Geoscience Reference
In-Depth Information
where
cients, respectively; w is the
medium moisture content; h is the radiative layer depth; T
s
is the soil temperature;
T
bs
is the soil brightness temperature. Yakimov (1996) proposed the following
decision rule based on the calculation of mathematical average M
*
and standard
deviation
ʺ
and
ʱ
are the radiation and attenuation coef
*
for series of T
b
for forested area using wavelength 2.25 cm. Value M
*
is compared with threshold d = 7.2514
˃
*
+ 249.876. If M
*
˃
≥
d then considered
fire dangerous. If M
*
< d then
forested area has average or higher forest
re
dangerous of considered area is low.
The forest
fire scale depends on the series of factors such as water content in
vegetations, topography, and wind direction. Water content in the plants is the most
important factor regulating the speed of forest
fire evolution. Maki et al. (2004) and
Rahimzadeh-Bajgiran et el. (2012) studied relations between different indexes of
forest
fires and determined that the most perspective indexes are:
•
vegetation dryness index (TVDI);
•
temperature vegetation dryness index (TVDI);
•
water de
cit index (WDI); and
•
normalized difference water index (NDWI).
To assess the water content in vegetation it is needed to use indicators FMC (fuel
moisture content) and EWT (equivalent water thickness):
¼ FW
DW
cm
2
FMC
=
100
%
¼ FW
DW
ð
Þ=
FW
;
EWT g
=
ð
Þ=r;
is the area of leaves in crown (cm
2
), FW is the wet weight of vegetations
(g) and DW is the dry weight of vegetation (g).
Indicators NDWI, WDI and VDI are calculated by means of the following
formulae:
where
˃
=
;
NDWI ¼ T
b
;
NIR
T
b
;
SNIR
T
b
;
NIR
þ
T
b
;
SNIR
WDI ¼ 1
D
1
=D
2
;
VDI ¼ 1
D
3
=D
4
;
where T
b,NIR
is the brightness temperature registered in near IR range, T
b,SNIR
is the
brightness temperature registered in shot-wave IR range,
ʔ
1
is the distance on the
phase plane (NDWI, NDVI) between conditions when water de
cit (D) and excess
(C) take place under low-density vegetation,
ʔ
2
is the distance between conditions
of dense (A) and low-density (B) vegetation,
ʔ
4
are distances between
measured NDVI and NDWI and between sides of parallelogram with corners in
points A, B, C and D (dense vegetation and moisture excess) on plane (NDWI,
NDVI) under NDVI = const, respectively. Index NDWI is calculated basing on the
measurements received from satellite SPOT in channels 430
-
470, 610
-
680, 780
-
890 and 1,580
ʔ
3
and
1,750 nm. NDVI value is usually assessed with use of measure-
ments received by means of sensors in red and near IR ranges from satellite
Landsat.
-
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