AVHRR monitoring of vegetation ? res in the tropics: toward the development of a global product

• has accompanied and played important roles in the development of the vegetation of our planet. Since control by humans about half a million years ago, ,lt has been a major tool in hunting, forest conversion, ngriculture, and pasture renewal, just to mention a few ies related to vegetation changes. Since the pioneer work of Crutzen et al. (1979) environmental effects re'ening from vegetation fires have increasingly become ,nsubject of scientific interest. With tropical deforesta6on taking place at unprecedented rates, they rapidly :changed into a worldwide scientific and public envi;tormenta' concern. Two recent publications (Levine 1991; Cnitzen and Goldammer 1993) contam n hundreds of references that extensively document the current importance of biomass burning. Of particular interest is the estimate of the location and extent of vegetation burning on the planet, which hu yet to be made. Supposedly, 60% of the pantropical savannas in the world, about 15 x 10 6 km2, iy be affected by fire every year (Goldammer 1993). 'tropical deforestation, norrnally attained through the of fire, was estimated at 15.4 x 10 4 km2 for the last ;decade (Singh 1993); unknown immense areas of >mal forests are also destroyed by tires. On a global buis, carbon emissions from biomass burning could râceown for 30-80% of the fossil fuel burning rate of 3.7 x 1015 g of carbon per year, with significant effects ;in biogeochemical cycles and possibly also on the mate (Crutzen and Andreae 1990). Vegetation fires occur on ali continents (e.g., -Andreae 1993) and difficulties in their detection with ,totwentional ways for most of the world makes syslematic remote sensing from space the only possibility a,for their comprehensive study. Because of its daily cov! tuge, low (1.1 km) resolution irnages of the Advanced yery High Resolution Radiometer (AVH R R) aboard tlte National Oceanographic and Atmospheric AdSinistration (NOAA) series satellites (Kidwell 1991) Nive been used in thermal detection of active fires in ,rlaytime since the early work of Matson and Dozier ,0981) pointed to this possibility. An AVHRR-based , real-time operational program for firefighting and monitoring has existed for many years on a regional scale (Setzer and Pereira 1991a) and an AVHRR global tire product is under consideration (IGBP 1992) following Malingreau (1990) and Malingreau et al. (1993). However, little validation of AVI1RR fire detection algoritluns is found in the literature; different authors use different methods and commonly refer to fires detected and mapped without any field verification. More recently Setzer et al. (1994) describe AVHRR responses to two large forest tires in Amazonia, and Belward et al. (1993) to five rires in West African savannas; in opposition to algorit hms using a multichannel approach, these data sets indicate that AVHRR's channel 3 (3.55-3.93 mim) alone is enough and the best to detect active fires. In the following text we present evideriee from 330 cases of vegetation fires in NOAA-11/AVHR R images of three continents. Of significant importance, these tires were detected in daytime and not nighttime images, representing the most needed and difficult conditions for their detection. General statistics of the results are given but we concentrate on the main practical problems of fire detection with AVHRR, suggesting techniques toward a global fire product.