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Evapotranspiration is one of the largest fluxes in the terrestrial water cycle, and also impacts the surface energy budget and the carbon cycle. In this dissertation, I explore how the surface energy budget contributes to ET uncertainty, I apply new satellite soil moisture observations to improve global ET estimates, and I link the carbon and water cycles from space to characterize how vegetation responds to stressful conditions.First, the differences of ground heat flux models are evaluated against 88 locations with in situ observations. I discuss the mechanisms which control ground heat flux and quantify how errors in this measurement have the potential to impact evapotranspiration. A new optimized model is presented to reduce this potential uncertainty.Second, I apply integrated observations of soil moisture and evapotranspiration to improve a satellite-based evapotranspiration algorithm. I demonstrate how observations from soil moisture improve evapotranspiration estimates in water-limited regions and use this new model with soil moisture observations from the Soil Moisture Active Passive (SMAP) mission to compute evapotranspiration globally. I compare the new model with the original model and quantify how evapotranspiration is partitioned. This is the first global satellite-derived evapotranspiration dataset to incorporate water availability limitations from SMAP.Finally, I link independent measurements of the carbon and water cycle from space. I use satellite derived transpiration and new observations of solar induced fluorescence from the Orbiting Carbon Observatory -2 (OCO-2) to characterize how vegetation responds to hotter and drier climate perturbations. Overall, this dissertation advances remote sensing evapotranspiration algorithms through quantifying the uncertainty contribution from ground heat flux models and provides a new relationship to link soil moisture observations to evapotranspiration. Additionally, I present the first study to apply transpiration and solar induced fluorescence from OCO-2 to explore how vegetation responds to hotter and drier conditions by regulating the carbon and water cycles. This dissertation delivers new ideas of how to leverage earth observing satellites to advance ET science and address knowledge gaps in the earth system.