513
Soil and Vegetation Moisture Changes for Forest Fire Events
For both soil moisture and leaf water content, an increase in each is connected with
a reflectance reduction. The sensitivities of MODIS bands 6 (1.64 µm) and 7 (2.13
µm) responding to the moisture change, however, are definitely different. As shown
in Figure 21.1, the slope between the 1.64- and 2.13-μm channels becomes steeper as
soil moisture increases but flatter as leaf water content increases. On the basis of the
characteristic “slope variation” in response to different kinds of moisture changes, the
NMDI is defined by using three wavelengths, one in the NIR centered approximately
at 0.86 µm and two in the SWIR centered at 1.64 and 2.13 µm, respectively:
NMDI
m
m
m
m
m
=
⋅
−
⋅
− ⋅
⋅
+
⋅
−
R
R
R
R
0
1
2
0
1
86
86
13
86
86
µ
µ
µ
µ
µ
(
)
(
R
R R 2 13
⋅ µm)
,
(21.2)
where R represents the reflectance at the wavelengths denoted by the subscripts.
To show that the NMDI can be used to monitor both soil and vegetation moisture
contents from space, its sensitivities to bare soil or weak vegetation as well as heavy
vegetation have been investigated.
Simulations are obtained by the coupled soil–leaf–canopy reflectance models
with varying soil moisture from low to high values and an LAI range of 0.01–2. For
bare soil or weakly vegetated areas with an LAI equal to 0.01, higher values of the
NMDI indicate increasingly severe soil drought: the NMDI decreases from high values around 0.85 for extremely dry soil to low values around 0.15 for wet soil with soil
moisture content higher than 0.3 (Figure 21.2a). The NMDI stops responding to soil
moisture change starting from an LAI equal to 2; that is, no soil background effects
are found on the NMDI for any soil moisture range. Therefore, for a vegetation canopy with an LAI equal to or higher than 2, which means heavily vegetated areas, the
NMDI turns to be a complete index for estimating vegetation water content: lower
NMDI values indicate increasingly severe vegetation drought (Figure 21.2b).
21.3.2 validation
21.3.2.1 Soil Drought Monitoring
The soil moisture condition falls into three classes according to the volumetric soil
moisture range: dry, 0–0.1; intermediate, 0.1–0.2; or wet, >0.2 (Idso et al. 1975;
LAI
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
0.1
0.2
0.3
0.4
0.5
0
0.1
0.2
0.3
0.4
0.5
Soil moisture (Vol)
NMDI
0.2
0.3
0.4
0.5
0.6
0.7
0.8
NMDI
0.01
0.5
1
2
LAI
Cw (cm)
(a)
(b)
0.5
1
2
3
4
5
6
FIGURE 21.2 Sensitivity of the NMDI to (a) soil moisture, and (b) leaf water content.
Soil and Vegetation Moisture Changes for Forest Fire Events
For both soil moisture and leaf water content, an increase in each is connected with
a reflectance reduction. The sensitivities of MODIS bands 6 (1.64 µm) and 7 (2.13
µm) responding to the moisture change, however, are definitely different. As shown
in Figure 21.1, the slope between the 1.64- and 2.13-μm channels becomes steeper as
soil moisture increases but flatter as leaf water content increases. On the basis of the
characteristic “slope variation” in response to different kinds of moisture changes, the
NMDI is defined by using three wavelengths, one in the NIR centered approximately
at 0.86 µm and two in the SWIR centered at 1.64 and 2.13 µm, respectively:
NMDI
m
m
m
m
m
=
⋅
−
⋅
− ⋅
⋅
+
⋅
−
R
R
R
R
0
1
2
0
1
86
86
13
86
86
µ
µ
µ
µ
µ
(
)
(
R
R R 2 13
⋅ µm)
,
(21.2)
where R represents the reflectance at the wavelengths denoted by the subscripts.
To show that the NMDI can be used to monitor both soil and vegetation moisture
contents from space, its sensitivities to bare soil or weak vegetation as well as heavy
vegetation have been investigated.
Simulations are obtained by the coupled soil–leaf–canopy reflectance models
with varying soil moisture from low to high values and an LAI range of 0.01–2. For
bare soil or weakly vegetated areas with an LAI equal to 0.01, higher values of the
NMDI indicate increasingly severe soil drought: the NMDI decreases from high values around 0.85 for extremely dry soil to low values around 0.15 for wet soil with soil
moisture content higher than 0.3 (Figure 21.2a). The NMDI stops responding to soil
moisture change starting from an LAI equal to 2; that is, no soil background effects
are found on the NMDI for any soil moisture range. Therefore, for a vegetation canopy with an LAI equal to or higher than 2, which means heavily vegetated areas, the
NMDI turns to be a complete index for estimating vegetation water content: lower
NMDI values indicate increasingly severe vegetation drought (Figure 21.2b).
21.3.2 validation
21.3.2.1 Soil Drought Monitoring
The soil moisture condition falls into three classes according to the volumetric soil
moisture range: dry, 0–0.1; intermediate, 0.1–0.2; or wet, >0.2 (Idso et al. 1975;
LAI
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
0.1
0.2
0.3
0.4
0.5
0
0.1
0.2
0.3
0.4
0.5
Soil moisture (Vol)
NMDI
0.2
0.3
0.4
0.5
0.6
0.7
0.8
NMDI
0.01
0.5
1
2
LAI
Cw (cm)
(a)
(b)
0.5
1
2
3
4
5
6
FIGURE 21.2 Sensitivity of the NMDI to (a) soil moisture, and (b) leaf water content.