Reference Evapotranspiration: the foundations of the design parameters

In simple words, evapotranspiration (ET) is the combination of the evaporation rate and the transpiration rate (for further explanation, check out my previous article “Evapotranspiration, what is it?”). But, for the same location and conditions, there could be as many different ET values as different crop types and varieties are. In order to work with these values in your irrigation design, it is necessary to reference the ET to a single crop, becoming suitable to work with it everywhere. Now, let’s find out how to calculate it.

The reference evapotranspiration value, ETo, has been well studied and accepted by the scientific community for long time, in order to standardise the ET value and allow us to reference it and compare it worldwide. A hypothetical grass surface in a specific environmental and agronomic condition was selected as point of reference. In other words, it allows us to easily compare the ET in two different places, consequently, everyone can speak the same language. For you to calculate it, you need to determine the climatic parameters of your crop location.

The reference evapotranspiration (ETo) is the evapotranspiration of a standard grass cover field, and sets a standardized value for ET values worldwide. 

MEASURING THE CLIMATE

To determine the ET, there is a specific device which can directly measure it, the lysimeter. It considers the weight changes or equations that indirectly estimate it according to the climatic parameters available (from the weather station), whose equations have been checked and validated in many different places.

The lysimeter installation, management and acquisition might present some difficulties and result in an expensive solution. This system is just employed for special purposes, such as research institutes, regional or governmental centers, even in some specific agro-climatologic monitoring studies.

On the other hand, weather stations are common devices and well widespread in many places all-over the World. They make it easier and quicker to indirectly determine the ETo values out of the climatic parameter measured and gathered by those climatic devices.

REFERENCE EVAPOTRANSPIRATION IN YOUR REGION

The amount of water that your crops needs per year mainly depends on the local climate throughout the crop growing stages.

Since you probably cannot predict the weather a year in advance, it is necessary to look at the previous year's weather. If you can get access to the climate historical data of the last 10 or 20 years in your region, you could easily make an estimation of the average climate values and foresee the future weather. Note that, however, the total ETo will change if comparing two consecutive years. The point is to adequately select the right value to (1) allow the irrigation installation watering the plants in the most critical period, and (2) optimize initial costs while avoiding oversizing.

One of the most used and widespread methods to calculate the ETo rate is the FAO Penman-Monteith equation. It is based on the climatic parameters (radiation, wind, air temperature and relative humidity) and has proven to be valid and accurate mostly everywhere.

There are several alternative methods that require less climatic data acquisition, but those results are not that precise and are limited to specific locations.

The above image represents the mean daily ETo values in four different months (January, April, July and October) throughout the World, for the period 1979-1998, calculated by the Penman-Monteith formula.

In the image, you can appreciate how the ETo values greatly vary throughout the year as it gets closer to the Poles; while the places along the Equator remain more constant all year round. Moreover, the highest ETo rates tend to be concentrated during summer period around the Tropics, easily recognized on the map where the dark red color areas are (July in the Northern Hemisphere and January in the Southern Hemisphere). You can clearly see that the higher and longer the radiation is, the greater ETo results!

The highest ETo values are normally located around the tropics during the summer period, but greatly decrease at other time of the year; while the ETo remains more constant throughout the year in the Equator area.

Is the above data, shown on the map, valid?

Well, since it is a wide period of ETo data, it could make you think it is a reliable source. Nevertheless, up to date information is always preferred for better final results. Further, how can you get the ETo values for the rest of the year?

It is true thought that nowadays, climate change is making even more unpredictable weather patterns, where temperatures and precipitation are changing in terms of intensity and frequency, as the following graph shows.

Prior to starting with the drip irrigation design, it becomes essential to accurately determine the ETo values for your project location. This way, the system will be correctly dimensioned, and will be able to meet the water demand during the most critical periods of the year, considering that greater climatic disturbances will probably happen in the coming time.

In the following post we’ll show you how the ETo can be easily determined with the minimum effort!

Stay tuned!

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