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Mesures physiques et signatures en télédétection

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C.S.T. Daughtry 1 , J.E. McMurtrey III 1 , and E.W. Chappelle 2
'USDA ARS Remote Sensing Research Lab
Building 7 BARC-West, 10300 Baltimore Ave
Beltsville, MD 20705-2350 USA
2 NASA Goddard Space Flight Center
Greenbelt, MD 20771 USA
Crop residues, the portion of the crop left in the field after harvest, can be an important management factor in
controlling soil erosion. Current methods for quantifying crop residue cover are tedious and somewhat subjective.
There is a need for new methods to quantify residue cover that are rapid, accurate, and objective. Scenes with
known amounts of crop residue were illuminated with long-wave ultraviolet (UV) radiation and fluorescence images
were recorded with an intensified-video camera fitted with a 453-488 nm band-pass filter. A light-colored soil and
a dark-colored soil were used as background for the weathered soybean stems. Residue cover was determined by
counting the proportion of the pixels in the image with fluorescence values greater than a threshold. Soil pixels had
the lowest gray levels in the images. The values of the soybean residue pixels spanned nearly the full range of the
8-bit video data. Classification accuracies typically were within 3% (absolute units) of measured cover values.
Video imaging can provide an intuitive understanding of the fraction of the soil covered by residue.
KEY WORDS: Blue fluorescence, crop residue, litter, soil, video, image analysis
In order for a farm manager to utilize crop residues effectively for controlling soil erosion, he must be able to
quantify the residue cover in his fields. Current methods for quantifying crop residue cover are tedious and
somewhat subjective (Laflen et al., 1981; Morrison et al., 1993). There is a need for new methods to quantify
residue cover that are rapid, accurate, and objective.
For nearly 20 years, scientists have tried with limited success to discriminate soils and crop residue based
cm their reflectance spectra (e.g., Aase and Tanaka, 1991; Baumgardner et al., 1985; Gausman et al., 1975). In the
visible (400-700 nm) and near infrared (700-1300 nm) wavelength regions reflectance differences between soils and
residues change as soil moisture changes and as residues weather and decompose. Residues may be brighter or
darker than a given soil, even within a single field (Daughtry et al., 1993; McMurtrey et al., 1993). Similar
difficulties are expected in the shortwave infrared (1300-2400 nm) wavelength region, where water absorption
dominates the spectral properties. In the shortwave infrared, changes in moisture content of the soil and residue will
likely affect discrimination. Thus, reflectance techniques to quantify crop residue cover will need frequent
calibrations or adjustments to discriminate accurately between soil and crop residues in the field.
One promising, yet not fully explored, technique analyzes the curve shapes of the reflectance spectra of soils
and crop residues. Dulaney et al. (1992) used the wavelength of the maximum first derivative plus the reflectance
factors in a visible and a near infrared band to discriminate a wide range of soils and crop residues. Dulaney et al.
(1992) used spectral reflectance data acquired at 5 nm intervals for their analyses. However it is unclear whether
the technique would be sucdessful using broader band spectral data.
McMurtrey et al. (1993) first proposed that the fluorescence induced by a nitrogen laser, emitting at 337
Dm, could be used to discriminate residues and soils. The fluorescence intensities of the crop residues were 2 to 10
tunes greater than the fluorescence of the soils. Subsequent work by Daughtry et al. (1993) showed that the
ultraviolet-induced fluorescence of crop residues was a broad band phenomena with an emission maxima wi thin the
420-495 nm band for an excitation band of 350-420 nm. Most soils have low intensity emissions over the same
wavelength range. They also measured the fluorescence of recendy harvested- and weathered-residues of com (Zea