3.2 Factors affecting evapotranspiration rates

Evapotranspiration (ET) is the combined loss of water from a surface through evaporation and transpiration. Understanding what controls ET rates is central to hydrology because it determines how much precipitation actually becomes runoff, recharge, or soil moisture storage. The factors break into three main categories: meteorological drivers, vegetation characteristics, and soil/land cover effects.

Meteorological Factors and Vegetation Characteristics

Meteorological factors of evapotranspiration

Temperature directly controls how much water vapor the air can hold. Warmer air has a higher saturation vapor pressure, which increases the evaporative demand. This is why ET rates spike in hot, arid environments like deserts compared to cooler climates at the same humidity level.

Humidity determines the vapor pressure gradient between the evaporating surface and the atmosphere. That gradient is the driving force for ET.

• Low humidity (dry air) creates a steep gradient, enhancing ET. Arid regions see high potential ET for this reason.
• High humidity reduces the gradient, slowing ET. In tropical rainforests, the air is often near saturation, which limits how fast water can leave surfaces and leaves.

Wind speed controls how quickly water vapor is carried away from the surface. Without wind, a thin layer of moist air builds up just above the surface, reducing the vapor pressure gradient.

• Higher wind speeds enhance turbulent mixing, replacing that moist boundary layer with drier air and increasing ET (common in coastal and open plain environments).
• Calm conditions allow the moist boundary layer to persist, suppressing ET even when other conditions favor it.

Solar radiation supplies the energy needed for the phase change from liquid water to vapor. Vaporizing water requires roughly 2.45 MJ/kg (the latent heat of vaporization), so without sufficient radiant energy, ET is energy-limited regardless of how dry or windy conditions are. Equatorial regions receive the most consistent solar input, which is one reason they support high ET rates year-round.

Vegetation effects on transpiration

Leaf Area Index (LAI) is the total one-sided area of leaves per unit of ground surface area ($\text{LAI} = \frac{\text{total leaf area}}{\text{ground area}}$). A dense forest might have an LAI of 6–8, while sparse grassland might be below 1. Higher LAI means more stomatal surface exposed to the atmosphere, so transpiration generally increases with LAI, up to a point where mutual shading and limited soil moisture become constraints.

Stomatal resistance is how much a plant restricts gas exchange through its leaf pores (stomata). This is the plant's main valve for controlling water loss.

• When stomata are open, resistance is low and transpiration proceeds freely.
• When stomata close (high resistance), transpiration drops sharply. Plants close stomata in response to water stress, high vapor pressure deficit, and elevated $CO_2$ levels.
• This is why drought-stressed crops transpire far less than well-watered ones, even under identical weather conditions.

Soil Moisture and Land Cover Effects

Soil moisture and evapotranspiration

ET rates are highest when soil moisture is readily available. As the soil dries, two things happen:

1. Plants close their stomata to conserve water, reducing transpiration. Visible wilting is an extreme sign of this response.
2. Capillary rise of water to the soil surface slows, reducing direct soil evaporation. Severely dry soils may crack, further limiting upward water movement.

Soil water potential quantifies how tightly water is held by the soil matrix. It's expressed as a negative pressure (e.g., $-1.5 \text{ MPa}$ at the permanent wilting point). The more negative the potential, the harder plants must work to extract water. Desert-adapted plants can extract water at much more negative potentials than most crops.

Soil texture controls water retention and movement:

• Sandy soils drain quickly and hold less water. Moisture depletes fast after rainfall, so ET drops off rapidly between rain events.
• Clay soils retain more water but have lower hydraulic conductivity, meaning water moves slowly to roots. Plants in clay soils may have water nearby but struggle to access it quickly enough during peak demand.

Land cover impact on evapotranspiration

Different land covers produce very different ET rates, even under the same climate.

• Forests vs. grasslands: Forests generally have higher ET because of greater LAI, deeper root systems that access more soil water, and larger aerodynamic roughness that enhances turbulent vapor exchange. The Amazon rainforest, for example, recycles a large fraction of its rainfall back to the atmosphere through ET.
• Crop management directly modifies ET in agricultural settings. Irrigation raises soil moisture and increases ET substantially (center-pivot systems can push ET close to potential rates). Mulching and crop residue left on the surface shade the soil, reducing soil evaporation by 20–50% depending on coverage.
• Urbanization generally lowers ET. Impervious surfaces like roads and parking lots prevent infiltration and leave little surface water to evaporate. Reduced vegetation limits transpiration. The urban heat island effect does increase evaporative demand, but the lack of available moisture means actual ET stays low. Urban parks and green roofs are partial exceptions, maintaining localized ET within otherwise impervious landscapes.