Τhe field work was designed to provide fire frequencies in the Gambia at the four daily satellite overpasses of two orbiting NOAA (National Oceanic and Atmospheric Administration) satellites. The AVHRR (Advanced Very High Resolution Radiometers) of each of the two satellites scan over each surface area twice a day, once during the day and once at night, with a nominal time displacement of around 6 hours. Over a 2-week period in March 1988, 26 Gambian foresters reported all continuing and burned-out savanna fires which had occurred at the NOAA overpass times along pre-determined routes. The foresters drove along the routes twice a day, once just before sunset and once just after dawn. They reported a total of 115 fires which showed a strong diurnal cycle. The minimum fire frequency occurred at the 2.30 a.m. local time overpass (NOAA-9). At the 8.30 a.m. pass (NOAA-lO) the fire frequency was 6.3 times higher; at the 2.30 p.m. pass (NOAA-9) 8.5 times higher; and at the 8.30 p.m. pass (NOAA-lO) it was 2.8 times higher.

Further, I have analysed eight pairs of consecutive NOAA-lO evening and NOAA-II early-morning images of Guinea-Bissau, the Gambia, Senegal and parts of Mauritania, Mali and Guinea from November 1989 to May 1990 using a physically based bispectral interpretation model. The model uses A VHRR thermal night-time data in the 3.55-3.92-μm and 10.2-11.2-μm bands and has been tailored for night-time detection of savanna fires. These pairs enabled analysis of the relative frequencies of fires at around 10 p.m. and 3 a.m. local time. The relative fire frequencies between the two night-time passes were very similar to those obtained in the field; namely 2.7 to 1 for the late-evening pass compared to the early-morning pass.

Even though most savanna fires in West Africa are not controlled, this does not necessarily imply that night-time fire frequencies are close to day-time frequencies, as suggested by Cahoon et al. At least in the region covered by my study, the weather conditions counteract biomass burning at night because of increased air humidity and associated dew deposition on the vegetation, reduced wind activity and lowered air temperature. Under low wind speeds the normal duration of savanna fires, most of which are started, intentionally or accidentally, by the rural population, is determined by the time before the fire front is trapped by natural or artificial barriers. This usually happens within 12 hours, or before dawn the next day.

Tags: fire Gambia Senegal West Africa