MODIS雪产品的计算及其与NSIDC雪产品的比较--以青藏高原雪覆盖为例

以青藏高原上的两个区域、选择不同时相的MODIS影像，运用NDSI、NDVI等指标，结合MODIS云腌膜数据，计算得到MODIS的雪产品；并将其与美国NSIDC的雪产品进行比较。结果表明，计算得到的MODIS雪产品中雪的覆盖区域与NSIDC雪产品中的雪覆盖区域基本一致，但计算得到的雪覆盖面积比对应的NSIDC雪产品中的雪覆盖面积略大3％。计算得到的雪产品与NSIDC雪产品在同样经纬度范围内图像位置有偏差。范围越小，位置偏差越明显。对图像进行经度偏移后，取共同区域进行像素差值比较，结果，计算得到的雪产品与NSIDC雪产品判断一致的像素约83．12％。     

Portable Instrument for Normalized Difference Vegetation Index

By using four specially designed narrow bandpass filters and photodetectors in the instrument,the incident and reflected radiances of sun light on the vegetation are optically sensed, at the red and near infrared bands, then the normalized difference vegetation index(NDVI) is processed by a microprocessor.Compared with conventional spectrometer measuring method of NDVI, the instrument is easy to be used,compact, light and low-cost.     

Quantification of aboveground rangeland productivity and anthropogenic degradation on the Arabian Peninsula using Landsat imagery and field inventory data

The productivity of semi-arid rangelands on the Arabian Peninsula is spatially and temporally highly variable, and increasing grazing pressure as well as the likely effects of climatic change further threatens vegetation resources. Using the Al Jabal al Akhdar mountains in northern Oman as an example, our objectives were to analyse the availability and spatial distribution of aboveground net primary production (ANPP) and the extent and causes of vegetation changes during the last decades with a remote sensing approach. A combination of destructive and non-destructive biomass measurements by life-form specific allometric equations was used to identify the ANPP of the ground vegetation (< 50 cm) and the leaf and twig biomass of phanerophytes. The ANPP differed significantly among the life forms and the different plant communities, and the biomass of the sparsely vegetated ground was more than 50 times lower (mean = 0.22 t DM ha^− 1) than the biomass of phanerophytes (mean = 12.3 t DM ha^− 1). Among the different vegetation indices calculated NDVI proved to be the best predictor for rangeland biomass.Temporal trend analysis of Landsat satellite images from 1986 to 2009 was conducted using a pixel-based least square regression with the annual maximum Normalized Differenced Vegetation Index (NDVI_max) as a dependent variable. Additionally, linear relationships of NDVI_max and annual rainfall along the time series were calculated. The extent of human-induced changes was analysed using the residual trends method. A strongly significant negative biomass trend detected for 83% of the study area reflected a decrease in annual rainfall but even without clear evidence of deforestation of trees and shrubs, human-induced vegetation degradation due to settlement activities were also important.     

Satellite passive microwave remote sensing for monitoring global land surface phenology

Vegetation phenology characterizes seasonal life-cycle events that influence the carbon cycle and land-atmosphere water and energy exchange. We analyzed global phenology cycles over a six year record (2003-2008) using satellite passive microwave remote sensing based Vegetation Optical Depth (VOD) retrievals derived from daily time series brightness temperature (T_b) measurements from the Advanced Microwave Scanning Radiometer on EOS (AMSR-E) and other ancillary data inputs. The VOD parameter derives vegetation canopy attenuation at a given microwave frequency (18.7 GHz) and varies with canopy height, density, structure and water content. An error sensitivity analysis indicates that the retrieval algorithm can resolve the VOD seasonal cycle over a majority of global vegetated land areas. The VOD results corresponded favorably (p < 0.01) with vegetation indices (VIs) and leaf area index (LAI) information from satellite optical-infrared (MODIS) remote sensing, and phenology cycles determined from a simple bioclimatic growing season index (GSI) for over 82% of the global domain. Lower biomass land cover classes (e.g. savannas) show the highest correlations (R = 0.66), with reduced correspondence at higher biomass levels (0.03 < R < 0.51) and higher correlations for homogeneous land cover areas (0.41 < R < 0.83). The VOD results display a unique end-of-season signal relative to VI and LAI series, and may reflect microwave sensitivity to the timing of vegetation biomass depletion (e.g. leaf abscission) and associated changes in canopy water content (e.g. dormancy preparation). The VOD parameter is independent of and synergistic with optical-infrared remote sensing based vegetation metrics, and contributes to a more comprehensive view of land surface phenology.     

HJ-1-CCD与MODIS数据的NDVI比较分析

利用辐射传输模型对HJ-1-CCD数据进行大气校正并反演出植被指数,和MODIS标准产品数据进行对比,并对典型区域的结果相关性和差异性进行了统计分析.结果显示:①当下垫面为相对均一的地表时,如耕地、有植被覆盖的山地、水体等,两传感器反演出的植被指数相关性(r)和相对差异分别为耕地(0.74,10％)、山地(0.69,12％)、水体(0.78,5％);②当下垫面为较为复杂的地表时,如建筑用地(含居民住宅区)等,两传感器的相关性(r)仅为0.58、相对差异达到18％;③在估算小范围区域且地表覆盖较为复杂的下垫面植被指数时,HJ-1-CCD因其相对较高的空间分辨率,能有效减弱混合像元的影响,进而提供更加丰富的植被指数信息.     

On the relationship of NDVI with leaf area index in a deciduous forest site

Numerous studies have reported on the relationship between the normalized difference vegetation index (NDVI) and leaf area index (LAI), but the seasonal and annual variability of this relationship has been less explored. This paper reports a study of the NDVI-LAI relationship through the years from 1996 to 2001 at a deciduous forest site. Six years of LAI patterns from the forest were estimated using a radiative transfer model with input of above and below canopy measurements of global radiation, while NDVI data sets were retrieved from composite NDVI time series of various remote sensing sources, namely NOAA Advanced Very High Resolution Radiometer (AVHRR; 1996, 1997, 1998 and 2000), SPOT VEGETATION (1998-2001), and Terra MODIS (2001). Composite NDVI was first used to remove the residual noise based on an adjusted Fourier transform and to obtain the NDVI time-series for each day during each year.The results suggest that the NDVI-LAI relationship can vary both seasonally and inter-annually in tune with the variations in phenological development of the trees and in response to temporal variations of environmental conditions. Strong linear relationships are obtained during the leaf production and leaf senescence periods for all years, but the relationship is poor during periods of maximum LAI, apparently due to the saturation of NDVI at high values of LAI. The NDVI-LAI relationship was found to be poor (R² varied from 0.39 to 0.46 for different sources of NDVI) when all the data were pooled across the years, apparently due to different leaf area development patterns in the different years. The relationship is also affected by background NDVI, but this could be minimized by applying relative NDVI.Comparisons between AVHRR and VEGETATION NDVI revealed that these two had good linear relationships (R²=0.74 for 1998 and 0.63 for 2000). However, VEGETATION NDVI data series had some unreasonably high values during beginning and end of each year period, which must be discarded before adjusted Fourier transform processing. MODIS NDVI had values greater than 0.62 through the entire year in 2001, however, MODIS NDVI still showed an “M-shaped” pattern as observed for VEGETATION NDVI in 2001. MODIS enhanced vegetation index (EVI) was the only index that exhibited a poor linear relationship with LAI during the leaf senescence period in year 2001. The results suggest that a relationship established between the LAI and NDVI in a particular year may not be applicable in other years, so attention must be paid to the temporal scale when applying NDVI-LAI relationships.     

基于多时相MODIS监测冬小麦的种植面积

论文基于时相和波谱信息，利用MODIS数据监测了北京冬小麦的种植面积。首先，基于地形高度对地物光谱反射值的影响，借助DEM数据对研究区域进行划分。其次，在分析并提取北京地区主要农作物时间谱曲线特征的基础上，设计决策函数，成功提取了北京地区冬小麦的种植面积。最后，比较并分析了非遥感数据对监测精度的影响。研究结果表明，（1）时相信息可以极大的提高农作物种植面积的监测精度。（2）辅助数据的利用，使决策函数的设计更具有针对性，监测结果更可靠。（3）多源多时相遥感数据在农作物种植面积的提取中具有明显的技术优势和重要的应用潜力。     

基于MODIS NDVI的中国东部植被覆盖时空变化特征

基于2001~2010年逐月MODIS NDVI产品,采用Mann-Kendall检验和Hurst指数,研究中国东部及5个子区(东北区、黄淮海区、长江中下游区、江南区和华南区)植被覆盖的时空动态特征。结果表明:10 a期间中国东部植被覆盖以不显著改善和不显著退化特征为主,前者稍占优势,4个季节中秋季改善状况最显著;未来态势主要表现为不显著改善且未来将持续改善和不显著退化且未来将持续退化两种特征。植被覆盖为不显著退化且将持续退化的区域主要分布在大型城市或城市群周围;5个子区中,除江南区年内主要表现为不显著退化且将持续退化特征外,其余4区均主要表现为不显著改善且未来将持续改善特征。     

On the need to observe vegetation canopies in the near-infrared to estimate visible light absorption

This paper examines the rationale for and implications of using a near-infrared band to estimate the absorption of visible light by vegetation canopies. The benefits of using near-infrared observations have already been documented extensively in the literature, notably in the context of applications based on vegetation indices. These include, for instance, a degree of normalization with respect to undesirable perturbing factors. Our intent here is twofold: provide the theoretical basis to justify using measurements outside the main absorption band of vegetation for the purpose of retrieving canopy properties, and uncover the implications of doing so. On the basis of simple radiation transfer considerations, we conclude that near-infrared observations are critical to ensure the accurate retrieval of absorption estimates in the visible domain, and that observations within the absorption band help control the perturbing effect of the soil background.The analytical approach implemented here is conceptually similar to a scale analysis which permits us assessing the most significant contributions to the absorption and scattering processes in the vast majority of geophysical situations. Our final conclusions derived from a series of intermediate steps that need to be performed first. To this end, we illustrate in Section 2 the fact that a suitably-defined one-dimensional radiation transfer model can always be setup to represent accurately the reflected, transmitted and absorbed fraction of vertical fluxes in any vegetation volume at medium spatial resolutions (100 m or lower), and this irrespective of the local variability exhibited by the canopy attributes. This finding is exploited throughout the paper to show that 1) measurements performed in the near-infrared band are needed to ensure a large dynamic range in albedo for dense canopy conditions, by contrast to the visible domain, 2) measurements in the visible domain are effective to remove the contribution due to the background below vegetation for low to intermediate LAI conditions. This is made possible thanks to the soil line concept and the spectral invariance of the interception process, and 3) the estimation of visible light absorption in a canopy on the basis of combinations of spectral bands (as implemented in traditional vegetation indices) hinges on spectral correlations between variables, most notably those controlling the absorbing and scattering properties of the soil and leaves. A series of implications and consequences is drawn from our analysis and, in particular, the suggestion to adopt modern interpretation techniques, superseding the commonly used vegetation index approaches. These advances allow us to improve on current approaches, in particular by lifting some of the hypotheses associated with approaches based on combinations of spectral bands.     

黄河三角洲NDVI变化与气候因子的相关分析

利用MOD IS/Terra资料,得到了黄河三角洲地区2000~2004年逐月NDVI统计资料,详细分析了NDVI的时空变化特征,同时研究了NDVI与降水、气温的关系。结果表明:一年中,黄河三角洲NDVI具有明显的周期变化规律,但近年来黄河三角洲的NDVI年际间变化不明显;黄河三角洲地区的气候变化,尤其是气温、降水与NDVI存在着一定的相关性,但相对来说当月NDVI对气温的响应较强,而降雨则对滞后1个月的NDVI的影响较大。