected by the soil moisture content and vegetation.

Evolution and changes of permafrost on the Qinghai-Tibet Plateau during the Late Quaternary

Due to the uplift of Qinghai-Tibet Plateau (QTP), the cryosphere gradually developed on the higher mountain summits after the Neocene, becoming widespread during the Late Quaternary. During this time, permafrost on the QTP experienced repeated expansion and degradation. Based on the remains and cross-correlation with other proxy records such as those from glacial landforms, ice-core and paleogeography, the evolution and changes of permafrost and environmental changes on the QTP during the past 150, 000 years were deduced and are presented in this paper. At least four obvious cycles of the extensive and intensive development, expansion and decay of permafrost occurred during the periods of 150-130, 80-50, 30-14 and after 10.8 ka B.P.. During the Holocene, fluctuating climatic environments affected the permafrost on the QTP, and the peripheral mountains experienced six periods of discernible permafrost changes:(1) Stable development of permafrost in the early Holocene (10.8 to 8.5-7.0 ka B.P.); (2) Intensive permafrost degradation during the Holocene Megathermal Period (HMP, from 8.5-7.0 to 4.0-3.0 ka B.P.); (3) Permafrost expansion during the early Neoglacial period (ca. 4, 000-3, 000 to 1, 000 a B.P.); (4) Relative degradation during the Medieval Warm Period (MWP, from 1, 000 to 500 a B.P.); (5) Expansion of permafrost during the Little Ice Age (LIA, from 500 to 100 a B.P.); (6) Observed and predicted degradation of permafrost during the 20 th and 21 st century. Each period differed greatly in paleoclimate, paleoenvironment, and permafrost distribution, thickness, areal extent, and ground temperatures, as well as in the development of periglacial phenomena. Statistically, closer dating of the onset permafrost formation, more identification of permafrost remains with richer proxy information about paleoenvironment, and more dating information enable higher resolution for paleo-permafrost reconstruction. Based on the scenarios of persistent climate warming of 2.2~2.6℃ in the next 50 years, and in combination of the monitored trends of climate and permafrost changes, and model predictions suggest an accelerated regional degradation of plateau permafrost. Therefore, during the first half of the 21 st century, profound changes in the stability of alpine ecosystems and hydro (geo) logical environments in the source regions of the Yangtze and Yellow rivers may occur. The foundation stability of key engineering infrastructures and sustainable economic development in cold regions on the QTP may be affected.

The extraction of watershed characteristics of the Source Area of Yellow River based on SRTM DEM with ArcGIS

The principles and procedures for extracting the watershed characteristics using the Shuttle Radar Togograhy Mission (SRTM) Digital Elevation Model (DEM) data in ArcGIS9.3 was introduced in this paper, and its application in the Source Area of Yellow River (SAYR) was used as a case analysis. Much emphasis was put on the details of watershed generation, such as filling the depression of SRTM DEM data, extracting flow directions, determining flow accumulation, generating and classifying river networks, generating watershed and dividing sub-watersheds, as well as operating all them with SRTM DEM in the ArcGIS 9.3 environment. The results then were compared with those existing empirical coarse region maps of the SAYR from documentations and literatures. This method resulted higher efficiency and precision for watershed characteristics extraction in ArcGIS, and avoided some artificial errors. Additionally, it is labor saving and less time-consuming. The generation of watershed characteristics lays foundations for quantitative analysis on further studies in alpine ecology, geological environments, and hence facilitates modeling and mapping of permafrost in the SAYR.