Understanding Evapotranspiration: What It Is and How We Measure It

This is an informative guide to Evapotranspiration (ET), covering its definition, measurement methods, and practical applications in irrigation scheduling. We'll explore the fundamental concepts, making them accessible and useful for both researchers and commercial growers.

What is Evapotranspiration?

Evapotranspiration (ET) is a rate (e.g. inch per day) at which soil and plants release water into the atmosphere. ET is the sum of water loss through the processes of evaporation and transpiration. Evaporation is the water loss from either soil or plant surfaces. Transpiration occurs when water vapor moves out of leaf stomata (tiny pores on plant leaf surface).

You might be asking, 'Why bother measuring ET?' The answer lies in its power to inform irrigation decisions and provide insights into crop productivity. Let's explore how.

Irrigation scheduling

Irrigation management is about determining when and how much water to apply for irrigation. If we know how much water is used by plants, we can calculate how much water to put back in the soil to avoid plant water stress (how much to irrigate). When our calculations show that soil water has dropped below a minimum level it is time to irrigate.

The purpose of irrigation is to keep soil moisture between limits. The soil should not be too wet or too dry.

Estimate yield

For irrigated crops with full ground cover, most of ET is from transpiration. Transpiration losses are directly linked to crop yield (growth and productivity). This is because the pathway for transpiration and photosynthesis is the same. Both processes occur at the same rate through stomates (tiny pores on plant leaves). If there is no enough water in the soil, stomates close causing a decrease in transpiration and photosynthesis.

How do we measure it?

Estimation of ET

ET is driven by weather factors (solar radiation, temperature, humidity, wind) that determine the drying demand of the atmosphere. ET is usually estimated using complex mathematical equations and local weather data. These values are estimation of what is called reference ET (ET of a well-watered reference crop like alfalfa or grass) and need to be adjusted using a crop coefficient (Kc). Kc depends on the type of crop, planting date, and crop development stage. For example, larger canopies consume more water than smaller canopies and this is reflected in Kc. The adjusted reference ET (reference ET multiplied by Kc) is called crop ET (ETc).

ET is driven by weather factors (solar radiation, temperature, humidity, wind) and can be estimated using mathematical models.

Measurement of ET using Atmometer (ET gauge)

An atmometer mimics plants and like a mini-weather station can provide reference ET at a reasonable cost and with little effort. The value of ET is shown on a tube in front of a ruler on the atmometer. Growers or consultants can use an ET gauge to monitor how crop water based on their changing local weather.

Measurement of ET Using a Multi-Sensor Unit

A computer vision system like the BINA Pro is a good example of a system that uses a variety of sensors to calculate crop coefficient, reference ET and crop water use. Growers and researchers can use the unit to conduct daily measurements of crop ET and decide on how much water to apply to their plants.

ET and soil water balance

Consider the soil in your field as a bucket. It has a limited capacity for holding water (called soil water-holding capacity). If we put too much water in it, it’s not going to hold it and it will spill out (lost to deep percolation). Soil water balance can tell how much water to put back in the bucket (irrigate). Daily ET amounts (i.e. crop water use) are withdrawals from this bucket (soil storage) and rainfall or irrigation are a deposit to this storage. Account for these inputs and outputs and you know how much water is in the soil. Irrigate when soil water content drops below a minimum level.

An illustration of soil water balance

Challenges of Using ET in Research

Using Evapotranspiration (ET) in research isn't without its challenges. Here's a look at common obstacles and practical solutions to ensure accurate and reliable data.

Microclimate variability

Weather is highly variable and measurements taken a few miles from the field may not be representative of that field’s microclimate. Microclimate is simply local weather. Reference ET, therefore, needs to be estimated using local weather data.

Estimation errors

Mathematical models used to estimate ET have small estimation errors. The soil water balance equation may also have estimation errors because we cannot necessarily account for all the inputs and outputs. ET-based irrigation scheduling using the water balance approach should be verified periodically by checking soil moisture in the field with a soil moisture sensor.

Conclusion

Evapotranspiration is a crucial factor in understanding plant water use and optimizing irrigation. By understanding how ET is measured and applied, whether through simple atmometers or advanced multi-sensor systems, growers and researchers can make informed decisions about water management. Remember that accurate ET estimation relies on local weather data and periodic soil moisture checks. Ultimately, mastering ET helps ensure healthy crops and efficient water use.