Determination of the fire regime in the middle of savanna in Madagascar from a MODIS time series
Résumé
In savannah areas where fire is a predominant practice, the localization and the monitoring of burned areas are essential information to develop an accurate management system of this ecosystem. This problem is addressed through the analysis of time series of medium spatial-resolution remotely sensed images. The authors studied the savannah localized on the Marovoay watershed, on the north-west coast of Madagascar. These present the particularity of a low vegetation cover. The methodological developments proposed in this article concern more specifically two domains: the spectral signature of burned areas and the method for mapping these areas. First, a spectral analysis is undertaken to assess the capacity of six spectral indexes (NDVI, SAVI, BAI, GEMI, Albedo and Brightness Index) to discriminate burned areas. Results show that the Brightness Index is the best. Then, a method for mapping annual burned areas at the scale of the watershed is developed. It is based on the processing of a MODIS eight-day composite times series over the period 2000-2007. Analysis of the intra-annual variations of the Brightness Index leads to the definition of two spatial indicators - annual and seasonal - elaborated to localize the burned areas and to determine the period at which the fire occurs within the fire season. The ROC curves are used as a threshold tool to define the optimal value of the burned area indicator that minimizes both omission and commission errors. The validation of the MODIS burned areas maps consists of the comparison with burned areas maps produced with high spatial-resolution remotely sensed images. The quality of the results is good, with a Kappa index between 0.74 and 0.76 and a cartographic index between 76% and 79%.