| The source region of the Yellow River is the starting point of the Yellow River’s hydrological cycle. It is one of the main areas with the alpine wetland on Qinghai-Tibet Plateau, and is also an important part of the national nature reserve of three river sources. My case study focuses on this area. Its purpose is to reveal how the changes in the ecological environment in this area affects the water cycle, and the major influence factors of the soil moisture. Remote sensing observation, field acquisition, and site observations, have been adopted as the methodology. The main themes of my analysis are: the spatial-temporal pattern of land cover change, the space and time differences of regional hydrothermal pattern, the temporal changing pattern, and spatial heterogeneity of soil moisture in the source region of the Yellow River between 2000 and 2010. The influencing factors on the soil moisture have been explored and reported here from three aspects respectively: land use, vegetation coverage, and climate.(1) From 2000 to 2010, the main type of land covering the examined area is grassland, but most grasslands are on the degradation. The area of grassland restoration is far smaller than that of grassland degradation. The southeast region is better-covered, whereas the northwest region is less-covered.(2) During the study period, the temperature in the region in the trend of rising(with minor fluctuation), and the rising is most obvious in springs. From the perspective of spatial distribution, the average annual and seasonal temperature decrease gradually from the east to the west; the rate of change increase from the north to the south. The precipitation in the area present the rising trend in fluctuation, but the change of precipitation is not remarkable and the increasing trend of precipitation in summer is the most significant. The precipitation decrease from the north to the south.(3) It has been verified that soil moisture in 1km can improve the quality of 25 km soil moisture data obviously. From 2000 to 2010, the percentage of soil moisture in the area is in an upward trend, with both annual and seasonal increase. The content order of four seasons is as follows: summer> autumn >spring> winter. The annual mean soil moisture is in escalation from northwest area to southeast area.(4) In the spatial range from 1km to 25 km, soil moisture has little variability. At the same time, the variable coefficient of soil moisture will increase as the spatial scale increases. The variable coefficient rises more quickly when spatial scale is small and stays more evenly if the spatial scale grows. Meanwhile, the changes of variable coefficient are different when the soil moisture is in the grassland covered with different plants and observed at different spatial scales.(5)Different types of land-cover vegetation make soil on the surface a great difference in water content. It is the highest when ratio of vegetation coverage is highest. In the unchanged trajectory, the water content of the surface soil in the area increased with grassland coverage. Besides, when the coverage is larger, the water content of the surface soil is higher, and the annual increase of the soil moisture is greater. In the trajectory of grassland restoration, the order of water content is as follows: high coverage grassland> moderate coverage grasslands> low coverage grasslands. Although the water content in these three types of land have increased, the rising percentages are different: moderate coverage grasslands> high coverage grassland >low coverage grasslands. In the trajectory of grassland restoration, the order of water content is as follows: moderate coverage grasslands> shrub>low coverage grasslands> sandy land.(6) The vegetation coverage directly affects the capacity of water storage in the surface soil. The moisture content of the surface soil increased with vegetation coverage. The capacities of water storage in the surface soil in meadow and steppe were excellent, but the water storage capacity of meadow is greater than that of steppe.(7) On an annual scale, the soil moisture of the source area of the Yellow River is largely affected by the temperature, but the influence of precipitation on soil moisture is not significant. On a seasonal scale, the climate factors that influenced the soil moisture varied in different seasons. In spring, the key influencing factor is the temperature; in summer and autumn, the key influencing factors are temperature and precipitation. In winter, there is no influencing factor, which is expressed in the fact that the effect of temperature and precipitation to soil moisture is not obvious. |
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