| In the world, the Qinghai-Tibet plateau (QTP) is the only region where the permafrost is present in mid latitude. The permafrost regions of the Qinghai-Tibet plateau are located in the headwaters of two major rivers, the Yangtze and Yellow Rivers, in China. Obviously, these regions, like the Eurasian arctic rivers, serve an important hydrological function in safeguarding freshwater supplies and maintaining watershed ecological security. Due to the great sensitivity to climate change in source area, make it a hot climate change research area. In order to reveal the influence the changes of vegetation cover and snow cover on hydrological processes in the permafrost watershed, this paper chose Fenghuoshan permafrost watershed as the case study region, which located in the headwater region of Yangtze River. This study investigated the influence of variation in vegetation cover on soil moisture content and temperature of alpine meadow and swamp meadow. Meanwhile, the thermal regime, moisture distribution and heat-water coupling of the active layers under different vegetation cover in alpine meadow and swamp meadow were described. Furthermore, the variation of slope surface hydrological cycle under climate change and vegetation cover changes of alpine grassland were discussed. At last, the influence of freezing and thawing cycle on runoff processes of river were determined in watershed scale. The results are as follows:1. Seasonal snow is one of the important factors to influence the permafrost development. The variations of snow accumulation and ablation lead to the shallow soil water-thermal processes happened changes. The changes of soil water-thermal processes produced important influence to the active soil layer. The results show that the thawing and freezing started-time of active soil were later and the duration of freezing were longer with snow cover than that without snow both in alpine meadow and swamp areas. With the snow cover, the active soil temperature variation rate decreased while the soil moisture variation rate increased. The snow cover plays important roles in restraining the soil temperature change and accelerating the moisture change in active soil layer. The impacts of snow cover on soil temperature and moisture are greater in autumn and summer than that in winter and spring, and the influence is more dominant in melting process than in freezing process. Snow cover results in reducing the annual maximum soil temperature and rising the annual minimum soil temperature in alpine swamp. On the contrary, both the annual maximum and minimum soil temperature increase under snow cover in alpine meadow. By comparing the temperature differences between the annual maximum and minimum temperature of swamp and meadow soil with or without snow cover, it shows that the snow cover has greater influence on swamp soil temperature than on meadow soil.2. As vegetation cover in the alpine swamp increased, the soil water thaw-rise time and the soil water freeze-fall time occurred earlier; however, the opposite response occurred in alpine meadows. Low vegetation covers are also linked to higher thermal diffusivity and thermal conductivity in the soils. As vegetation cover decreased, soil water and heat circulation in the active layer increased, and the response to temperature of the water distribution across the soil profile and meteorological factors were heightened. The maintenance of a high vegetation cover on alpine meadow and swamp reduces the impact of heat cycling on the permafrost, may minimize the impact of climate change and helps preserve the microenvironment of the soil. Alpine meadow and swamp with higher vegetation cover was beneficial for protecting permafrost. Based on these long-term observations, we defined a simple soil water-heat coupling function to demonstrate the differences in the soil water-heat relationships θv=Ac/{1+exp [B (T-△T0)]}+△θ0of areas with varying degrees of vegetation coverage. In the soil water-heat coupling function, three parameters,△θ0, Ac, and B, were directly affected by changes in vegetation cover during the thawing and freezing periods. The coupled soil thermal and water relationship can be used in the future as an initial step in improving hydrology and land surface models. 3. Increasing of temperature makes the biomass of the alpine swamp significantly higher than that in control point, which implied that short-term warming has positive effect for vegetation biomass of the alpine swamp. In alpine swamp ecosystems, the entirely frozen period of shallow active soil layer was all shortened and the fully thawed period was extended under the temperature increasing. Meanwhile the soil moisture increased considerably with temperature warming at the same depth, the soil moisture decreased with the increase of soil depth. Contrasting the alpine meadow, the alpine swamp does not appear soil dry layer in soil profile. With climate warming, the alpine swamp will degenerate to the alpine meadow. When the vegetation covers decrease, the soil water-thermal process will be changed significantly. That will have a direct impact on runoff season distribution and components of runoff process in permafrost catchment.4. According to the soil moisture dynamic regularity and characteristic of active layer in freezing-thawing period, the interaction mode of runoff generation under saturated condition and from excess rain in permafrost regions was put forward in this study. Under the influence of the freeze-thaw process, the relationship between rainfall and runoff were obvious delayed. The water cycle of the frozen soil that affects the rainfall-runoff relations has obvious seasonal fluctuation, and its main parameters (freezing and thawing depth, etc.) have seasonal dynamics. With high vegetation coverage proportion increased, the runoff coefficient presented the trend of decreasing in the alpine meadow region. However, as vegetation cover in the alpine swamp increased, the runoff coefficient appeared a trend of increasing. The conclusions are consistent with the changes of total runoff in the Yangtze source region.5. No matter the level of flood runoff in spring or summer, the active freezing-thawing process was one of the most important factors influencing the runoff processes. The temperature of active soil layer was the primary factor controlling runoff variation and components during the spring flood stage, summer low flow stage (spring flood recession) and autumn low flow stage (summer flood recession). The spring flood runoff was character a high runoff coefficient and low direct runoff ratio. Runoff increased logarithmically with an increase in the thawed depth up to60cm. In the spring flood recession period, the recession gradient was relatively small and the recession process was generally rapid, and the deep ground temperature and soil moisture played a primary role. Autumn runoff was the recession process of the summer flood, and presented an exponential decline with a decrease in air and active soil layer temperature. Precipitation played a minor role in spring flood runoff and autumn runoff, and exhibited limited effect on direct runoff. The major factors, identified by PCA (principal component analysis), i.e. soil moisture, soil temperature, air temperature were selected to construct the multivariate regression relationship with runoff. The results show that the contribution rate of the three major factors to runoff were varied mostly too, but the soil moisture and the soil temperature were the main contributors to runoff. The vegetation degradation was another factor to drive the variation in soil thawing-freezing processes except the climate warming. The decrease of vegetation cover can accelerate the active soil thawing, and therefore rapidly reduce surface runoff in thawing and freezing season. Meanwhile, different vegetation cover had a comparatively large impact on the interrelations of storm flow across different spatial scales. |
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