Environmental Engineering Reference
In-Depth Information
The Beer-Lambert law states that the amount of radiation either absorbed by or
transmitted through a homogeneous substance depends on the path length through
that substance. Leaves are not homogeneous; however, the effective path length of
water in a leaf, called the equivalent water thickness (EWT), may be calculated
from leaf transmittances using the Beer-Lambert law (Gausman et al.
1970
). Leaf
water content (LWC, Table
20.1
) is a slightly more accurate term compared to
EWT, because water volume is determined by the difference of leaf fresh and dry
weights, and the density of water has a small dependency on temperature. Leaf Area
Index (LAI) of
x
may be thought of as a stack of
x
leaves (Miller et al.
1992
); LWC
multiplied by LAI is the canopy water content (CWC, Table
20.1
). Reflectance
parallels transmittance through leaves (Gates et al.
1965
), so leaf and canopy
reflectances may be used to estimate LWC and CWC.
There are two terms (Table
20.1
) that are often used interchangeably with LWC
and CWC; these are the vegetation water content (VWC) and fuel moisture content
(FMC). Except for grass species and very small plants, there is usually more liquid
water in stems compared to leaves, and there is some water in flowers and fruits.
Active and passive remote sensing at microwave wavelengths is sensitive to all of
the water in plants (Jackson et al.
2004
; Entekhabi et al.
2010
). VWC is a term
which better describes the total amount of water in vegetation aboveground,
approximately equal to the sum of LWC and stem water content (Table
20.1
).
Leaves from different species, or from the same species but under different
growth conditions, have different leaf thicknesses and LWC (Abrams and Kubiske
1990
). Leaf dry matter content (
C
m
, Table
20.1
) is the leaf dry weight per leaf area
and may vary up to 100-fold for different species (Poorter et al.
2009
). FMC is the
ratio of LWC/
C
m
and is used to estimate the potential for wildfire ignition and
spread (Burgan
1988
; Chuvieco et al.
2002
,
2010
; Yebra et al.
2008
). Canopy FMC
is equal to leaf FMC because LAI cancels out from the numerator and denominator.
FMC and RWC are both expressed as percentages; occasionally, studies will
measure FMC or LWC/
W
f
but report the data as RWC.
20.3 Multi-temporal NDVI and Drought Stress
NDVI (Rouse et al.
1974
; Tucker
1979
) distinguishes vegetation and soils based on
the high reflectance of foliage at near-infrared wavelengths and the low reflectance
of foliage at red wavelengths. The low reflectance of red light by vegetation is
caused by a very large chlorophyll absorption coefficient; so changes in red
reflectance are small for large changes of chlorophyll content. Therefore, NDVI
cannot be used to estimate foliar chlorophyll concentration under most conditions
(Lichtenthaler et al.
1996
). Depending on the plant species, dehydration may
increase leaf reflectance at visible and red-edge wavelengths (Fig.
20.2
), due to
some chlorophyll degradation (Carter
1991
,
1993
; Govender et al.
2009
). However,
many different stresses also affect leaf chlorophyll content (Carter
1993
), so if a
decrease in chlorophyll content is detected by remote sensing, the cause of the stress
cannot be determined.
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