Sihai Liang

Analysis of groundwater flow in mountainous, headwater catchments with permafrost

Headwater catchments have a direct impact on the water resources of downstream lowland regions as they supply freshwater in the form of surface runoff and discharging groundwater. Often, these mountainous catchments contain expansive permafrost that may alter the natural topographically controlled groundwater flow system. As permafrost could degrade with climate change, it is imperative to understand the effect of permafrost on groundwater flow in headwater catchments. This study characterizes groundwater flow in mountainous headwater catchments and evaluates the effect of permafrost in the context of climate change on groundwater movement using a three-dimensional, finite element, hydrogeologic model. The model is applied to a representative headwater catchment on the Qinghai-Tibet Plateau, China. Findings suggest that increased annual air temperatures may increase groundwater discharge to streams, which has implications for ecosystem health and the long-term availability of water resources to downstream regions.

Characteristics and causes of vegetation variation in the source regions of the Yellow River, China

The eco-environment in the source region of the Yellow River in western China has been experiencing deterioration in the past decades. Vegetation affected by climate variables and anthropogenic activities is indicative of eco-environment well-being. To quantify temporal and spatial variations of vegetation coverage and analyse potential causes for the variations, we analysed the normalized difference vegetation index (NDVI), temperature and precipitation data from 2000 to 2007. We found that altitude and topographic aspects have a strong influence on vegetation coverage. Altitudes between 4500 and 4800 m and shady aspects provide more favourable environments for vegetation growth. Data show stronger vegetation growth within the temperature range of 4.5–5.5°C. Vegetation growth generally increases with precipitation. At higher elevations of 4800–5200 m, however, despite high precipitation rates, lower temperatures restrict growth. Local hydrology conditions are found to directly influence vegetation variations. Vegetation degradation increases with distance from surface water boundaries up to 4 km, but groundwater might serve as a reliable source for preventing vegetation from degrading. Finally, we found that the percentage of degradation decreases with increasing distance from residential loci up to 24 km, which suggests that overgrazing can be a lead cause for localized vegetation degradation. Findings of this study may have a broad implication in assessing vegetation variation and grassland restoration.

Vegetation phenology and its variations in the Tibetan Plateau, China

ABSTRACT Understanding the vegetation phenology and its variations in the Tibetan Plateau is critical to the study of ecological responses to global climate change. In this study, several pre-processed methods or techniques were applied to filter the Global Inventory Modelling and Mapping Study’s Normalized Difference Vegetation Index (GIMMS NDVI) data from 1982 to 2006, and construct the daily NDVI series. Firstly, vegetation and non-vegetation were determined by NDVI quantity contour, and cloud-covered pixels were also eliminated by NDVI change characteristics in a year. Then, the NDVI series were filtered by three-standard deviation and Savitzky-Golay method. Finally, the Savitzky–Golay method was employed to fit and construct the daily NDVI series. These methods guarantee a more reliable subsequent calculation of subsequent vegetation phenology. The vegetation phenology parameters including the start of growth season (SOG), the end of growth season (EOG), the lengths of growth season (LOG) and the absolute increase in vegetation (AIV), defined as the difference between the maximum NDVI and the NDVI for SOG in a year, were derived from the daily NDVI series based on the maximum ratio threshold method and their variations were analysed. The results showed that the SOGs were gradually delayed from the southeast to the northwest of the Tibetan Plateau, but the distribution pattern of the EOGs was opposite to that of the SOGs. From 1982 to 2006, SOGs were advanced approximately 3–18 days and EOGs delayed around 0–24 days in the southeast, whereas AIVs decreased around 0–0.3. In the northwest, these phenology parameters followed inverse trends compared with those of the southeast. Over the 25-year period, LOG changes had no constructive or active effects on the vegetation absolute increase. These complex phenological shifts were mainly due to the spatial differences in the environmental changes. However, in some extent, they might be related to the vegetation itself, such as its fractional cover. These findings may help to understand the alpine vegetation responds to climate change in the Tibetan Plateau.