摘要:

A ground data-collection programme was initiated to establish a calibration between the normalized difference vegetation index (NDVI) from the NOAA Advanced Very High Resolution Radiometer (AVHRR) and grassland biomass. Thirty sites were selected representing a range of Sahclian vegetation communities in the Gourma region of Mali and monitored during the 1984 growing season. The sites were 1 km square and located within larger areas of homogeneous terrain. The herbaceous and woody strata were sampled every fourteen days, and above-ground green biomass and rainfall data were collected. Ground and airborne radiometer data were recorded to facilitate interpretation of the satellite data, and aerial photographs were taken to provide estimates of tree and shrub density. AVHRR LAC and GAC data were acquired and a thermal cloud mask was applied to the data. NDVI values were extracted for the ground sites and correlation analysis performed. Low correlation coefficients were calculated for the ground measured green biomass and satellite NDVI (0,67). The correlation between the maximum NDVI and the total biomass produced during the season was 0,73. A value of 0,05 was determined as the NDVI associated with the minimum vegetation cover identifiable by the satellite (100 kg/ha). Explanation is given for the possible causes for such low correlations, including the very low biomass production associated with the 1984 drought conditions, atmospheric haze and dust and poor locational accuracy of the satellite data

ISSN:0143-1161    

DOI:10.1080/01431168608948951

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Field measurements of the cover and biomass of live and dead herbaceous vegetation, the cover of trees and shrubs and the area of bare ground were made for rangelands in three study sites in eastern Botswana between September 1983 and April 1984. The sites were selected to be representative ofTerminalia sericea, Cotophospermum mopane and Combretum apiculatum-Acacia nigrescenswoodland savannas, which, taken together, occupy a large part of eastern and northern Botswana. Mean herbaceous biomass varied from 0 to 563 kg ha−1, cover from 0 to 21 per cent and bare ground from 57 to 85 per cent. The mean tree canopy cover in each site was approximately 30 per cent, with a range of 0-50. Landsat miiltispectral scanner (MSS) data were obtained for May, August, November 1983, January and March-April 1984. Nine MSS pixels were registered with 20 sample plots in each site and the ratios of mean band-7 to band-5 digital numbers were calculated. The variation in these ratios between the three sites and four dates on which the data were acquired was analyzed with respect to the field measurements.

ISSN:0143-1161    

DOI:10.1080/01431168608948952

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) global-area coverage (GAC) data for the visible and near-infrared bands were used to investigate the relationship between the normalized difference vegetation index (NDVI) and the herbaceous vegetation in three representative rangeland types in eastern Botswana. Regressions between Landsat MSS band-7/band-5 ratios and field measurements of the cover of the live parts of herbaceous plants, above-ground biomass of live herbaceous plants and bare ground were used in conjunction with MSS data in order to interpolate the field data to 144 km2areas for comparison with blocks of nine AVHRR GAC pixels. NOAA NDVI data were formed into 10-day composites in order to remove cloud cover and extreme off-nadir viewing angles. Both individual NDVI composite data and multitemporal integrations throughout the period May 1983-April 1984 were compared with the field data.

ISSN:0143-1161    

DOI:10.1080/01431168608948953

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Normalized difference vegetation index data obtained from polar-orbiting meteorological satellites were used to compare the growing or rainy seasons of 1984 and 1985 for the Sahelian zone of Africa. A substantial difference was found between these two years, with 1985 generally having higher normalized difference vegetation index values indicating higher levels of primary production in 1985 than in 1984. 1 km data were compared for Senegal, Mali, Niger and Sudan, and 7 km data were compared for sub-Saharan Africa. The qualitative comparison of these data suggests the use of similar data to assist in centralized monitoring of rangeland conditions, to identify areas of deficiencies in primary production and provide synoptic information in support of regional drought monitoring

ISSN:0143-1161    

DOI:10.1080/01431168608948954

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Vegetation growing periods for 1983-84 were determined for 28 sites in Ethiopia using data from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA series of meteorological satellites. Results offer promise for drought early warning from space. A strong correlation (r=0·99) was found between rates of change of the normalized difference vegetation index (NDVI) derived from the AVHRR data and threshold values of a soil moisture index at the beginning and ends of growing periods. The moisture index (P + S)/ETprelates precipitation P, stored soil moisture S and potential evapotranspiration ETpin a simple moisture balance (LPG) model that requires inputs of standard monthly meteorological data. A moisture threshold of (P + S)/ETp= 0·5 was used to identify the beginning and end of the growing periods and to calibrate the time series of NDVI responses. Trends also detected in values of the NDVI during vegetation growth cycles suggest useful minima exist at the beginning and end of growing periods. Below respective minima of 0·10 and 0·22, growing periods are unlikely to have been initiated or to continue during a declining growth stage. Correlation analysis indicated a relation between moisture index and NDVI, with NDVI lagging in time, in most cases, by 5 or less weeks during the initial growth stage and 6 or more weeks during declining growth

ISSN:0143-1161    

DOI:10.1080/01431168608948955

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

National Oceanic and Atmospheric Administration (NOAA) satellite data from the Advanced Very High Resolution Radiometer (AVHRR) sensor were analysed to document the vegetation biomass dynamics associated with the regional desert-locust upsurge in West Africa during 1980/81, which affected an area of some 600 000 km2in Mali, Niger and Algeria. Comparisons were made among locust population survey reports, rainfall records from eighteen stations in the same area, and the satellite data in vegetation index format. The satellite-recorded temporal and spatial distributions of desert vegetation biomass were closely correlated with both the locust population surveys and the available rainfall data. An attempt was made to develop a quantitative relationship between a satellite-derived potential breeding activity factor (PBAF) and the observed desert locust populations. Analysis of the multitemporal satellite data set indicates that, had the NOAA/AVHRR vegetation index data been operationally available in June 1980, effective preventive control measures would have only been necessary for an area of 600 km2.

ISSN:0143-1161    

DOI:10.1080/01431168608948956

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor