Abstract
Field spectroradiometers integrated in automated systems at Eddy Covariance (EC) sites are a powerful tool for monitoring and upscaling vegetation physiology and carbon and water fluxes. However, exposure to varying environmental conditions can affect the functioning of these sensors, especially if these cannot be completely insulated and stabilized. This can cause inaccuracy in the spectral measurements and hinder the comparison between data acquired at different sites. This paper describes the characterization of key sensor models in a double beam spectroradiometer necessary to calculate the Hemispherical-Conical Reflectance Factor (HCRF). Dark current, temperature dependence, non-linearity, spectral calibration and cosine receptor directional responses are modeled in the laboratory as a function of temperature, instrument settings, radiation measured or illumination angle. These models are used to correct the spectral measurements acquired continuously by the same instrument integrated outdoors in an automated system (AMSPEC-MED). Results suggest that part of the instrumental issues cancel out mutually or can be controlled by the instrument configuration, so that changes induced in HCFR reached about 0.05 at maximum. However, these corrections are necessary to ensure the inter-comparison of data with other ground or remote sensors and to discriminate instrumentally induced changes in HCRF from those related with vegetation physiology and directional effects.
Highlights
Linking gas exchange measurements taken at single-point Eddy Covariance (EC) sites with spatial information provided by remote sensing is key to globally quantify and monitor the “breathing” of the planet [1].the connection between these data sources is challenging due to the existence of spatial and temporal mismatches
Nbias linearly decreases with T and N0 is weaker than Nbias at low temperatures
Different instrumental sources of error in the computation of Hemispherical-Conical Reflectance Factor (HCRF) have been characterized in laboratory under different temperatures and configuration settings
Summary
Linking gas exchange measurements taken at single-point EC sites with spatial information provided by remote sensing is key to globally quantify and monitor the “breathing” of the planet [1].the connection between these data sources is challenging due to the existence of spatial and temporal mismatches. Unattended ground-set optical sensors have the advantage of overcoming the temporal mismatch existing between the continuous micrometeorological measurements acquired by the EC systems and the periodic overpass of remote sensors This way, information relative to the optical properties of vegetation can be directly related with the biospheric carbon and water fluxes, and used to upscale the flux information from site to local, regional and global scales [1,2]. Though the use of single or multi-spectral sensors at EC sites is more frequent due to their low cost and easy installation [3], hyperspectral sensors (spectroradiometers) are being gradually installed at these sites [2,3] These sensors sample radiation in narrow and overlapping bands continuously arranged along the spectral domain, typically covering the visible and near infra-red (NIR) regions. On one hand, such detailed optical information can be related with the physiological and biochemical status of vegetation [4,5,6,7,8,9,10,11]; on the other it can be flexibly matched with the spectral bands of other remote sensors [12,13,14]
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