The utility of NOAA AVHRR data for vegetation studies in semi-arid regions : a minor field study in the Hoanib catchment of Namibia
Sammanfattning: The relationship between field measurements of vegetation in a semi arid region and NDVI from the 1 km NOAA AVHRR was investigated within this study. The Hoanib catchment in the northwestern Namibia was chosen as study area. The field measurements were carried out during the month of February 1999. Different NDVI values and combinations of NDVI were used and it was found that the integrated NDVI until the time of the field measurements correlated best to the grass vegetation. No significant correlations were found with the woody vegetation. The utility of non-destructive measurements of vegetation was also examined. Since destructive measurements of woody vegetation are both time-consuming and harmful to the environment, two different non-destructive methods were compared. It was found that the result of the two methods differed considerably for species and growth forms that were not recorded in the areas where the equations were established. The relationship between NDVI and grass measured with destructive or non-destructive methods was also compared. It was found that a non-destructive method returned almost as high correlation values as the destructive method. The relationship between NDVI and vegetation for all plots (with a large precipitation gradient) were compared to the relationship for the most easterly plots (with a small precipitation gradient) to draw attention to the fact that a trend in the data set improves the correlation values. This was also the case in the study. Finally, a Monte Carlo simulation was used to study the sensitivity of the calculated net primary production, using the Monteith equation, to variations in the e, a and b parameters. It was found that the NPP was very sensitive to variations in e but less sensitive to variations in a and b.
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