VIs generally have the scientific requirement of contrasting an absorbing leaf
spectral feature with a non-absorbing one in a globally robust manner to capture
essential plant biophysical phenomena with adequate fidelity. Most VIs will
combine a chlorophyll-absorbing spectral band in the Red with a non-absorbing
band in the near-infrared (NIR) to depict canopy greenness, or area-averaged
canopy photosynthetic activity. Vegetation water indices, on the other hand, may
combine a water absorbing leaf spectral feature in the shortwave infrared (SWIR)
with the NIR to provide measures of canopy moisture content (Ceccato et al.
2002a, b; Gao 1996).
As a greenness measure, VIs encapsulate leaf- and whole-canopy optical
properties expressed through several biophysical quantities related to foliage
amount and structure (vegetation fraction, leaf angles, and leaf area) and physiological (pigments, water) status of a canopy. VIs have been widely used as
proxies in the assessment of canopy biophysical/biochemical variables, including
leaf area index (LAI), fraction of absorbed photosynthetically-active radiation
(f APAR ), chlorophyll content, green vegetation fraction (F veg ), biomass, and canopy
biophysical processes (Sellers 1985; Field et al. 1995; Gitelson et al. 2003; Glenn
et al. 2008). Several comprehensive reviews on the use of VIs to assess ecological
properties are found in Kerr and Ostrovsky (2003), Pettorelli et al. (2005), and
Huete and Glenn (2011).
1.2 Theoretical Description of Vegetation Indices
The theoretical basis for vegetation indices is derived from an examination of leaf,
soil, woody, and senesced plant spectral reflectance signatures (Fig. 1.1). The
reflected energy from a green leaf is very low in the visible portion of the spectrum
due to high absorption of photosynthetically active radiation (PAR) by leaf pigments, primarily in the Blue (470 nm) and Red (670 nm), whereas nearly all of the
NIR radiation is scattered (reflected and transmitted) in a manner dependent on
leaf type, morphology, and cellular structure. The resulting contrast between Red
and NIR reflectances is a sensitive measure of variations in leaf physiology and
structure, with maximum contrast occurring in healthy, structurally-developed
leaves and minimal contrast in stressed and senesced leaves. At the canopy level,
maximum NIR-Red contrasts occur in densely foliated canopies containing vigorous leaves, with lower contrasts found in stressed and open canopies, and the
least contrasts occur in defoliated or sparse canopies.
1.2.1 Index Formulations
There are a variety of ways in which two or more spectral bands may be combined
to quantify the NIR-Red contrast as a measure of canopy greenness or moisture.
This has resulted in a multitude of VI formulas and variants that include two-band
2
A. Huete et al.
spectral feature with a non-absorbing one in a globally robust manner to capture
essential plant biophysical phenomena with adequate fidelity. Most VIs will
combine a chlorophyll-absorbing spectral band in the Red with a non-absorbing
band in the near-infrared (NIR) to depict canopy greenness, or area-averaged
canopy photosynthetic activity. Vegetation water indices, on the other hand, may
combine a water absorbing leaf spectral feature in the shortwave infrared (SWIR)
with the NIR to provide measures of canopy moisture content (Ceccato et al.
2002a, b; Gao 1996).
As a greenness measure, VIs encapsulate leaf- and whole-canopy optical
properties expressed through several biophysical quantities related to foliage
amount and structure (vegetation fraction, leaf angles, and leaf area) and physiological (pigments, water) status of a canopy. VIs have been widely used as
proxies in the assessment of canopy biophysical/biochemical variables, including
leaf area index (LAI), fraction of absorbed photosynthetically-active radiation
(f APAR ), chlorophyll content, green vegetation fraction (F veg ), biomass, and canopy
biophysical processes (Sellers 1985; Field et al. 1995; Gitelson et al. 2003; Glenn
et al. 2008). Several comprehensive reviews on the use of VIs to assess ecological
properties are found in Kerr and Ostrovsky (2003), Pettorelli et al. (2005), and
Huete and Glenn (2011).
1.2 Theoretical Description of Vegetation Indices
The theoretical basis for vegetation indices is derived from an examination of leaf,
soil, woody, and senesced plant spectral reflectance signatures (Fig. 1.1). The
reflected energy from a green leaf is very low in the visible portion of the spectrum
due to high absorption of photosynthetically active radiation (PAR) by leaf pigments, primarily in the Blue (470 nm) and Red (670 nm), whereas nearly all of the
NIR radiation is scattered (reflected and transmitted) in a manner dependent on
leaf type, morphology, and cellular structure. The resulting contrast between Red
and NIR reflectances is a sensitive measure of variations in leaf physiology and
structure, with maximum contrast occurring in healthy, structurally-developed
leaves and minimal contrast in stressed and senesced leaves. At the canopy level,
maximum NIR-Red contrasts occur in densely foliated canopies containing vigorous leaves, with lower contrasts found in stressed and open canopies, and the
least contrasts occur in defoliated or sparse canopies.
1.2.1 Index Formulations
There are a variety of ways in which two or more spectral bands may be combined
to quantify the NIR-Red contrast as a measure of canopy greenness or moisture.
This has resulted in a multitude of VI formulas and variants that include two-band
2
A. Huete et al.