Vegetation density plays an important role in the water and energy balance. Satellite based passive
microwave instruments have shown an ability to monitor the total above-ground vegetation biomass at global
scales. A recently developed approach to retrieving vegetation optical depth (VOD, an index of vegetation density)
from microwave emissions can be used for all bands in the microwave domain, allowing data collected by different
satellites (e.g. Special Sensor Microwave/Imager (SSM/I from middle 1987), Tropical Rainfall Measuring Mission
(TRMM) Microwave Imager (TMI from 1998) and Advanced Microwave Scanning Radiometer – Earth Observing
System (AMSR-E from middle 2002)) to yield a long-time series. However, differences in measurement
specifications prevent merging the data directly. Here we develop a merged product by adjusting SSM/I and TMI
products against the reference sensor (AMSR-E) using the cumulative distribution frequency matching approach.
Results of Mann-Kendall trend analysis on the merged VOD product during 1988–2008 show that northwest
Australia experienced considerable increases in vegetation density, whereas southeast Australia experienced
considerable declines. Gridded rainfall and temperature products were used to assess climate induced changes
during the study period over Australia. By performing multiple linear regression analysis over varying periods of
precipitation, temperature and annual maximum monthly VOD, we identify the proportion of VOD change that is
explained by precipitation and temperature, and distinguish the contribution of natural climate from human
activities on the long-term change. Expanding analysis to the global scale along these lines should increase our
understanding of the natural and anthropogenic impacts on terrestrial hydrology and vegetation dynamics.
Figure 1: Spatial distribution of changes (% / year) in vegetation density over the period from 1988 to
2008. The percentage is the annual change relative to the average over 1988 to 2008.
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Last updated 31st January 2013