The Invisible Water Loss
Evapotranspiration silently returns most terrestrial rainfall to the atmosphere. Plants draw soil water through roots and release it as vapor through stomatal pores, while bare soil and water surfaces lose moisture directly to the air. In many regions, ET exceeds 60% of annual precipitation — making it the dominant term in the water budget and the key to understanding drought, irrigation needs, and ecosystem health.
The Penman-Monteith Framework
John Monteith combined Penman's energy balance approach with a canopy resistance term to create the most physically complete ET equation. The FAO-56 version standardizes this for a reference grass crop, producing ET₀ values from four readily available weather variables. The radiation term drives ET in humid conditions; the aerodynamic (wind and VPD) term dominates in arid climates.
Temperature, Humidity, and VPD
The saturation vapor pressure increases exponentially with temperature — following the Clausius-Clapeyron relation. A 10°C warming roughly doubles the atmosphere's moisture capacity. When actual humidity does not keep pace, the vapor pressure deficit widens, pulling more water from surfaces. This simulation visualizes how each climate variable contributes to the total ET rate and how changing conditions shift the energy-aerodynamic balance.
Global Water Stress
As temperatures rise under climate change, potential ET increases even where precipitation remains stable, tightening water budgets. Agriculture accounts for 70% of global freshwater withdrawals, most of it replacing ET losses. Accurate ET estimation — whether by Penman-Monteith, remote sensing, or eddy covariance — is essential for irrigation efficiency, drought monitoring, and sustaining river flows for ecosystems and communities downstream.