Evapotranspiration

Evapotranspiration is the combined flux of water vapour from the land surface to the atmosphere through two pathways: evaporation from soil and water bodies, and transpiration through plant stomata. Globally, evapotranspiration returns approximately 60% of all land precipitation to the atmosphere, making it the dominant term in the terrestrial water budget. But it is more than a hydrological accounting entry. Evapotranspiration is a thermodynamic process that couples the energy, water, and carbon cycles: the same solar energy that drives photosynthesis also drives evaporation, and the same stomatal pores that admit CO₂ also release water vapour. This coupling means that any perturbation to one cycle — whether through deforestation, irrigation, or climate change — perturbs all three. The ratio of transpiration to total evapotranspiration (the T/ET ratio) is a sensitive indicator of ecosystem health and land-use change, yet it remains poorly constrained in most climate models.