Deriving seasonal dynamics in ecosystem properties of semi-arid savannas using in situ based hyperspectral reflectance.

This paper investigates how seasonal hyperspectral reflectance data (between 350 and 1800 nm) can be used to infer ecosystem properties for a semi-arid savanna ecosystem in West Africa using a unique in situ based dataset. Relationships between seasonal dynamics in hyperspectral reflectance, and ecosystem properties (biomass, gross primary productivity (GPP), light use efficiency (LUE), and fraction of photosynthetically active radiation absorbed by vegetation (FAPAR)) were analysed. Reflectance data (ρ) were used to study the relationship between normalised difference spectral indices (NDSI) and the measured ecosystem properties. Finally, also 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 761nm used for estimating chlorophyll fluorescence (GPP, and LUE), and blue wavelengths (FAPAR). The NDSI with the strongest correlation to: (i) biomass combined red edge reflectance (ρ₇₀₅) with green reflectance (ρ₅₈₇), (ii) GPP combined wavelengths at the peak of green reflection (ρ₅₁₈, ρ₅₅₆), (iii) the LUE combined red (ρ₆₈₈) with blue reflectance (ρ₄₃₆), and (iv) FAPAR combined blue (ρ₃₉₉) and near infrared (ρ₁₂₉₅) wavelengths. NDSI 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. [ABSTRACT FROM AUTHOR]