Evapotranspiration
Evapotranspiration is the combined biophysical and thermodynamic flux of water vapor from Earth's land surface to the atmosphere via direct abiotic evaporation and plant stomatal transpiration.
Evapotranspiration (ET) is the combined thermodynamic and ecophysiological flux of water transferred from the terrestrial surface into the atmosphere as water vapor, integrating abiotic evaporation from soils, open water bodies, and wet vegetation canopies with biotic transpiration through plant stomata. Serving as the central nexus linking the global water cycle, the surface energy balance, and the terrestrial carbon cycle, terrestrial evapotranspiration returns approximately 65,000 to 70,000 km3 of water per year—roughly 60% to 65% of all continental precipitation—back to the troposphere. Because converting liquid water into vapor absorbs the latent heat of vaporization (λ ≈ 2.45 MJ/kg at 20 °C), global evapotranspiration consumes roughly 38–40 W/m2 (over half of the net solar radiative energy absorbed by continents), functioning as Earth's primary land-surface air conditioner and driving regional precipitation recycling across biomes such as the Amazon rainforest.
Partitioning the Flux: How Do Evaporation and Transpiration Differ?
Total terrestrial evapotranspiration (ET) is the sum of three physically distinct moisture pathways:
- Plant Transpiration (T): The cohesive transport of soil water along the soil–plant–atmosphere continuum (SPAC) from root xylem vessels to leaf mesophyll cell walls, followed by vapor diffusion through microscopic epidermal pores (stomata) into the boundary layer. Isotopic water-vapor partitioning (using δ18O and δ2H signatures) and satellite synthesis studies demonstrate that transpiration accounts for 60% to 80% of total terrestrial ET (~61 ± 15% globally).
- Canopy Interception Evaporation (Ei): The direct evaporation of rainwater or snow intercepted on leaf and bark surfaces before reaching the ground, representing ~10% to 20% of terrestrial ET.
- Soil and Open-Water Evaporation (Es): The direct phase change of capillary water within the upper 5–15 cm of soil pores or surface water bodies, accounting for ~15% to 25% of terrestrial ET.
Potential (PET) vs. Actual Evapotranspiration (AET)
In hydrometeorology (introduced by Charles Warren Thornthwaite in 1948), a fundamental distinction is drawn between Potential Evapotranspiration (PET) and Actual Evapotranspiration (AET):
- Potential Evapotranspiration (PET) (or Reference Evapotranspiration, ET0) represents the atmospheric evaporative demand: the maximum rate at which water would be removed from a well-watered, uniform reference surface (defined by the FAO as a hypothetical green grass crop of 0.12 m height, surface resistance of 70 s/m, and albedo of 0.23) when soil moisture is non-limiting.
- Actual Evapotranspiration (AET) is the real water flux achieved under ambient environmental constraints. Within the Budyko hydrological framework, ecosystems are categorized along an aridity index (PET / Precipitation): in energy-limited regimes (such as humid equatorial rainforests or high-latitude maritime forests), soil water is abundant and AET approaches PET, governed primarily by net solar radiation; in water-limited regimes (such as semi-arid steppes and deserts), AET is strictly capped by precipitation and soil moisture availability.
The Penman-Monteith Equation: What Controls Surface Latent Heat Flux?
In 1948, Howard Penman combined the surface radiative energy balance with aerodynamic turbulent mass transfer into a single physical equation, which John Monteith extended in 1965 by incorporating physiological canopy and aerodynamic resistances. The Penman-Monteith equation computes the latent heat flux (_λ_ET, in W/m2) as:
where:
- λ is the latent heat of vaporization (~2.45 × 106 J/kg),
- Δ is the slope of the saturation vapor pressure curve with respect to temperature (d_e_s/d_T_, in kPa/°C),
- R_n is net radiation at the canopy surface and _G is soil heat conduction flux (W/m2),
- ρa and cp are the density and specific heat capacity of air at constant pressure,
- VPD = (es − ea) is the vapor pressure deficit of the air (kPa),
- γ is the psychrometric constant (~0.066 kPa/°C at sea level), and
- ra and rs are the bulk aerodynamic resistance and bulk canopy (stomatal) resistance (s/m).
Surface Energy Partitioning and the Bowen Ratio
At any land surface, net absorbed radiative energy (Rn − G) must be partitioned between sensible heat flux (H)—which directly warms the overlying air via convection—and latent heat flux (_λ_ET)—which cools the surface via water vaporization. The ratio of sensible to latent heat is the Bowen ratio (β = H / _λ_ET). Over well-watered forests and irrigated croplands, β ≈ 0.1–0.3, meaning 75% to 90% of solar energy is channeled into evaporative cooling rather than air heating. Conversely, during soil moisture droughts or after deforestation, stomatal closure drives rs → ∞, collapsing _λ_ET and surging the Bowen ratio above β > 2.0–5.0, which directly amplifies regional heatwaves via land–atmosphere climate feedback loops.
| Terrestrial Biome / Land Cover | Mean Annual ET (mm/yr) | Transpiration Fraction (_T_/ET) | Typical Bowen Ratio (β = _H_/_λ_ET) | Dominant Biophysical Limiting Factor |
|---|---|---|---|---|
| Tropical Evergreen Rainforest | 1,200 – 1,500 mm/yr | 70% – 85% | 0.15 – 0.30 | Energy-limited (cloud cover & net radiation Rn) |
| Temperate Deciduous & Mixed Forest | 550 – 850 mm/yr | 65% – 75% | 0.30 – 0.60 | Seasonally energy-limited; summer VPD & soil water |
| Irrigated Croplands (C3/C4) | 700 – 1,100 mm/yr | 60% – 80% | 0.15 – 0.35 | Atmospheric demand (VPD and wind speed) |
| Boreal Forest (Taiga) | 250 – 450 mm/yr | 55% – 65% | 0.60 – 1.20 | Low temperature, high stomatal resistance, short season |
| Semi-Arid Shrubland & Savanna | 200 – 400 mm/yr | 40% – 55% | 1.50 – 4.00 | Strictly water-limited (soil matric potential Ψsoil) |
| Subtropical Hot Desert | < 50 – 100 mm/yr | 10% – 25% | 5.00 – 10.0+ | Extreme water limitation; bare soil evaporation dominates |
Vapor Pressure Deficit (VPD) and Stomatal Optimality: How Do Plants Respond to Warming and CO<sub>2</sub>?
Clausius-Clapeyron Scaling and Atmospheric Thirst
The thermodynamic driving force for vapor diffusion out of leaf stomata is the Vapor Pressure Deficit (VPD)—the difference between the saturation vapor pressure inside the leaf's intercellular air spaces (es(T), where relative humidity is ~99%) and the actual vapor pressure of the ambient atmosphere (ea). According to the Clausius-Clapeyron relation, saturation vapor pressure increases exponentially with temperature at approximately 7% per 1 °C of warming:
If ambient relative humidity remains constant or declines over continents during global warming, VPD rises exponentially, dramatically increasing atmospheric evaporative pull on plant vascular systems.
Stomatal Conductance vs. CO2 Fertilization
Vascular plants face a continuous evolutionary trade-off: opening stomata allows atmospheric carbon dioxide (CO2) to diffuse inward for photosynthesis (catalyzed by RuBisCO), but simultaneously loses ~200 to 500 molecules of H2O per molecule of CO2 fixed. Under elevated atmospheric CO2 (>420 ppm), many C3 and C4 plants exhibit partial stomatal closure (reducing stomatal conductance gs) to increase their intrinsic water-use efficiency (WUE = A / T). However, across global forests, this physiological water-saving effect is countered by two opposing trends: (1) CO2 fertilization stimulates higher Leaf Area Index (LAI, "global greening"), increasing total transpiring surface area, and (2) rising VPD under warming forces higher transpiration per unit stomatal aperture until hydraulic cavitation risk forces complete stomatal shutdown.
Continental Moisture Recycling: How Do "Flying Rivers" Sustain Downwind Biomes?
Evapotranspiration does not merely remove water from land; it actively generates continental precipitation down-wind via moisture recycling. In the Amazon basin, deep-rooted rainforest trees extract groundwater from depths of 10–18 meters during the dry season, transpiring ~20 billion metric tons of water vapor daily into the lower troposphere. Trade winds emerging from the Atlantic atmospheric circulation carry this transpired moisture westward across the basin, where a single water molecule is evaporated and re-precipitated 5 to 6 times before encountering the Andes Mountains and deflecting southward as low-level jets ("flying rivers") that water the agricultural heartlands of southern Brazil, Paraguay, Uruguay, and northern Argentina. Disrupting this biotic pump through deforestation risks crossing a regional hydrological tipping point.
Frequently Asked Scientific Questions
What is the difference between potential evapotranspiration (PET) and actual evapotranspiration (AET)?
Potential evapotranspiration (PET) measures the atmosphere's maximum evaporative demand—how much water would evaporate and transpire from a standard, fully watered vegetative surface given current solar radiation, temperature, humidity, and wind. Actual evapotranspiration (AET) is the amount of water that actually enters the atmosphere, which is often far lower than PET when soils dry out and plants close their stomata.
Why does high Vapor Pressure Deficit (VPD) stress plants even when soil moisture is available?
Under the Clausius-Clapeyron relation, warming air holds ~7% more water vapor per °C, causing Vapor Pressure Deficit (VPD) to spike. When atmospheric evaporative demand exceeds the maximum hydraulic conductivity of plant roots and xylem vessels, water tension inside the xylem drops to critical negative pressures, triggering air-bubble embolisms (cavitation) and forcing plants to close their stomata and halt photosynthesis.
How do scientists measure evapotranspiration at ecosystem and global scales?
At the ecosystem scale (1 km2 footprint), micrometeorological eddy covariance flux towers measure high-frequency (10–20 Hz) vertical wind eddies and water vapor concentrations to compute direct latent heat flux (_λ_ET). At continental and global scales, satellites such as NASA's MODIS, Landsat, and ECOSTRESS solve the surface energy balance by measuring land surface temperature (thermal infrared cooling) and vegetation indices.