Evaporation and transpiration are the two main processes that move water from Earth's surface into the atmosphere. Together, they account for a large share of the water cycle's return flow and directly influence local climate, soil moisture, and water resource availability. This section covers the physics behind both processes, the energy balance that drives them, and the factors that control their rates.

Physical processes of evapotranspiration

Evaporation is a phase change: liquid water molecules at a surface gain enough kinetic energy to escape into the atmosphere as vapor. The rate of evaporation depends on the vapor pressure gradient between the water surface and the overlying air. A large gradient (moist surface, dry air) drives fast evaporation; a small gradient (humid air close to saturation) slows it down. Common evaporating surfaces include lakes, oceans, wet soil, and water intercepted by vegetation after rainfall.

Transpiration is the loss of water vapor from plants through stomata, tiny pores mostly found on leaf undersides. Plants pull water from the soil through their roots, move it up through the xylem, and release it as vapor through open stomata. Stomatal opening is actively regulated by the plant in response to light, temperature, humidity, and soil moisture. Different species transpire at very different rates: a corn field in midsummer transpires far more per unit area than a patch of cacti in the desert.

Both processes share the same fundamental requirement: energy must be supplied to break the hydrogen bonds holding liquid water molecules together (the latent heat of vaporization). Both are also governed by the same atmospheric conditions: temperature, humidity, wind speed, and available radiation.

Physical processes of evapotranspiration, 12. Surface Energy Balance and Evapotranspiration – Rain or Shine

Energy balance in evapotranspiration

The energy that drives evapotranspiration has to come from somewhere, and tracking where it comes from is what the surface energy balance does. The governing equation is:

$R_n = LE + H + G$

Each term is typically expressed in W/m²:

Net radiation ( $R_n$ ) is the total energy available at the surface after accounting for incoming shortwave and longwave radiation minus what's reflected and emitted back. A positive $R_n$ means the surface is gaining energy, which can then be partitioned among the other three fluxes.
Latent heat flux ( $LE$ ) is the energy consumed by evapotranspiration. This is the term most directly tied to water loss. Higher $LE$ means more water is evaporating or transpiring.
Sensible heat flux ( $H$ ) is the energy transferred between the surface and the atmosphere through direct heating of the air (driven by temperature differences). When $H$ is large relative to $LE$ , the surface heats the air rather than evaporating water. Unstable atmospheric conditions (warm surface, cooler air above) enhance turbulent mixing and can boost both $H$ and $LE$ .
Ground heat flux ( $G$ ) is the energy conducted into or out of the soil. During the day, $G$ is typically positive (heat flows downward into the ground); at night it reverses as the soil releases stored heat. Over a full day, $G$ is usually small compared to the other terms, but it matters for short-term calculations.

The key takeaway: $R_n$ sets the energy budget. How that budget is split between $LE$ , $H$ , and $G$ determines how much evapotranspiration actually occurs.

Physical processes of evapotranspiration, Evapotranspiration - Wikipedia

Evaporation vs. transpiration mechanisms

Though both move water into the atmosphere, evaporation and transpiration are controlled by different factors:

Evaporation is driven primarily by atmospheric demand and surface water availability:

The vapor pressure deficit (VPD) of the air is the main driver. VPD is the difference between the saturation vapor pressure at the surface temperature and the actual vapor pressure of the air.
Surface properties matter too. A dark lake (low albedo) absorbs more radiation than a light-colored sandy soil, providing more energy for evaporation. Rough surfaces generate more turbulence, which mixes drier air down to the surface and speeds evaporation.
Evaporation can occur from open water bodies, bare soil, and water sitting on leaf surfaces after rain (interception).

Transpiration adds a biological control layer on top of the physical drivers:

Stomatal conductance is the ease with which water vapor passes through stomata. Plants open stomata to take in $CO_2$ for photosynthesis but lose water in the process. When soil moisture drops or atmospheric demand is extreme, many plants partially close their stomata to conserve water, reducing transpiration even if atmospheric conditions would otherwise support high rates.
Root depth and distribution determine how much soil water a plant can access.
Plant characteristics like leaf area index, species type, and growth stage all affect transpiration rates. A mature oak with a large canopy transpires much more than a seedling.

Key controlling factors for both processes:

Meteorological variables: temperature, humidity, wind speed, solar radiation
Surface characteristics: water availability, vegetation cover, soil type, albedo
Biological factors (transpiration only): plant species, stomatal behavior, root distribution, growth stage

Energy and Heat in Evapotranspiration

Role of latent heat

The latent heat of vaporization ( $\lambda$ ) is the energy needed to convert one kilogram of liquid water into vapor without changing its temperature. At 20°C, $\lambda \approx 2.45$ MJ/kg. This value decreases slightly as temperature rises (molecules already have more kinetic energy, so less additional energy is needed to escape the liquid phase). The same amount of energy is released when water vapor condenses back to liquid, which is why condensation warms the surrounding air.

Latent heat flux ( $LE$ ) links the energy world to the water world:

$LE = \lambda \times ET$

where $ET$ is the evapotranspiration rate (in kg/m²/s, which is numerically equivalent to mm/s since 1 kg of water spread over 1 m² is 1 mm deep). This equation lets you convert between an energy measurement (W/m²) and a water depth measurement (mm/day), which is how most hydrologists report ET.

Evaporative cooling is a direct consequence of latent heat consumption. When water evaporates from a surface, it pulls energy from that surface, lowering its temperature. This is why:

Sweating cools your skin
Areas near large lakes or oceans have more moderate temperatures than inland areas at the same latitude
Irrigated fields are cooler than adjacent dry fields on a hot day

In humid regions with ample water supply, a large fraction of $R_n$ goes to $LE$ , keeping surface temperatures relatively moderate. In arid regions, little water is available to evaporate, so most of $R_n$ goes to $H$ , heating the air and producing the high surface temperatures typical of deserts. This partitioning between $LE$ and $H$ is one of the most important concepts in surface hydrology and land-atmosphere interactions.

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