Examinations of snow and vegetation covers under global warming in arid and semi arid regions
The important role that snow cover plays in the hydrological cycle has drawn a lot of concerns from environmental researchers, especially in cold, arid, and high altitude areas, where snow melt is the primary source of water. A slight change of snow cover in high mountain area can result in great chain-reactive influence on water availability and sustainability in low altitude areas and drainage basins. Temperature and precipitation are the two defining factors that control the distribution of snow cover and its change. Therefore, how these two parameters impact the snow cover and its distribution is of great important. On the other hand, as an important indicator in global climate change, vegetation plays a linkage role in bridging land surface and the atmosphere. Predicting vegetation response to precipitation and temperature anomalies, particularly during droughts, is of great importance in semi-arid regions, because ecosystem and hydrologic processes depend on vegetation conditions. In the background of global CO2 concentration increasing, CO 2 fertilization effect may result in non-negligible interfere on vegetation cover dynamics other than temperature and precipitation. In the dissertation, I tested two hypotheses: (1) temperature and precipitation contributions on snow cover distribution are altitude dependent in cold and arid region and (2) CO2 fertilization effect is increasing in arid environments. My research focused on Heihe River basin areas: Upper Heihe River basin and Lower Heihe River basin. Data used include satellites data and ground station data, such as MODIS snow cover, land surface temperature, and normalized difference vegetation index (NDVI), shuttle radar topography mission digital elevation models (SRTM DEM), ground measured snow depth, air temperature, and precipitation. Multivariate regression analysis and statistical analysis are used to examine the relationships between climate factors, snow cover, vegetation index, and elevation and their significance levels. In the Upper Heihe River basin, there is an altitude dependence between climate factors and snow cover area (SCA). It is found that altitude of 3800m defines the climate factor contributions. In area below this altitude, the snow accumulation is mainly controlled by precipitation (positively) while snow melting is controlled by both temperature (negatively) and precipitation (negatively); in area above this altitude, the snow accumulation is not highly related with climate factors, but snow melting is related with both temperature (negatively) and precipitation (positively). In arid regions, C3 vegetation is assumed to be more sensitive to precipitation and CO2 fertilization than C4 vegetation. In the second part of this dissertation, normalized difference vegetation index (NDVI) is used to examine vegetation growth in the arid Lower Heihe River Basin (LHRB), northwestern China, for the past 3 decades. The results indicate that maximum NDVI (MNDVI) of the area increases over the years and is significantly correlated with precipitation (R=0.47 and p <0.01), not temperature (R= -0.04). The upper limit of C3 vegetation cover of the area shows a yearly rising trend of 0.6% or an overall increase of 9% over the period of 25 years, primarily due to the CO2 fertilization effect (CO2 rising 14%) over the same period. C3 dominant areas can be potentially distinguished by both MNDVI asynchronous seasonality and a significant relation between MNDVI and cumulative precipitation. This study provides a potential tool of identifying C3 vegetation from C4 vegetation and confirms the CO2 fertilization effect in this arid region.