Annual and Interannual Variability of Forest Fires in Tropical South America and their association with the Normalized Difference Vegetation Index (NDVI) during 2000-2010

We study the space-time dynamics of the Normalized Difference Vegetation Index (NDVI), in tropical South America at annual and interannual timescales, and their association with forest fires for the period spanning from 2000 to 2010. NDVI is defined as the ratio between (NIR-Red) and (NIR+Red), where NIR is the spectral response in the band of the near infra -red (0.73 μm -1.1μm) and Red is the spectral response in the red band (0.55 μm to 0.68 μm). NDVI data correspond to the MOD13A2 MODIS sensor (Moderate Resolution Imaging Spectroradiometer) with a 1-km spatial resolution, and a 16-day temporal resolution, using the reference GRS80 ellipsoid and sinusoidal map projections. Regarding information about forest fires, thermal anomalies are used from the Landsat TM imagery (MOD14A2 MODIS sensor) with a 1 km spatial resolution and 8-day temporal resolution. The average seasonal cycles of both variables are estimated, and the interannual variability is estimated in association with the occurrence of El Niño/Southern Oscillation (ENSO), as quantified by the Multivariate ENSO Index (MEI), provided by the National Oceanic and Atmospheric Administration (NOAA) for the decade. All analysis are performed for the Colombian-Venezuelan Llanos, and four regions defined over the Amazon River basin, namely Western, Central, Southern, and Eastern. Fires over the Colombian-Venezuelan Llanos exhibit a marked bi-modal annual cycle, with maxima during September and April, and minima during December-January and June-August, whereas a uni-modal annual cycle in Amazonia, with maximum during August-October and minimum during February-April. Such fires dynamics are closely associated with the annual cycle of NDVI. Results for the interannual variability allow us to conclude that forest fires are tightly associated with dry periods, but also with those regions subject to agricultural expansion. Forest fires exhibit an increase of around 30% with respect to normal years, whereas forest fires diminish (around 15%) with respect to normal years.