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Coupled Retrieval of the Three Phases of Water from Spaceborne Imaging Spectroscopy Measurements
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  • Niklas Bohn,
  • Theres Kuester,
  • Karl Segl,
  • Luis Guanter,
  • Rene Preusker
Niklas Bohn
Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences

Corresponding Author:[email protected]

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Theres Kuester
Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences
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Karl Segl
Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences
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Luis Guanter
Polytechnic University of Valencia
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Rene Preusker
Free University of Berlin
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Measurements of reflected solar radiation by imaging spectrometers allow to quantify water in different states (solid, liquid, gas) thanks to the discriminative absorption lines in the solar spectrum. We developed a retrieval method to quantify the amount of water in each of the three states from spaceborne imaging spectroscopy data, such as those from the German EnMAP mission. Our retrieval couples atmospheric radiative transfer simulations from the MODTRAN5 radiative transfer code to a surface reflectance model based on the Beer-Lambert law. The model is inverted on a per-pixel basis using a maximum likelihood estimation formalism. Based on a unique coupling of the canopy reflectance model HySimCaR and the EnMAP end-to-end simulation tool EeteS, we performed a sensitivity analysis by comparing the retrieved values with the simulation input leading to an R2 of 0.991 for water vapor and 0.965 for liquid water. Furthermore, we applied our algorithm to airborne AVIRIS-C data to demonstrate the ability to map snow/ice extents as well as to a CHRIS-PROBA dataset for which concurrent field measurements of canopy water content were available. The comparison between our retrievals and the ground measurements showed an overall R2 of 0.80 for multiple crop types and a remarkable clustering in the regression analysis indicating a dependency of the retrieved water content from the physical structure of the vegetation. In addition, our algorithm is able to produce smoother and more physically-plausible water vapor maps than the ones from the band ratio approaches used for multispectral data, since biases due to background reflectance are reduced. The demonstrated potential of imaging spectroscopy to provide accurate quantitative measures of water from space will be further exploited using upcoming spaceborne imaging spectroscopy missions like PRISMA or EnMAP.
Jun 2020Published in Remote Sensing of Environment volume 242 on pages 111708. 10.1016/j.rse.2020.111708