Agriculture Reference
In-Depth Information
15
30
flash/km
2
/yr
0.01
0.1
1
10
100
Fig. 7.10
Annualized lightning flash rate density over South Africa for the period January 1, 1999
to December 31, 2004, collected from Lightning Image Sensor data on the TRMM satellite.
Lightning flash densities vary from
10 flashes km
2
yr
1
in the eastern part of the Cape Floristic Region. Peak lightning activity is 26.4 flashes km
2
yr
1
over the Drakensberg escarpment and the highveld plateau. (Figure from Collier
et al.
2006
.)
1km
2
yr
1
in the southwest to
∼
∼
Even if there is sufficient fuel, and weather conditions are suitable for burning,
fires have to be ignited. Natural sources of ignition in the Cape region include
lightning and rockfalls. Fires ignited by rock falls are rare, igniting only 4% of
fires of natural origin over a 50-yr period in the Swartberg mountains (Horne
1981
). Within South Africa, ground-based measures of lightning showed that the
fynbos and karoo (arid shrubland) biomes have the lowest average lightning
frequencies, with savannas having intermediate values and montane grasslands
the highest (Manry & Knight
1986
). The entire western Cape region had a
lightning flash density of <1 flash km
2
yr
1
. In contrast, the C
4
montane
grasslands of the summer rainfall region had the highest lightning incidence in
the country with 10-14 flashes km
2
y
1
. In the Swartberg mountains to the east
of the Cape region, 10 to 20 thunderstorms occur per year. Lightning days build
up from a low of 1.2 in winter, 4.4 in spring, peaking in summer (6.2) and autumn
(5.9) (Seydack
et al.
2007
). Satellite-based studies show a gradient of increasing
lightning strikes from lowest in the west (
1km
2
yr
1
), increasing in the south-
ern Cape with highest flash densities in the eastern Cape (Collier
et al.
2006
).
Maximum flash densities in South Africa (26.4 km
2
yr
1
) were recorded over the
Drakensberg mountains and the highveld plateau of the interior (
Fig. 7.10
), which
are mostly C
4
grasslands. The relationship between flash densities and fire starts
has not been studied in South Africa.
Despite the low incidence of thunderstorms relative to the rest of the country,
lightning is a common source of ignition of fynbos fires, especially in mountain
fynbos. In the Swartberg mountains, fires lit by natural causes (mostly lightning)
accounted for 78.6% of the total area burned between 1986 and 2002, a period
∼