Abstract
This research reports the findings of a Landsat Next expert review panel that evaluated the use of narrow shortwave infrared (SWIR) reflectance bands to measure ligno-cellulose absorption features centered near 2100 and 2300 nm, with the objective of measuring and mapping non-photosynthetic vegetation (NPV), crop residue cover, and the adoption of conservation tillage practices within agricultural landscapes. Results could also apply to detection of NPV in pasture, grazing lands, and non-agricultural settings. Currently, there are no satellite data sources that provide narrowband or hyperspectral SWIR imagery at sufficient volume to map NPV at a regional scale. The Landsat Next mission, currently under design and expected to launch in the late 2020’s, provides the opportunity for achieving increased SWIR sampling and spectral resolution with the adoption of new sensor technology. This study employed hyperspectral data collected from 916 agricultural field locations with varying fractional NPV, fractional green vegetation, and surface moisture contents. These spectra were processed to generate narrow bands with centers at 2040, 2100, 2210, 2260, and 2230 nm, at various bandwidths, that were subsequently used to derive 13 NPV spectral indices from each spectrum. For crop residues with minimal green vegetation cover, two-band indices derived from 2210 and 2260 nm bands were top performers for measuring NPV (R2 = 0.81, RMSE = 0.13) using bandwidths of 30 to 50 nm, and the addition of a third band at 2100 nm increased resistance to atmospheric correction residuals and improved mission continuity with Landsat 8 Operational Land Imager Band 7. For prediction of NPV over a full range of green vegetation cover, the Cellulose Absorption Index, derived from 2040, 2100, and 2210 nm bands, was top performer (R2 = 0.77, RMSE = 0.17), but required a narrow (≤20 nm) bandwidth at 2040 nm to avoid interference from atmospheric carbon dioxide absorption. In comparison, broadband NPV indices utilizing Landsat 8 bands centered at 1610 and 2200 nm performed poorly in measuring fractional NPV (R2 = 0.44), with significantly increased interference from green vegetation.
Highlights
Hyperspectral field spectrometer data published in Dennison et al [43] were used to simulate and subsequently evaluate five shortwave infrared (SWIR) bands and associated non-photosynthetic vegetation (NPV) indices relevant to the Landsat mission, focusing on 916 source spectra collected by Daughtry and
These results are consistent with visual inspection of the example spectra presented in Figure 1, which show increased deviation between original and atm spectra beyond the 20 nm bandwidth for the 2040 nm and 2100 nm bands
In this study we used a dataset of 916 reflectance spectra from agricultural targets, with associated measures of percent cover by NPV, soil, and green vegetation, to evaluate various narrowband SWIR indices employed to measure fractional NPV cover under a variety of surface moisture conditions
Summary
The presence of crop residues (non-photosynthetic stalks, stubble, and other senescent plant material) on the surface of agricultural soils supports a variety of environmental services, serving to decrease soil erosion, increase soil organic matter, and improve soil health [1]. Accurate measurement and mapping of crop residue cover can contribute to a better understanding of implementation of conservation practices within agricultural landscapes. Crop residue transect surveys are frequently conducted by State and Federal agencies, and by farmers, to monitor the adoption and effectiveness of conservation tillage practices. Crop residue cover has been quantified by use of in-field line-point transects [6,7], or more recently by line-point analogs using photographic analysis [8], and most recently using spectral analysis of reflectance from the field surface [9,10,11]
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