Abstract
Abstract. This paper investigates how hyperspectral reflectance (between 350 and 1800 nm) can be used to infer ecosystem properties for a semi-arid savanna grassland in West Africa using a unique in situ-based multi-angular data set of hemispherical conical reflectance factor (HCRF) measurements. Relationships between seasonal dynamics in hyperspectral HCRF and ecosystem properties (biomass, gross primary productivity (GPP), light use efficiency (LUE), and fraction of photosynthetically active radiation absorbed by vegetation (FAPAR)) were analysed. HCRF data (ρ) were used to study the relationship between normalised difference spectral indices (NDSIs) and the measured ecosystem properties. Finally, the effects of variable sun sensor viewing geometry on different NDSI wavelength combinations were analysed. The wavelengths with the strongest correlation to seasonal dynamics in ecosystem properties were shortwave infrared (biomass), the peak absorption band for chlorophyll a and b (at 682 nm) (GPP), the oxygen A band at 761 nm used for estimating chlorophyll fluorescence (GPP and LUE), and blue wavelengths (ρ412) (FAPAR). The NDSI with the strongest correlation to (i) biomass combined red-edge HCRF (ρ705) with green HCRF (ρ587), (ii) GPP combined wavelengths at the peak of green reflection (ρ518, ρ556), (iii) LUE combined red (ρ688) with blue HCRF (ρ436), and (iv) FAPAR combined blue (ρ399) and near-infrared (ρ1295) wavelengths. NDSIs combining near infrared and shortwave infrared were strongly affected by solar zenith angles and sensor viewing geometry, as were many combinations of visible wavelengths. This study provides analyses based upon novel multi-angular hyperspectral data for validation of Earth-observation-based properties of semi-arid ecosystems, as well as insights for designing spectral characteristics of future sensors for ecosystem monitoring.
Highlights
Hyperspectral measurements of the Earth’s surface provide relevant information for many ecological applications
In dense and erectophile canopies, hemispherical conical reflectance factor (HCRF) increases with sensor viewing and solar zenith angles, because a larger fraction of the upper vegetation canopy is viewed/illuminated, whereas the shadowed lower part of the canopy contributes less to the measured signal as shown previously by several studies (Huete et al, 1992, 2014; Jin et al, 2002; Kimes, 1983)
A recognised advantage of normalised difference spectral indices (NDSIs) calculations is that errors/biases that are similar in both wavelengths included in the index are suppressed by the normalisation
Summary
Hyperspectral measurements of the Earth’s surface provide relevant information for many ecological applications. An important tool for spatial extrapolation of ecosystem functions is to study how spectral properties are related to in situ measured ecosystem properties. These relationships found the basis for upscaling using Earth observation (EO) data. Continuous in situ measurements of hyperspectral reflectance in combination with ecosystem properties are thereby essential for improving our understanding of the functioning of the observed ecosystems. For example, been found between information in the reflectance spectrum and ecosystem properties such as leaf area index (LAI), fraction of photosynthetically active radiation (PAR) absorbed by the vegetation (FAPAR), light use efficiency (LUE), biomass, vegetation primary productivity, vegetation water content, and nitrogen and chlorophyll content Tagesson et al.: Deriving seasonal dynamics in ecosystem properties of semi-arid savanna grasslands
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