Emerging satellite observations for diurnal cycling of ecosystem processes

Diurnal cycling of plant carbon uptake and water use, and their responses to water and heat stresses, provide direct insight for assessing ecosystem productivity, agricultural production and management practices, carbon and water cycles, and feedbacks to the climate. Temperature, light, atmospheric water demand, soil moisture, and leaf water potential vary over the course of the day, leading to diurnal variations in stomatal conductance and ecosystem processes. Earth observations from polar-orbiting satellites are incapable of studying these diurnal variations. We review the emerging satellite observations that have the potential for studying how plant functioning and ecosystem processes vary over the course of the diurnal cycle. The recently launched ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) and Orbiting Carbon Observatory-3 (OCO-3) provide land surface temperature (LST), evapotranspiration (ET), and solar-induced fluorescence (SIF) data at different times of day. New generation operational geostationary satellites such as Himawari-8 and the GOES-R series can provide continuous, high-frequency data of LST, solar radiation, and gross primary production (GPP). Future satellite missions such as GeoCarb, TEMPO, and Sentinel-4 are also planned to have diurnal SIF sampling capability. We explore the unprecedented opportunities for characterizing and understanding how GPP, ET, and water use efficiency (WUE) vary over the course of the day in response to temperature and water stresses and management practices. We also envision that these emerging observations will revolutionize studies of ecosystem processes and that these observations and findings can inform agricultural and forest management and lead to improvement of Earth system models and climate projections.