What is evapotranspiration (ET) and why is it the most important factor in irrigation?

In modern agriculture, precise water management is impossible without understanding the term evapotranspiration (ET). According to current guidelines from the Food and Agriculture Organization of the UN (FAO), it is the key parameter for determining crop water requirements.

What is evapotranspiration?

Evapotranspiration is a combined process of water transfer from the Earth's surface to the atmosphere, consisting of two parts:

1. Evaporation: The conversion of liquid water into water vapor and its removal from surfaces such as soil, wet vegetation, lakes, and rivers.
2. Transpiration: The process of water evaporation from plant tissues, primarily through small openings on the leaves called stomata.

At the beginning of the growing season, almost 100% of ET is due to soil evaporation. When the crop fully covers the ground, over 90% of the process is accounted for by transpiration.

What does evapotranspiration depend on?

According to FAO 56 (revised 2025), the intensity of evapotranspiration is determined by four main groups of factors:

1. Meteorological Parameters (Weather)

These are the primary drivers of the process. The energy for evaporation comes from solar radiation and air temperature. The rate of evaporation also depends on air humidity (the drier the air, the greater the vapor pressure deficit) and wind speed, which carries away moisture-saturated air from the surface.

2. Crop Factors

Different plants have varying water consumption even under identical meteorological conditions. Important factors here include:

• Crop type and variety: Leaf anatomy and number of stomata.
• Developmental stage: Consumption changes from sowing to maturity.
• Plant height and aerodynamic resistance: Taller and "rougher" crops (like maize) often have higher ET than shorter ones.
• Albedo (reflectivity): How much solar energy is absorbed by the leaf surface.

3. Soil Conditions and Management

Soil moisture availability is critical. When it is limited, the plant closes its stomata to protect itself, which drastically reduces evapotranspiration (water stress). Other factors include soil salinity, the presence of soil crusts, soil fertility, and fertilizer use.

4. Management Practices

The use of mulching (organic or plastic) can reduce soil evaporation by 30-80%. Additionally, windbreaks reduce wind speed and, consequently, ET in the protected area.

Why is this important for irrigation?

Calculating evapotranspiration allows farmers to apply balanced irrigation – adding only the amount of water that has actually been lost.

If you irrigate above ET levels, you risk:

• Deep Percolation: Water passes below the root zone and leaches expensive nitrogen fertilizers.
• Salinization: Accumulation of salts due to improper management.
• Economic Losses: Increased electricity costs for pumps and unnecessary labor.

Source: FAO Irrigation and Drainage Paper No. 56, Revised Edition 2025.

‍