祁连山亚高山灌丛林叶面积指数与冠层氮、磷的关系

氮和磷作为植物体内重要的生命元素,在植物群落的生长发育过程中发挥着重要的作用。为了明确祁连山亚高山灌丛林叶面积指数与冠层氮、磷之间的关系,本文通过对祁连山亚高山灌丛林不同植被类型(箭叶锦鸡儿、高山吉拉柳、金露梅)及不同放牧处理(羊群、牦牛,未放牧)条件下灌丛群落的叶面积指数(LAI)与叶片氮积累量(TFN)、叶片磷积累量(TFP)比较发现,在整个亚高山灌丛群落中,LAI与TFN和TFP之间都有较强的相关性,并且TFN和TFP比值的变化表明不同植被类型叶片的生长都受到N、P的共同限制,只是随着LAI的增加,高山吉拉柳主要受到氮素的限制,箭叶锦鸡儿主要受到磷素的限制,而金露梅则受到N、P的共同限制;在不同放牧条件下,单位面积LAI对应的TFN的值较高而TFP的值较低,说明动物通过对植被的啃食可能会改变群落的模式,在一定程度上限制磷的摄入。LAI、N、P之间的耦合关系表明了亚高山灌丛群落的LAI在物种组成、放牧和冠层密度上存在差异,但仍然受到N和P的约束。研究结果有利于探索水分限制条件下祁连山灌丛林生态系统植物叶片与养分元素之间关系,对于研究干旱区高寒灌丛生态系统在全球气候变化中的作用及其对全球气候变化的响应与反馈,具有重要的理论价值和实践意义。     

How forest gaps shaped plant diversity along an elevational gradient in Wolong National Nature Reserve?

Journal of Geographical Sciences - Understanding the underlying ecological processes that control plant diversity within (α-diversity) and among (β-diversity) forest gaps is important for...     

中国碳强度关键影响因子的机器学习识别及其演进

碳强度影响因子数量众多,通过在众多因子中评估其重要性以识别出关键影响因子进而解析碳强度关键因子的变化规律,是中国2030年碳强度能否实现比2005年下降60%~65%目标的科学基础。传统的回归分析方法对于评估众多因子的重要性存在多重共线性等问题,而机器学习处理海量数据则具有较好的稳健性等优点。本文从能源结构、产业结构、技术进步和居民消费等方面选取了56个中国碳强度影响因子指标,采用随机森林算法基于信息熵评估了1980-2014年逐年各项因子的重要性,通过指标数量与信息熵的对应关系统一筛选出每年重要性最大的前22个指标作为相应年度关键影响因子,最终依据关键影响因子的变化趋势划分了3个阶段作了演进分析。结果发现：1980-1991年,碳强度的关键因子主要以高耗能产业规模及占比、化石能源占比和技术进步为主;1992-2007年,中国经济进入快车道增长时期,服务业占比和化石能源价格对碳强度的影响作用开始显现,居民传统消费的影响作用在增大;2008年全球金融危机后,中国进入经济结构深化调整时期,节能减排力度大大增强,新能源占比和居民新兴消费的影响作用迅速显现。为实现2030年碳强度下降60%~65%目标,优化能源结构和产业结构,促进技术进步,提倡绿色消费,强化政策调控是未来需要采取的主要措施。     

How changes of groundwater level affect the desert riparian forest ecosystem in the Ejina Oasis,Northwest China

Groundwater is a key factor controlling the growth of vegetation in desert riparian systems. It is important to recognise how groundwater changes affect the riparian forest ecosystem. This information will not only help us to understand the ecological and hydrological process of the riparian forest but also provide support for ecological recovery of riparian forests and water-resources management of arid inland river basins. This study aims to estimate the suitability of the Water Vegetation Energy and Solute Modelling(WAVES) model to simulate the Ejina Desert riparian forest ecosystem changes,China, to assess effects of groundwater-depth change on the canopy leaf area index(LAI) and water budgets, and to ascertain the suitable groundwater depth for preserving the stability and structure of desert riparian forest. Results demonstrated that the WAVES model can simulate changes to ecological and hydrological processes. The annual mean water consumption of a Tamarix chinensis riparian forest was less than that of a Populus euphratica riparian forest, and the canopy LAI of the desert riparian forest should increase as groundwater depth decreases. Groundwater changes could significantly influence water budgets for T. chinensis and P. euphratica riparian forests and show the positive and negative effects on vegetation growth and water budgets of riparian forests. Maintaining the annual mean groundwater depth at around 1.7-2.7 m is critical for healthy riparian forest growth. This study highlights the importance of considering groundwater-change impacts on desert riparian vegetation and water-balance applications in ecological restoration and efficient water-resource management in the Heihe River Basin.     

Hydrodynamic performance of a T-shaped floating breakwater

The comprehensive utilization of floating breakwaters, specially acting as a supporting structure for offshore marine renewable energy explorations, has received more and more attention recently. Based on linear water-wave theory, the hydrodynamic performance of a T-shaped floating breakwater is semi-analytically investigated through the matched eigenfunction expansion method (MEEM). Auxiliary functions, to speed up the convergence and improve the accuracy in the numerical computations, are introduced to represent the singular behavior of fluid field near the lower salient corners of the structure. The effects of the height and installation position of the vertical screen on the reflection and transmission coefficients, dynamic response and wave forces are examined. It is found that the presence of the screen shifts the resonance frequency of RAO for both surge and pitch modes to the low-frequency area, while has no effect on heave mode. The identical added masses, damping and transmission coefficients can be obtained in the cases where the screen holds the same distance away from the longitudinal central axis of the upper box-type structure. Moreover, a relatively small pitch response can be achieved in a wide wave–frequency range, when the breakwater is Γ-shaped.     

An efficient and robust approach to predict ship self-propulsion coefficients

Achieving a reliable and accurate numerical prediction of the self-propulsion performance of a ship is still an open problem that poses some relevant issues. Several CFD methods, ranging from boundary element methods (BEM) to higher-fidelity viscous Reynolds averaged Navier–Stokes (RANS) based solvers, can be used to accurately analyze the separate problems, i.e. the open water propeller and the hull calm water resistance. However, when the fully-coupled self-propulsion problem is considered, i.e. the hull advancing at uniform speed propelled by its own propulsion system, several complexities rise up. Typical flow simplifications adopted to speed-up the simulations of the single analysis (hull and propeller separately) lose their validity requiring a more complex solver to tackle the fully-coupled problem. The complexity rises up further when considering a maneuver condition. This aspect increases the computational burden and, consequently, the required time which becomes prohibitive in a preliminary ship design stage.The majority of the simplified methods proposed in literature to include propeller effects, without directly solve the propeller flow, in a high-fidelity viscous solver are not able to provide all the commonly required self-propulsion coefficients. In this work, a new method to enrich the results from a body force based approach is proposed and investigated, with the aim to reduce as much as possible the computational burden without losing any useful result. This procedure is tested for validation on the KCS hull form in self-propulsion and maneuver conditions.     

CFD modelling of a small–scale fixed multi–chamber OWC device

Wave Energy Converters (WECs) have excellent potential as a source of renewable energy that is yet to be commercially realised. Recent attention has focused on the installation of Oscillating Water Column (OWC) devices as a part of harbor walls to provide advantages of cost–sharing structures and proximity of power generation facilities to existing infrastructure. In this paper, an incompressible three–dimensional CFD model is constructed to simulate a fixed Multi–Chamber OWC (MC–OWC) device. The CFD model is validated; the simulation results are found to be in good agreement with experimental results obtained from a scale physical model tested in a wave tank. The validated CFD model is then used for a benchmark study of 96 numerical tests. These investigate the effects of the PTO damping caused by the power take–off (PTO) system on device performance. The performance is assessed for a range of regular wave heights and periods. The results demonstrate that a PTO system with an intermediate damping can be used for all chambers in the MC–OWC device for most wave period ranges, except for the long wave periods. These require a higher PTO damping. An increased incident wave height reduces the device capture width ratio, but there is a noticeable improvement for long wave periods.     

机载多光谱LiDAR的随机森林地物分类

机载多光谱LiDAR技术利用激光进行探测和测距,不仅可以快速获取地面物体的三维坐标,还可以获得多个波段的地物光谱信息,可广泛用于地形测绘、土地覆盖分类、环境建模、森林资源调查等。本文提出了多光谱LiDAR的随机森林地物分类方法。该方法通过对LiDAR强度数据和高程数据提取分类特征,完成多光谱LiDAR的随机森林地物分类;并分析随机森林的特征贡献度特性,采用后向特征选择方法实现分类特征选择。通过对加拿大Optech Titan多光谱LiDAR数据的试验表明：随机森林方法可以获得较好的地物分类精度,而且可以适当地去除部分冗余和相关的特征,从而有效提高分类精度。     

利用加权整体最小二乘DSM提取DEM的方法

随着无人机航摄软硬件技术的快速发展,通过倾斜摄影三维建模快速获取DSM的成本越来越低。在小比例尺的地形图测量中,DSM若能通过快速处理得到较为精确的DEM,即可节省大量的人力物力。针对林地范围内DSM数据的特点,采用加权整体最小二乘法对DEM进行提取。     

基于Sentinel-1B SAR数据的洪水提取和监测

洪涝灾害是我国最严重的气象灾害之一,及时准确的洪灾监测是防灾减灾的重要前期工作和基础。本文利用sentinel-1B雷达数据,以黑瞎子岛为研究区,联合使用OSTU阈值分割法和随机森林面向对象分类法针对像素统计单波形、双波形、多波形SAR影像提取洪水要素,实现对洪灾淹没面积的时序监测,为灾情监测提供数据和技术支撑。