分类： 地球科学 >> 地球科学史 提交时间： 2020-10-20 合作期刊: 《干旱区科学》
摘要： Determining the mechanisms controlling the changes of wet and dry conditions will improve our understanding of climate change over the past hundred years, which is of great significance to the study of climate and environmental changes in the arid regions of Central Asia. Forest trees are ecologically significant in the local environment, and therefore the tree ring analysis can provide a clear record of regional historical climate. This study analyzed the correlation between the tree ring width chronology of Juniperus turkestanica Komarov and the standardized precipitation evapotranspiration index (SPEI) in Northwest Tajikistan, based on 56 tree ring samples collected from Shahristan in the Pamir region. Climate data including precipitation, temperature and the SPEI were downloaded from the Climate Research Unit (CRU) TS 4.00. The COFECHA program was used for cross-dating, and the ARSTAN program was used to remove the growth trend of the tree itself and the influence of non-climatic factors on the growth of the trees. A significant correlation was found between the radial growth of J. turkestanica trees and the monthly mean SPEI of February–April. The monthly mean SPEI sequence of February–April during the period of 1895–2016 was reconstructed, and the reconstruction equation explained 42.5% of the variance. During the past 122 a (1895–2016), the study area has experienced three wetter periods (precipitation above average): 1901–1919, 1945–1983 and 1995–2010, and four drier periods (precipitation below average): 1895–1900, 1920–1944, 1984–1994 and 2011–2016. The spatial correlation analysis revealed that the monthly mean SPEI reconstruction sequence of February–April could be used to characterize the large-scale dry-wet variations in Northwest Tajikistan during the period of 1895–2016. This study could provide comparative data for validating the projections of climate models and scientific basis for managing water resources in Tajikistan in the context of climate change.
分类： 地球科学 >> 地理学 提交时间： 2021-12-03 合作期刊: 《干旱区科学》
SUN Lingxiao
YU Yang
GAO Yuting
ZHANG Haiyan
YU Xiang
HE Jing
WANG Dagang
Ireneusz MALIK
Malgorzata WISTUBA
YU Ruide
摘要： Net primary productivity (NPP) of the vegetation in an oasis can reflect the productivity capacity of a plant community under natural environmental conditions. Owing to the extreme arid climate conditions and scarce precipitation in the arid oasis regions, groundwater plays a key role in restricting the development of the vegetation. The Qira Oasis is located on the southern margin of the Taklimakan Desert (Tarim Basin, China) that is one of the most vulnerable regions regarding vegetation growth and water scarcity in the world. Based on remote sensing images of the Qira Oasis and daily meteorological data measured by the ground stations during the period 2006–2019, this study analyzed the temporal and spatial patterns of NPP in the oasis as well as its relation with the variation of groundwater depth using a modified Carnegie Ames Stanford Approach (CASA) model. At the spatial scale, NPP of the vegetation decreased from the interior of the Qira Oasis to the margin; at the temporal scale, NPP of the vegetation in the oasis fluctuated significantly (ranging from 29.80 to 50.07 g C/(m2•month)) but generally showed an increasing trend, with the average increase rate of 0.07 g C/(m2•month). The regions with decreasing NPP occupied 64% of the total area of the oasis. During the study period, NPP of both farmland and grassland showed an increasing trend, while that of forest showed a decreasing trend. The depth of groundwater was deep in the south of the oasis and shallow in the north, showing a gradual increasing trend from south to north. Groundwater, as one of the key factors in the surface change and evolution of the arid oasis, determines the succession direction of the vegetation in the Qira Oasis. With the increase of groundwater depth, grassland coverage and vegetation NPP decreased. During the period 2008–2015, with the recovery of groundwater level, NPP values of all types of vegetation with different coverages increased. This study will provide a scientific basis for the rational utilization and sustainable management of groundwater resources in the oasis.
分类： 地球科学 >> 地理学 提交时间： 2021-10-11 合作期刊: 《干旱区科学》
YU Yang
CHEN Xi
Ireneusz MALIK
Malgorzata WISTUBA
CAO Yiguo
HOU Dongde
TA Zhijie
HE Jing
ZHANG Lingyun
YU Ruide
ZHANG Haiyan
SUN Lingxiao
摘要： Central Asia is located in the hinterland of Eurasia, comprising Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, and Tajikistan; over 93.00% of the total area is dryland. Temperature rise and human activities have severe impacts on the fragile ecosystems. Since the 1970s, nearly half the great lakes in Central Asia have shrunk and rivers are drying rapidly owing to climate changes and human activities. Water shortage and ecological crisis have attracted extensive international attention. In general, ecosystem services in Central Asia are declining, particularly with respect to biodiversity, water, and soil conservation. Furthermore, the annual average temperature and annual precipitation in Central Asia increased by 0.30°C/decade and 6.9 mm/decade in recent decades, respectively. Temperature rise significantly affected glacier retreat in the Tianshan Mountains and Pamir Mountains, which may intensify water shortage in the 21st century. The increase in precipitation cannot counterbalance the aggravation of water shortage caused by the temperature rise and human activities in Central Asia. The population of Central Asia is growing gradually, and its economy is increasing steadily. Moreover, the agricultural land has not been expended in the last two decades. Thus, water and ecological crises, such as the Aral Sea shrinkage in the 21st century, cannot be attributed to agriculture extension any longer. Unbalanced regional development and water interception/transfer have led to the irrational exploitation of water resources in some watersheds, inducing downstream water shortage and ecological degradation. In addition, accelerated industrialization and urbanization have intensified this process. Therefore, all Central Asian countries must urgently reach a consensus and adopt common measures for water and ecological protection.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-05-02
He, LQ [He, Liqun][ 1 ]
Ye, J [Ye, Jian][ 2 ]
Shuai, M [Shuai, Min][ 3 ]
Zhu, Z [Zhu, Zhuan][ 4 ]
Zhou, XF [Zhou, Xufeng][ 5 ]
Wang, YA [Wang, Yanan][ 4 ]
Li, Y [Li, Yang][ 4 ]
Su, ZH [Su, Zhihua][ 4 ]
Zhang, HY [Zhang, Haiyan][ 2 ]
Chen, Y [Chen, Ying][ 2 ]
Liu, ZP [Liu, Zhaoping][ 5 ]
Cheng, ZD [Cheng, Zhengdong][ 3 ]
Bao, JM [Bao, Jiming][ 4 ]
摘要： The recent emergence of liquid crystals of atomically thin two- dimensional (2D) materials not only has allowed us to explore novel phenomena of macroscopically aligned 2D nanomaterials but also has provided a route toward their controlled assembly into three-dimensional functional macrostructures. Using flow- induced mechanical alignment, we prepared flakes of graphene oxide (GO) in different orientational orders and demonstrated that GO liquid crystals (LC) can be used as rewritable media for reflec- tive displays without polarizing optics. With a wire or stick as a pen, we can make the surface of GO LC reflective and bright, and we can then manually draw lines, curves, and any patterns with dark appearance. The contrast between bright and dark features is due to anisotropic optical responses of ordered GO flakes. Since optical anisotropy is an intrinsic property of 2D structures, our observations and demonstration represent one of many potential applications of macroscopically aligned 2D nanomaterials.
点击量
点击量
下载量
评论
待评议
