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Agricultural drought is a frequent global phenomenon. Solar-induced chlorophyll fluorescence (SIF) is a by-product of photosynthesis that can be used to monitor vegetation growth and agricultural drought. The global 0.05° spatial resolution data set has been obtained using the data-driven algorithm method. However, the broken farmland is not conducive to regional agricultural drought monitoring. Hence, 0.05° SIF products should be downscaled. On this basis, a convolutional neural network (CNN) downscaled work was conducted in this article to obtain 0.008° spatial resolution SIF results. The downscaled SIF and land surface temperature (LST) data were used to establish the temperature fluorescence dryness index (TFDI). The new TFDI was subsequently used for monitoring agricultural drought in Henan province (China) during the corn-growing season (from June to October 2013–2017). Results showed that the downscaled SIF data exhibit a good correlation with gross primary productivity (GPP) from the Moderate Resolution Imaging Spectroradiometer (MODIS) than 0.05° SIF products. During the study period, the soil moisture fluctuation corresponded well with precipitation, and the value of TFDI had an opposite fluctuation with soil moisture. Meanwhile, the annual averaged TFDI had a high correlation with summer corn yield ($R = -0.84$). In conclusion, the SIF results through the CNN-based downscaled method were reliable, and the new TFDI was suitable for region agricultural drought monitoring. [ABSTRACT FROM AUTHOR]

A dual-field-of-view spectroradiometer system has been developed for measuring solar-induced chlorophyll fluorescence (SIF), from an unmanned aircraft system (UAS). This “AirSIF” system measures spectral reflectance in the visible and near-infrared wavelengths as well as SIF in far-red O2-A and red O2-B absorption features at high spatial resolution. It has the potential to support the interpretation and validation of canopy-emitted SIF observed by airborne, and future spaceborne sensors at coarser spatial resolutions, as well as simulated by radiative transfer models. In this contribution, we describe the AirSIF data collection and processing workflows and present a SIF map product of spatially explicit and geometrically correct spectroradiometer footprints. We analyze two possible sources of error in SIF retrieval procedure: a sensor-specific spectral artifact called etaloning and the uncertainty of incoming irradiance during UAS flight due to airframe motion (pitching and rolling). Finally, we present results from two SIF acquisition approaches: a continuous mapping flight and a stop&go flight targeting predefined areas of interest. The results are analyzed for a case study of Alfalfa and grass canopies and validated against ground measurements using the same system. [ABSTRACT FROM AUTHOR]

Numerous observations and modeling results have shown that there is noticeable directional variation in the solar-induced chlorophyll fluorescence (SIF), and this has not been well investigated. In this paper, 16 multiangular spectral observations were carried out on winter wheat to assess the bidirectional SIF emission. First, the bidirectional SIF emission was retrieved from the spectral measurements made by a high-performance QE Pro spectrometer and an automatic multiangle observation system using the 3FLD algorithm. The bidirectional shape of the SIF emission was found to be similar to that of the canopy reflectance in the solar principal plane, with a mean correlation coefficient of 0.94 and 0.97 at the O2-B and O2-A bands, respectively. The modified Rahman–Pinty–Verstraete (MRPV) model, a semiempirical bidirectional reflectance distribution function (BRDF) model, was then employed to describe the bidirectional variation in the SIF and reflectance with a mean root-mean-square-error value of 0.036 and 0.041 mW m−2sr−1nm−1 for the SIF at the O2-B and O2-A bands, respectively. Finally, both the bidirectional reflectance and SIF were BRDF corrected to nadir using the MRPV model. Most of the directional variation was successfully corrected by this method—the mean correction ratios were 87% and 81% for the reflectance at the O2-B and O2-A bands and 84% and 72% for the SIF at the O2-B and O2-A bands, respectively. Therefore, the SIF emission cannot be regarded as isotropic, and the high similarity between the bidirectional SIF and reflectance, together with the BRDF correction results, indicates that the bidirectional SIF emission can be adjusted using either the BRDF reflectance models or prior knowledge. [ABSTRACT FROM AUTHOR]