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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2018-600
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2018-600
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Dec 2018

Research article | 12 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Spatial variability of mean daily estimates of actual evaporation from remotely sensed imagery and surface reference data

Robert N. Armstrong1, John W. Pomeroy2, and Lawrence W. Martz2 Robert N. Armstrong et al.
  • 1Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Toowoomba, 4350, Australia
  • 2Centre for Hydrology, University of Saskatchewan, Saskatoon, S7N 1K2, Canada

Abstract. Land surface evaporation has considerable spatial variability that is not captured by point scale estimates calculated from meteorological data alone. Knowing how evaporation varies spatially remains an important issue for improving parameterisations of land surface schemes and hydrological models, and various land management practices. Satellite-based and aerial remote sensing has been crucial for capturing moderate to larger scale surface variables to indirectly estimate evaporative fluxes. However, more recent advances for field research via unmanned aerial vehicles (UAVs) now allows for the acquisition of more highly detailed surface data.

Integrating models that can estimate actual evaporation from higher resolution imagery and surface reference data would be valuable to better examine potential impacts of local variations in evaporation on upscaled estimates. This study introduces a novel approach for computing a normalised index from surface variables that can be used to obtain more realistic distributed estimates of actual evaporation. For demonstration purposes the Granger and Gray evaporation model (G–D) was applied at a complex parkland site in central Saskatchewan, Canada. Visible and thermal images and meteorological reference data required to parameterise the model was obtained at midday.

Normalised indexes (simple ratios) were computed at midday for albedo and net radiation. This allowed for single measured values albedo and mean daily net radiation to be scaled across high resolution images over a large study region. Albedo and net radiation estimates were within 5–10 % of measured values. An evaporation estimate for a grassed surface was 0.5 mm larger than eddy covariance measurements. The methods applied have two key advantages for estimating evaporation over previous remote sensing approaches, 1. Detailed daily estimates of actual evaporation were directly obtained using a physically-based evaporation model, and 2. Analysis of more detailed and reliable evaporation estimates may lead to improved methods for upscaling evaporative fluxes to larger scales.

Robert N. Armstrong et al.
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Status: final response (author comments only)
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Robert N. Armstrong et al.
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Short summary
Digital and thermal images taken near midday were used to scale daily point observations of key factors driving actual evaporation estimates across a complex Canadian prairie landscape. Point estimates of actual evaporation agreed well with observed values via eddy covariance. Impacts of spatial variations on areal estimates were minor and no covariance was found between model parameters driving the energy term. The methods can be applied further to improve land surface parameterisations.
Digital and thermal images taken near midday were used to scale daily point observations of key...
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