Changzhen Yan

Surveying sandy deserts and desertified lands in north-western China by remote sensing

Sandy deserts and desertified lands (SDDL) cover most of north‐western China, and desertification is a severe environmental issue in this region. In this study, we classify SDDL into mobile, semi‐mobile, semi‐anchored, and anchored classes, and divide the corresponding severity of desertification into four levels: extremely severe, severe, moderate, and slight. Using SDDL databases derived from Landsat TM and ETM images in 1986 and 2000, we discuss the evolution of SDDL and the processes and causes of desertification. The desertified area increased by 16 101 km2 between 1986 and 2000, and most of the desertified land was classified as severely or extremely severely desertified. However, 3437 km2 of SDDL were also rehabilitated during this period. The area of SDDL increased in the Qinghai and Xinjiang regions, and in the western part of Inner Mongolia, but decreased in the Shaanxi, Gansu, and Ningxia regions. The causes of desertification differed among regions, and different measures to control desertification are proposed for the different regions.

Change of lake area in the southeastern part of China’s Badain Jaran Sand Sea and its implications for recharge sources

Understanding the relationship between the changes in lake water volume and climate change can provide valuable information to the recharge sources of lake water. This is particularly true in arid areas such as the Badain Jaran Sand Sea, an ecologically sensitive area, where the recharge sources of lakes are heatedly debated. In this study, we determined the areas of 50 lakes (representing 70% of the total permanent lakes in this sand sea) in 1967, 1975, 1990, 2000 and 2010 by analyzing remote-sensing images using image processing and ArGIS software. In general, the total lake area decreased from 1967 to 1990, remained almost unchanged from 1990 to 2000, and increased from 2000 to 2010. Analysis of the relationship between these changes and the contemporaneous changes in annual mean temperature and annual precipitation in the surrounding areas suggests that temperature has significantly affected the lake area, but that the influence of precipitation was minor. These results tend to support the palaeo-water recharge hypothesis for lakes of the Badain Jaran Sand Sea, considering the fact that the distribution and area of lakes are closely related to precipitation and the size of mega-dunes, but the contemporaneous precipitation can hardly balance the lake water.

Aeolian desertification monitoring in the Source Region of Yellow River by remote sensing technology

The Source Region of Yellow River locates in the hinterland of Qinghai-Tibet Plateau, which is especially sensitive to global environment change because of its high elevation and cold environment. Under the influence of global warming, aeolian desertified land expanded rapidly in this area during the last decades. To assess the trends of aeolian desertification from 1975 to 2005, we employed remote sensing and GIS technology to monitor the extent of aeolian desertification at four times (1975, 1990, 2000, and 2005). The data sources included Landsat multi-spectral scanner (MSS) images acquired in 1975 with an 80-m spatial resolution, Enhanced Thematic Mapper (ETM+) images acquired in 2000 with a 30-m resolution, and Thematic Mapper (TM) images acquired in 1990 and 2005 with a 30-m spatial resolution. The remote sensing monitoring result shows that the area of aeolian desertification land increased rapidly between 1975 and 1990, kept stable between 1990 and 2000, and turned to decrease slightly between 2000 and 2005.

Dynamic monitoring of aeolian desertification based on multiple indicators in Horqin Sandy Land, China

Aeolian desertification has become one of the most serious environmental and socioeconomic problems facing the world today. Quantitative remote sensing technology is an important means to achieve the development trends of aeolian desertified land (ADL). To compensate for the shortcomings in the time scale of Landsat Thematic Mapper and other high-spatial-resolution remote sensing data, this study introduces Moderate Resolution Imaging Spectroradiometer time series data and products to invert the monitoring indicators of ADL. The QUEST (quick, unbiased, and efficient statistical tree) classification method was used to establish the extraction model of ADL based on multiple indicators. The ADL time series dataset was extracted from 2000 to 2015, and the characteristics of ADL and its spatial-temporal dynamics were analyzed. These results were combined with meteorological data and socioeconomic statistics to discuss the main factors influencing ADL. The results showed that, by the end of 2015, the total area of ADL was 32,633 km2, accounting for 26.02% of the study area. The slight, moderate, severe, and extremely severe ADL accounted for 51.39%, 34.11%, 10.31%, and 4.20%, respectively. The total area of ADL decreased significantly at a rate of 2388.60 km2 y-1 from 2000 to 2015. The decreasing area was dominated by the slight and moderate ADL. The reversal of ADL exhibited significant correlations with an increase of annual precipitation and a decrease of annual maximum wind velocity (p < 0.01). The impact of annual maximum wind velocity on ADL is more important than annual precipitation. Increases in population density and the number of livestock did not promote the development of ADL. A series of ecological protection projects and policies created advantageous conditions for the reversal of ADL. This research provides a new method for monitoring ADL and useful information for controlling and managing aeolian desertification in this region.

Temporal and spatial changes in the pattern of sandy desert and sandy land in northern China from 1975 to 2010 based on an analysis of Landsat images

ABSTRACT Changes in the area of sandy desert and sandy land (SDSL) result from changes in the processes that underlie aeolian desertification. The area of SDSL in northern China has changed greatly in recent decades. Given that SDSL covers large areas of northern China, it is important to understand the spatial and temporal trends in its development and recession for understanding the aeolian desertification. To provide this information, we reconstructed the underlying processes of SDSL from 1975 to 2010 using Landsat images and an analysis of the driving forces. The results indicated that the changes of SDSL have mainly occurred in the eastern part of northern China. The area of SDSL increased by 58,455 km2 from 1975 to 2000, increasing by 4.9%. In contrast, the sandy land has been restored, and the area of SDSL had a decreasing trend since 2000. It decreased by 17,600 km2 from 2000 to 2010, decreasing by 1.4%. These changes were principally driven by human activities and climate variability. Since changes of the area of SDSL were not significantly related to natural environmental factors such as rainfall and temperature, it is clear that the decreasing trend of SDSL has been mainly caused by human activities.

Quantitative retrieval of soil salt content based on measured spectral data

Choosing the Minqin Oasis, located downstream of the Shiyang River in Northwest China, as the study area, we used field-measured hyperspectral data and laboratory-measured soil salt content data to analyze the characteristics of saline soil spectral reflectance and its transformation in the area, and elucidated the relations between the soil spectral reflectance, reflectance transformation, and soil salt content. In addition, we screened sensitive wavebands. Then, a multiple linear regression model was established to predict the soil salt content based on the measured spectral data, and the accuracy of the model was verified using field-measured salinity data. The results showed that the overall shapes of the spectral curves of soils with different degrees of salinity were consistent, and the reflectance in visible and near-infrared bands for salinized soil was higher than that for non-salinized soil. After differential transformation, the correlation coefficient between the spectral reflectance and soil salt content was obviously improved. The first-order differential transformation model based on the logarithm of the reciprocal of saline soil spectral reflectance produced the highest accuracy and stability in the bands at 462 and 636 nm; the determination coefficient was 0.603, and the root mean square error was 5.407. Thus, the proposed model provides a good reference for the quantitative extraction and monitoring of regional soil salinization.

Land use and land cover change monitoring in the Source Region of Yellow River by remote sensing technology

The Source Region of Yellow River locates in the northeast of Qinghai-Tibet Plateau with an area of 131419 km2, which is especially sensitive to global warming because of its high elevation and cold environment. To assess the land use and land cover changes from 1975 to 2005, we employed remote sensing and GIS technology to monitor the area of land use and land cover at four times (1975, 1990, 2000, and 2005). The data sources included Landsat multi-spectral scanner (MSS) images acquired in 1975 with an 80-m spatial resolution, Enhanced Thematic Mapper (ETM+) images acquired in 2000 with a 30-m resolution, and Thematic Mapper (TM) images acquired in 1990 and 2005 with a 30-m spatial resolution. The monitoring result shows that, under the influence of global warming and human activities, the Source Region of Yellow River experienced an overall environmental degradation between 1975 and 2005. The environmental degradation includes deforestation, grassland degradation, cultivated land increasing, sandy land expansion and wetland shrinking.

Monitoring land use and land cover change in the source region of the Yangtze River using multi-temporal Landsat data

The Source Region of the Yangtze River locates in the Qinghai-Tibet Plateau with an area of 142299 km2, which is especially sensitive to global warming because of its high elevation and cold environment. To assess the land use and land cover changes from 1975 to 2005, we employed remote sensing and GIS technology to monitor the area of land use and land cover at four times (1975, 1990, 2000, and 2005). The data sources included Landsat multi-spectral scanner (MSS) images acquired in 1975 with an 80-m spatial resolution, Enhanced Thematic Mapper (ETM+) images acquired in 2000 with a 30-m resolution, and Thematic Mapper (TM) images acquired in 1990 and 2005 with a 30-m spatial resolution. The monitoring result shows that, the study area experienced an overall environmental degradation between 1975 and 2005. The environmental degradation includes deforestation, grassland degradation and sandy land expansion.