The variation of soil temperature and water content of seasonal frozen soil with different vegetation coverage in the headwater region of the Yellow River, China

The variation and distribution of temperature and water moisture in the seasonal frozen soil is an important factor in the study of both the soil water cycle and heat balance within the source region of the Yellow River, especially under the different conditions of vegetation coverage. In this study, the impact of various degrees of vegetation coverage on soil water content and temperature was assessed. Soil moisture (theta(v)) and soil temperature (T(s)) were monitored on a daily basis. Measurements were made under different vegetation coverage (95, 70-80, 40-50 and 10%) and on both thawed and frozen soils. Contour charts of T(s) and theta(v) as well as a theta(v)-T(s) coupling model were developed in order to account for the influence of vegetation cover and the interaction between T(s) and theta(v). It was observed that soil water content affected both the overall range and trend in the soil temperature. The regression analysis of theta(v) versus T(s) plots indicated that the soil freezing and thawing processes were significantly affected by vegetation cover changes. Vegetation coverage changes also caused variations in the theta(v)-T(s) interaction. The effect of soil water content on soil temperature during the freezing period was larger than during the thawing period. Moreover, the soil with higher vegetation coverage retained more water than that with lower coverage. In the process of freezing, the higher vegetation coverage reduced the rate of the reduction in the soil temperature because the thermal capacity of water is higher than that of soil. Areas with higher vegetation coverage also functioned better for the purpose of heat-insulating. This phenomenon may thus play an important role in the environmental protection and effective uses of frozen soil.