Monitoring Crop Photosynthesis through Satellite-based Measurements of Sun-Induced Fluorescence (CropSIF)

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Along with efforts to increase productivity of agricultural systems, there is a critical need for the development of new methods to monitor agricultural resources globally in a continuous and objective manner. In particular, crop photosynthesis (quantified as gross primary production, GPP) is an important variable for observation, as it places an upper limit for the supply of food and fuels by our agricultural systems and is a good indicator of crop yield and stress. Optical remote sensing techniques based on reflectance data have been used in the last decades to monitor agricultural resources. Spectral reflectance data enable the retrieval of canopy biochemical and structural parameters, which indicate potential photosynthesis and are the basis for the monitoring of vegetation condition and phenological stage at high spatial resolution. Based on this principle, the Sentinel-2 satellites (started in mid 2015) are expected to become the workhorse for operational crop monitoring in the next decades. However, it is recognized that remotely-sensed vegetation parameters based on spectral reflectance cannot capture the complex dynamics of a highly variable physiological measure such as photosynthesis. Complementing reflectance-based measurements, global space-based estimates of sun-induced chlorophyll fluorescence (SIF) became available recently. SIF has been shown to have a higher sensitivity to crop photosynthesis than any other existing remote sensing parameter or model. The launch of the Tropospheric Monitoring Instrument (TROPOMI) onboard the EU Copernicus Sentinel 5-Precursor by mid 2017 offers the possibility of monitoring SIF with a a much higher spatio-temporal resolution than any other mission,, which makes TROPOMI to have a game-changing potential for global monitoring of plant photosynthetic functioning in general, and crop productivity in particular. In combination, TROPOMI and Sentinel-2 will allow a global, observation-based picture of the photosynthetic functioning of crops, grasslands and rangelands with an unprecedented spatio-temporal resolution and accuracy. The CropSIF project will contribute to the assessment of agricultural productivity and climate impacts by taking advantage of this unique near-future observational scenario for crop monitoring. We will produce time-resolved maps of crop GPP, which will be subsequently exploited to study the effect of extreme climate events on crop productivity in several agricultural regions around the globe.