Evapotranspiration

What is evapotranspiration?

Evapotranspiration is the combined process by which water is lost from the soil through two primary mechanisms: evaporation directly from the soil surface and transpiration from the leaves of plants growing within that soil.¹

This process is responsible for the majority of water loss from soil during crop growth. Several environmental factors influence the rate at which this occurs, including:

• Solar radiation
• Atmospheric vapor pressure
• Temperature
• Wind
• Soil moisture levels¹

Because it significantly impacts water availability, calculating evapotranspiration rates is a critical component in the effective planning of irrigation schemes.¹

How do plants release water through transpiration?

Transpiration is the process by which plants lose water, primarily through small pores on their leaves called stomata. These openings are essential for photosynthesis, as they allow the plant to take in carbon dioxide and release oxygen; however, they also inevitably permit water vapor to escape into the atmosphere.²³

The process is regulated by guard cells that surround each stoma, opening or closing the pore in response to environmental stimuli like light, temperature, and water availability. When water evaporates from the leaf interior, it creates a “transpiration pull”—a negative pressure that draws water and dissolved nutrients upward from the roots through the xylem. This upward movement is supported by the cohesion hypothesis, which describes how the physical attraction between water molecules and the walls of the plant’s vascular tissue allows water to travel to the tops of even the tallest trees.²⁴⁵

What role does solar radiation play in the water cycle?

Solar radiation acts as the primary engine for the hydrologic cycle, providing the energy necessary to drive the continuous movement of water through the Earth’s atmosphere, oceans, and terrestrial environments.⁶¹

The role of solar energy in this process includes:

• Evaporation: Solar energy heats water in oceans, lakes, and soil, causing it to transition from a liquid to a gaseous state (water vapor). This process is the primary mechanism by which water enters the atmosphere.⁶¹
• Latent Heat Conversion: At the Earth’s surface, solar radiation is converted into latent heat. This energy is consumed during phase changes—such as evaporation and transpiration—rather than increasing the temperature of the air, which helps regulate global surface temperatures.⁷
• Atmospheric Circulation: Solar energy influences the patterns of moisture circulation in the air and currents in the ocean, which transport water vapor across the globe to eventually fall as precipitation water vapor across the globe to eventually fall as precipitation.⁷

Without this constant input of solar energy, the cycling of water through the biosphere would not occur in its current form.¹

How does irrigation management rely on evapotranspiration data?

Irrigation management relies heavily on evapotranspiration data because this process accounts for the majority of water lost from the soil during crop growth. By estimating these rates, agricultural planners can determine the precise amount of water required to sustain crops, ensuring efficient irrigation schemes.¹¹⁰

Management strategies often utilize the concept of “potential evapotranspiration”—the maximum rate of water loss possible under prevailing meteorological conditions without supply limitations. By comparing this to actual rates, which fluctuate based on soil moisture levels and plant physiological controls like stomata, managers can adjust water delivery to match the specific needs of the vegetation.¹⁰¹

While standard measurements like evaporation pans provide a baseline, more precise management often requires data from lysimeters, which allow for a detailed evaluation of the water balance. Understanding these variables—such as solar radiation, temperature, and wind—is essential for optimizing water use and preventing both under-irrigation and resource waste.¹⁰¹