Cropland Carbon Uptake Delayed and Reduced by 2019 Midwest Floods

While large-scale floods directly impact human lives and infrastructures, they also profoundly impact agricultural productivity. New satellite observations of vegetation activity and atmospheric CO2 offer the opportunity to quantify the effects of such extreme events on cropland carbon sequestration. Widespread flooding during spring and early summer 2019 delayed crop planting across the U.S. Midwest. As a result, satellite observations of solar-induced chlorophyll fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI) and the Orbiting Carbon Observatory (OCO-2) reveal a shift of 16~days in the seasonal cycle of photosynthetic activity relative to 2018, along with a 15% lower peak photosynthesis rate. We estimate the 2019 anomaly to have led to a reduction of -0.21 PgC in gross primary production (GPP) in June and July, partially compensated in August and September (+0.14 PgC). The growing season integral results in a 4% reduction in cropland GPP of the midwest, but a 3% increase for areas with cropland ratio less than 10%. Using an atmospheric transport model, we show that a decline of ~0.1 PgC in the net carbon uptake during June and July is consistent with observed CO2 enhancements of up to 10 ppm in the midday boundary layer from Atmospheric Carbon and Transport - America (ACT-America) aircraft and ~1 ppm in column-averaged dry-air mole fractions from OCO-2. This study quantifies the impact of floods on cropland productivity and demonstrates the potential of combining SIF with atmospheric CO2 observations to monitor regional carbon flux anomalies.