The Advanced Very High Resolution Radiometer (AVHRR) satellite-borne sensor is presently used, together with field data, to determine the geographic distributions of the seasonal exchange of CO2 between the earth's atmosphere and the terrestrial biota. The exchange functions thus obtained are validated in virtue of the ability to reproduce the observed annual cycle of atmospheric CO2 in a three-dimensional tracer transport model. The AVHRR is carried by the TIROS-N series of polar-orbiting satellites.
Regression analysis of in situ spectral reflectance of a blue grama grass canopy sampled with approximately equal amounts of standing live and standing dead vegetation has identified spectral regions between 0.35 and 0.80 μm where the total wet biomass, total dry biomass, and leaf water content can be spectrally estimated. The amount of total wet or dry biomass was best estimated in the 0.35 to 0.44 μm region of the spectrum. This resulted from a combination of a relatively greater carotenoid than chlorophyll retention in the recent standing dead vegetation and of carotenoid and chlorophyll pigments present in the photosynthetically active or live vegetation. The leaf water content, highly related to the amount of photosynthetically active vegetation present, was best estimated in the ∼0.46 to 0.50, 0.63 to 0.69, and 0.74 to 0.80 μm regions. This resulted from strong chlorophyll absorption in the ∼0.46 to 0.50 and 0.63 to 0.69 μm regions and the high reflectance of living vegetation in the 0.74 to 0.80 μm region.
The NOAA-7 polar orbiting, sun-synchronous, operational satellite carries the 5 channel Advanced Very High Resolution Radiometer (AVHRR). Data are acquired globally at a resolution of 4 km on a daily basis. This data provides the means of frequently monitoring global vegetation on continental scales. Techniques for compositing and cloud screening a vegetation index for Africa are presented. The sample data set covers 9 days beginning August 16, 1983 and is compared with a semi-operational vegetation index product produced by NOAA.
Many assumptions have been made about the nature and character of desertification in West Africa. This paper examines the history of this issue, reviews the current state of our knowledge concerning the meteorological aspects of desertification, and presents the results of a select group of analyses related to this question. The common notion of desertification is of an advancing “desert,” a generally irreversible anthropogenic process. This process has been linked to increased surface albedo, increased dust generation, and reduced productivity of the land. This study demonstrates that there has been no progressive change of either the Saharan boundary or vegetation cover in the Sahel during the last 16 years, nor has there been a systematic reduction of “productivity” as assessed by the water-use efficiency of the vegetation cover. While it also showed little change in surface albedo during the years analyzed, this study suggests that a change in albedo of up to 0.10% since the 1950s is conceivable.
The contribution that AVHRR data can make to resolving the controversy about the deforestation of the Amazon region is discussed. The most significant types of information which such data can supply are pointed out. A color composite is shown and discussed, showing how it points out areas of deforestation.
The first four LANDSAT-D thematic mapper sensors were evaluated and compared to: the return beam vidicon (RBV) and multispectral scanners (MSS) sensors from LANDSATS 1, 2, and 3; Colvocoresses' proposed 'operational LANDSAT' three band system; and the French SPOT three band system using simulation/intergration techniques and in situ collected spectral reflectance data. Sensors were evaluated by their ability to discriminate vegetation biomass, chlorophyll concentration, and leaf water content. The thematic mapper and SPOT bands were found to be superior in a spectral resolution context to the other three sensor systems for vegetational applications. Significant improvements are expected for most vegetational analyses from LANDSAT-D thematic mapper and SPOT imagery over MSS and RBV imagery.
Red and photographic-infrared spectral data collected on 21 dates over the growing season with a hand-held radiometer were quantitatively correlated with total dry-matter accumulation in winter wheat. The spectral data were found to be highly related to vigor and condition of the plant canopy. Two periods of drought stress and subsequent recovery from it were readily apparent in the spectral data. Simple ratios of the spectral radiance data compensated for variations in solar intensities and, when integrated over the growing season, explained 79% of the variation in total above-ground accumulation of dry matter. A satellite system is proposed to provide large-area assessment of total dry accumulation or net primary production from terrestrial vegetation.
Spaceborne remote sensing has led to great advances in the global monitoring of vegetation. For example, the NASA Global Inventory Modeling and Mapping Studies (GIMMS) group has developed widely used datasets from the Advanced Very High Resolution Radiometer (AVHRR) sensors as well as the Moderate Resolution Imaging Spectroradiometer (MODIS) map imagery and normalized difference vegetation index datasets. These data are valuable for analyzing vegetation trends and variability at the regional and global levels. Numerous studies have investigated such trends and variability for both natural vegetation (e.g., re-greening of the Sahel, shifts in the Eurasian boreal forest, Amazonian drought sensitivity) and crops (e.g., impacts of extremes on agricultural production). Here, a critical overview is presented on recent developments and opportunities in the use of remote sensing for monitoring vegetation and crop dynamics.
No abstract is provided for this article.
There is growing interest in employing hand-held radiometry as a nondestructive research tool in lieu of or support of more tedious vegetation measurements. The objective of this study was to evaluate such techniques on corn. The spectral radiances from corn plots 1.8 m in diameter were measured using a three-band radiometer elevated 3.7 m above the ground. The three spectral bands used corresponded to NASA'S Landsat-D Thematic Mapper bands TM3 (0.63–0.69 μm), TM4 (0.76–0.90 μm), and TM5 (1.55–1.75 μm). Periodically throughout the growing season a plot was selected and radiometrically measured then harvested for measurement of several agronomic variables. By the end of the growing season, a total of 43 plots had been measured with solar zenith angles ranging between 16 and 44°. Significant relationships were found between various combinations of the radiance data and the wet and dry total biomass, plant height, fraction of ground covered by plants, wet and dry green leaf biomass, green leaf area index, fraction of leaf chlorosis, and total plant water content. Some of these relationships were found to be redundant since several of the agronomic variables were highly correlated to one another. In addition, the TM5 band did not provide any marked improvement in the relationships to the agronomic variables. The relationships between the radiance data and agronomic variables represent a nondestructive remote sensing technique for researching the growth of corn canopies.
