Factors affecting evapotranspiration

Evapotranspiration (ET) is the combined loss of water by evaporation from soil and water surfaces and transpiration from plants. Its rate depends on four groups of factors.

1. Meteorological (climatic) factors

• Solar radiation: supplies the latent heat of vaporisation; the most important factor.
• Temperature: higher temperature raises the saturation vapour pressure and ET.
• Humidity: low humidity increases the vapour pressure deficit $(e_s - e_a)$ and therefore ET.
• Wind speed: removes the moist air layer near the surface, so ET increases with wind (up to a limit).
• Atmospheric pressure and sunshine hours (day length) also have an effect.

2. Plant factors

• Type of crop or vegetation, its leaf area, root depth and stomatal behaviour.
• Stage of growth and crop cover density (ET is small at sowing and maximum at full cover).
• Colour and roughness of the canopy (albedo).

3. Soil and water factors

• Soil moisture availability: when moisture falls below the wilting point, actual ET drops far below potential ET.
• Soil type, texture and water-holding capacity; depth of the water table.
• Quality of water (salinity lowers ET).

4. Management factors

• Method and frequency of irrigation, tillage, mulching and planting density.
• Presence of wind breaks and shading.

In design, ET is expressed as potential ET (PET) for a well-watered, fully covering crop; actual ET (AET) is then PET reduced for the soil-moisture and crop-stage effects.