Ecological stability is a core issue in ecological research and holds significant implications forhumanity. The increased frequency and intensity of drought and wet climate events resulting from climatechange pose a m...Ecological stability is a core issue in ecological research and holds significant implications forhumanity. The increased frequency and intensity of drought and wet climate events resulting from climatechange pose a major threat to global ecological stability. Variations in stability among different ecosystemshave been confirmed, but it remains unclear whether there are differences in stability within the sameterrestrial vegetation ecosystem under the influence of climate events in different directions and intensities.China's grassland ecosystem includes most grassland types and is a good choice for studying this issue.This study used the Standardized Precipitation Evapotranspiration Index-12 (SPEI-12) to identify thedirections and intensities of different types of climate events, and based on Normalized DifferenceVegetation Index (NDVI), calculated the resistance and resilience of different grassland types for 30consecutive years from 1990 to 2019 (resistance and resilience are important indicators to measurestability). Based on a traditional regression model, standardized methods were integrated to analyze theimpacts of the intensity and duration of drought and wet events on vegetation stability. The resultsshowed that meadow steppe exhibited the highest stability, while alpine steppe and desert steppe had thelowest overall stability. The stability of typical steppe, alpine meadow, temperate meadow was at anintermediate level. Regarding the impact of the duration and intensity of climate events on vegetationecosystem stability for the same grassland type, the resilience of desert steppe during drought was mainlyaffected by the duration. In contrast, the impact of intensity was not significant. However, alpine steppewas mainly affected by intensity in wet environments, and duration had no significant impact. Ourconclusions can provide decision support for the future grassland ecosystem governance.展开更多
针对移动机器人噪声模型不确定性导致定位算法鲁棒性弱、精度低的问题,提出一种基于奇异值分解(Singular Value Decomposition,SVD)的自适应无迹H_(∞)滤波定位算法。该算法利用无迹H_(∞)滤波融合多传感器数据估计移动机器人位姿,并通...针对移动机器人噪声模型不确定性导致定位算法鲁棒性弱、精度低的问题,提出一种基于奇异值分解(Singular Value Decomposition,SVD)的自适应无迹H_(∞)滤波定位算法。该算法利用无迹H_(∞)滤波融合多传感器数据估计移动机器人位姿,并通过自适应调节滤波器参数γ,提高了移动机器人的定位精度。同时为了提高算法的鲁棒性,采用SVD分解代替常规Cholesky分解,避免了误差协方差矩阵在数值迭代过程中出现负定的情况。实验结果表明:相较于扩展H_(∞)滤波和粒子滤波算法,基于SVD分解的自适应无迹H_(∞)滤波定位算法具有精度高、鲁棒性强的优势。展开更多
设计了一种改进型射频功率源输出功率控制系统,解决了现有射频功率源使用中存在的输出功率稳定性与控制精度不足等问题,预期将应用于中国聚变工程实验堆(China Fusion Engineering Test Reactor,CFETR)负离子源中性束系统(Negative Ion ...设计了一种改进型射频功率源输出功率控制系统,解决了现有射频功率源使用中存在的输出功率稳定性与控制精度不足等问题,预期将应用于中国聚变工程实验堆(China Fusion Engineering Test Reactor,CFETR)负离子源中性束系统(Negative Ion Based Neutral Beam Injection System,NNBI)。采用ARM+CPLD双核设计的软、硬件分离控制结构,保障输出功率控制算法运行效率;采用数字化信号控制方法,实现输出功率的高精度控制;通过精确采样射频功率源实际输出功率和闭环功率控制方法设计,实现输出功率的高稳定性控制。对射频功率源样机进行输出功率控制系统模拟负载测试,结果表明:在额定输出功率为50 kW时,输出功率的控制精度高于0.1%、稳定性波动小于0.5%、人机交互软件功能完善。该方案预期可以搭配阻抗匹配网络满足CFETR NNBI射频功率源对输出功率控制的性能要求。展开更多
选取2015年10月至2016年6月冬小麦生长期9个关键时相的GF-1 WFV影像为数据源,综合多时相的光谱特征、植被指数特征与纹理特征,设置4组特征组合方案进行对比分析;并根据特征重要性进行特征选择,得到最优的特征子集建立随机森林分类模型,...选取2015年10月至2016年6月冬小麦生长期9个关键时相的GF-1 WFV影像为数据源,综合多时相的光谱特征、植被指数特征与纹理特征,设置4组特征组合方案进行对比分析;并根据特征重要性进行特征选择,得到最优的特征子集建立随机森林分类模型,对河南省许昌市地物类型进行分类并实现冬小麦种植面积的提取。结果表明:在没有进行特征选择的情况下,4种特征组合中,综合多种特征类型的D组分类精度最高,经过特征选择后,各组分类精度均得到不同程度的提高,说明通过多种类型的特征变量综合与特征优选均可有效地提高分类精度;不同特征类型以及不同时相的特征变量对分类的贡献率不同,贡献率由大到小为植被指数、光谱指数、纹理特征,冬小麦生长季的2月、3月、5月、6月比其他月份对分类精度的贡献率更高;河南省许昌市冬小麦面积为2258.7 km 2,分类的总体精度达到95.18%,Kappa系数为0.9255,其中冬小麦的制图精度与用户精度均达到98.67%。展开更多
The Palmer drought severity index （PDSI）, standardized precipitation index （SPI）, and standardized precipitation evapotranspiration index （SPEI） are used worldwide for drought assessment and monitoring. However,...The Palmer drought severity index （PDSI）, standardized precipitation index （SPI）, and standardized precipitation evapotranspiration index （SPEI） are used worldwide for drought assessment and monitoring. However, substantial differences exist in the performance for agricultural drought among these indices and among regions. Here, we performed statistical assessments to compare the strengths of different drought indices for agricultural drought in the North China Plain. Small differences were detected in the comparative performances of SPI and SPEI that were smaller at the long-term scale than those at the short-term scale. The correlation between SPI/SPEI and PDSI considerably increased from 1- to 12-month lags, and a slight decreasing trend was exhibited during 12- and 24-month lags, indicating a 12-month scale in the PDSI, whereas the SPI was strongly correlated with the SPEI at 1- to 24-month lags. Interestingly, the correlation between the trend of temperature and the mean absolute error and its correlation coefficient both suggested stronger relationships between SPI and the SPEI in areas of rapid climate warming. In addition, the yield-drought correlations tended to be higher for the SPI and SPEI than that for the PDSI at the station scale, whereas small differences were detected between the SPI and SPEI in the performance on agricultural systems. However, large differences in the influence of drought conditions on the yields of winter wheat and summer maize were evident among various indices during the crop-growing season. Our findings suggested that multi-indices in drought monitoring are needed in order to acquire robust conclusions.展开更多
With global warming, the great changes in the patterns of plant growth have occurred. The conditions in early spring and late autumn have changed the process of vegetation photosynthesis, which are expected to have a ...With global warming, the great changes in the patterns of plant growth have occurred. The conditions in early spring and late autumn have changed the process of vegetation photosynthesis, which are expected to have a significant impact on net primary productivity(NPP) and affect the global carbon cycle. Currently, the seasonal response characteristics of NPP to phenological changes in dryland ecosystems are still not well defined. This article calibrated and analyzed the normalized difference vegetation index(NDVI)time series of Advanced Very-High-Resolution Radiometer(AVHRR) data from 1982 to 2015 in the Loess Plateau, China. The spatial and temporal distributions of vegetation phenology and NPP in the Loess Plateau under semihumid and semiarid conditions were investigated. The seasonal variation in the NPP response to vegetation phenology under the climate change was also analyzed. The results showed that, different from the northern forest, there was distinct spatial heterogeneity in the effect of climate change on the dynamic change in vegetation growth in the Loess Plateau: 1) an advance of the start of the growing season(SOS) and a delay of the end of the growing season(EOS) significantly increased the NPP in spring and autumn, respectively, in the humid southeast;2) in the arid northwest, the NPP did not significantly increase in spring and autumn but significantly decreased in summer.展开更多
基金the National Natural Science Foundation of China(42271289).
文摘Ecological stability is a core issue in ecological research and holds significant implications forhumanity. The increased frequency and intensity of drought and wet climate events resulting from climatechange pose a major threat to global ecological stability. Variations in stability among different ecosystemshave been confirmed, but it remains unclear whether there are differences in stability within the sameterrestrial vegetation ecosystem under the influence of climate events in different directions and intensities.China's grassland ecosystem includes most grassland types and is a good choice for studying this issue.This study used the Standardized Precipitation Evapotranspiration Index-12 (SPEI-12) to identify thedirections and intensities of different types of climate events, and based on Normalized DifferenceVegetation Index (NDVI), calculated the resistance and resilience of different grassland types for 30consecutive years from 1990 to 2019 (resistance and resilience are important indicators to measurestability). Based on a traditional regression model, standardized methods were integrated to analyze theimpacts of the intensity and duration of drought and wet events on vegetation stability. The resultsshowed that meadow steppe exhibited the highest stability, while alpine steppe and desert steppe had thelowest overall stability. The stability of typical steppe, alpine meadow, temperate meadow was at anintermediate level. Regarding the impact of the duration and intensity of climate events on vegetationecosystem stability for the same grassland type, the resilience of desert steppe during drought was mainlyaffected by the duration. In contrast, the impact of intensity was not significant. However, alpine steppewas mainly affected by intensity in wet environments, and duration had no significant impact. Ourconclusions can provide decision support for the future grassland ecosystem governance.
文摘针对移动机器人噪声模型不确定性导致定位算法鲁棒性弱、精度低的问题,提出一种基于奇异值分解(Singular Value Decomposition,SVD)的自适应无迹H_(∞)滤波定位算法。该算法利用无迹H_(∞)滤波融合多传感器数据估计移动机器人位姿,并通过自适应调节滤波器参数γ,提高了移动机器人的定位精度。同时为了提高算法的鲁棒性,采用SVD分解代替常规Cholesky分解,避免了误差协方差矩阵在数值迭代过程中出现负定的情况。实验结果表明:相较于扩展H_(∞)滤波和粒子滤波算法,基于SVD分解的自适应无迹H_(∞)滤波定位算法具有精度高、鲁棒性强的优势。
文摘设计了一种改进型射频功率源输出功率控制系统,解决了现有射频功率源使用中存在的输出功率稳定性与控制精度不足等问题,预期将应用于中国聚变工程实验堆(China Fusion Engineering Test Reactor,CFETR)负离子源中性束系统(Negative Ion Based Neutral Beam Injection System,NNBI)。采用ARM+CPLD双核设计的软、硬件分离控制结构,保障输出功率控制算法运行效率;采用数字化信号控制方法,实现输出功率的高精度控制;通过精确采样射频功率源实际输出功率和闭环功率控制方法设计,实现输出功率的高稳定性控制。对射频功率源样机进行输出功率控制系统模拟负载测试,结果表明:在额定输出功率为50 kW时,输出功率的控制精度高于0.1%、稳定性波动小于0.5%、人机交互软件功能完善。该方案预期可以搭配阻抗匹配网络满足CFETR NNBI射频功率源对输出功率控制的性能要求。
文摘选取2015年10月至2016年6月冬小麦生长期9个关键时相的GF-1 WFV影像为数据源,综合多时相的光谱特征、植被指数特征与纹理特征,设置4组特征组合方案进行对比分析;并根据特征重要性进行特征选择,得到最优的特征子集建立随机森林分类模型,对河南省许昌市地物类型进行分类并实现冬小麦种植面积的提取。结果表明:在没有进行特征选择的情况下,4种特征组合中,综合多种特征类型的D组分类精度最高,经过特征选择后,各组分类精度均得到不同程度的提高,说明通过多种类型的特征变量综合与特征优选均可有效地提高分类精度;不同特征类型以及不同时相的特征变量对分类的贡献率不同,贡献率由大到小为植被指数、光谱指数、纹理特征,冬小麦生长季的2月、3月、5月、6月比其他月份对分类精度的贡献率更高;河南省许昌市冬小麦面积为2258.7 km 2,分类的总体精度达到95.18%,Kappa系数为0.9255,其中冬小麦的制图精度与用户精度均达到98.67%。
基金supported by the Fundamental Research Funds for the Central Universities (GK201703049)the Major Project of High Resolution Earth Observation System, China
文摘The Palmer drought severity index （PDSI）, standardized precipitation index （SPI）, and standardized precipitation evapotranspiration index （SPEI） are used worldwide for drought assessment and monitoring. However, substantial differences exist in the performance for agricultural drought among these indices and among regions. Here, we performed statistical assessments to compare the strengths of different drought indices for agricultural drought in the North China Plain. Small differences were detected in the comparative performances of SPI and SPEI that were smaller at the long-term scale than those at the short-term scale. The correlation between SPI/SPEI and PDSI considerably increased from 1- to 12-month lags, and a slight decreasing trend was exhibited during 12- and 24-month lags, indicating a 12-month scale in the PDSI, whereas the SPI was strongly correlated with the SPEI at 1- to 24-month lags. Interestingly, the correlation between the trend of temperature and the mean absolute error and its correlation coefficient both suggested stronger relationships between SPI and the SPEI in areas of rapid climate warming. In addition, the yield-drought correlations tended to be higher for the SPI and SPEI than that for the PDSI at the station scale, whereas small differences were detected between the SPI and SPEI in the performance on agricultural systems. However, large differences in the influence of drought conditions on the yields of winter wheat and summer maize were evident among various indices during the crop-growing season. Our findings suggested that multi-indices in drought monitoring are needed in order to acquire robust conclusions.
基金Under the auspices of MOE(Ministry of Education in China)Project of Humanities and Social Sciences(No.20YJC840027)Natural Science Basic Research Program of Shaanxi,China(No.2021JQ-771,No.2021JQ-768)Soft Science Project of Xi’an Science and Technology Bureau,Shaanxi Province(No.2021-0013)。
文摘With global warming, the great changes in the patterns of plant growth have occurred. The conditions in early spring and late autumn have changed the process of vegetation photosynthesis, which are expected to have a significant impact on net primary productivity(NPP) and affect the global carbon cycle. Currently, the seasonal response characteristics of NPP to phenological changes in dryland ecosystems are still not well defined. This article calibrated and analyzed the normalized difference vegetation index(NDVI)time series of Advanced Very-High-Resolution Radiometer(AVHRR) data from 1982 to 2015 in the Loess Plateau, China. The spatial and temporal distributions of vegetation phenology and NPP in the Loess Plateau under semihumid and semiarid conditions were investigated. The seasonal variation in the NPP response to vegetation phenology under the climate change was also analyzed. The results showed that, different from the northern forest, there was distinct spatial heterogeneity in the effect of climate change on the dynamic change in vegetation growth in the Loess Plateau: 1) an advance of the start of the growing season(SOS) and a delay of the end of the growing season(EOS) significantly increased the NPP in spring and autumn, respectively, in the humid southeast;2) in the arid northwest, the NPP did not significantly increase in spring and autumn but significantly decreased in summer.
