Yang Zihui

GIS-assisted FEFLOW modeling of groundwater moving processes within the Minqin oasis in the lower reach of the Shiyang River, Northwest China

Oases are specific landscapes and play very important economic roles in arid Northwestern China. Yet, the peripheral ecological systems around the oases are equally important to the oasis economic systems in terms of protecting the oasis from desertification and ensuring sustainable development of oases. However, the rapid increases in the population and steady progress in the society and technology since 1950s have greatly expanded irrigated agricultural production in the oases, resulting in a dramatic decrease in stream flow and associated increase in the groundwater withdrawal in arid Northwestern China. The overexploitation of groundwater formed groundwater table depletion cones and eventually lowered the groundwater table not only in the oases but also in the periphery of the oasis, resulting in an overall decrease in soil water content. In response to the soil moisture decrease in the periphery, some plant species relying on groundwater resource have been dying out and other more mesic species have been replaced with more xerosere species. In this paper, the groundwater flowing processes and the water-table fluctuating processes during the past 40 years were simulated to obtain the spatial and temporal distribution of groundwater table by means of the GIS-assisted FEFLOW modeling based on the hydrogeologic data obtained in the Minqin Basin. Several conclusions can be drawn from the simulated results. Firstly, the discharge and recharge of groundwater retained balanced and the groundwater table depth in the periphery area of the oasis maintained 2-3 m deep during 1960-1965. Secondly, the utilization of the groundwater dropped the groundwater table within the oasis to 5-10 m, leading to the formation of big groundwater depletion cones that started to influence the groundwater table of the periphery area in the 1970s. Thirdly, the further overexploitation of the groundwater within the oasis has dropped the groundwater table to 10-20 m in the oasis and 7-10 m in the periphery since the middle 1980s

Monitoring of vegetation changes using multi-temporal NDVI in peripheral regions around minqin oasis, northwest china

Minqin Oasis encircled by the Badain Jaran Desert in north and Tengger Desert in east to south lies in the lower reach of Shiyang River Watershed, Northwest China. The peripheral desert ecological system around the oasis is quite important and absolutely necessary to the oasis economic systems in terms of protecting the oasis from desertification and ensuring sustainable development. However, the peripheral eco-system disturbed by human activities has been changed greatly since 1960s, bringing the great attention to the environment deterioration problems. In this study, Time series of normalized difference vegetation index (NDVI) derived from seven Landsat TM/ETM+ satellite images were used quantitatively to monitor the vegetation change in peripheral regions around the Minqin Oasis. Based on field observation and re-adjustment, the TM-based NDVI values from 0 to 0.21 were classified as natural and artificial vegetation such as herbage, desert shrub, and landscapes such as semi-fixed sand dunes, fixed sand dunes and sand dunes, while the NDVI values more than 0.22 as the crops and shelter belt beside the crops. The variations of NDVI values less than 0.21 indicating the natural and human planted vegetation were analyzed. Results show that the areas of NDVI values ranging from 0.09- 0.14 indicating semi-fixed sand dunes planted perennial herbage and desert shrub have decreased gradually. The human planted shrubs (NDVI 0.15-0.21) have reduced in short time occurring during 1987-1991. While the areas indicating shifting sand dunes and inter-dunes planted annual or perennial herbages and scrubs (NDVI less than 0.09) fluctuated. The correlation analysis between NDVI and annual precipitation shows that NDVI is statistically correlated with the variations in the annual precipitation, implying that the precipitation-controlled natural vegetation coverage in the margins of the Minqin oasis has overshadowed the made-planted vegetation in dominating the ecological landscape since late 1980s.

The succession and spatial structure of vegetation on desertified land in arid area

It is well known that man-assisted revegetation is one of the most effective methods to stabilize sand dunes, promote biodiversity, reduce the damages of desertification and hence reverse the course of desertification. The objectives of this study were to monitor the succession and changes in spatial structure of the vegetation after establishment of revegetation on windward sand dunes in arid area of Northwestern China, where the average annual rainfall is 160 mm. Field observations revealed that during the past 20 years, the main vegetation community has undergone a series of changes from herbages to herbages + Hedysarum scoparium to herbages + H. scoparium + Artemisia ordosica to Artemisia ordosica + herbages. In the diluvium and alluvium based sandy areas where the Nitraria tangutorum was the dominant species, the vegetation have had the succession from Nitraria tangutorum + herbages to Nitraria tangutorum + herbages + Artemisia ordosica to Artemisia ordosica + herbages + Nitraria tangutorum to Asterothamnus centrali + Stipa gobica + Artemisia ordosica. It is clear that, with the degradation of the man-assisted revegetation, e.g. H. scoparium, the natural species, such as Artemisia ordosica and herbages have dominated the communities. With the succession of the dominant species, the spatial structure of the plants has tended to concentrate to the height of 20-60 cm. Results revealed that the feasibility of sand dune fixation not only relates to the coverage of vegetation, but also to the spatial structure of the plants.