When we take on a new irrigation project, the first step is to determine the base for the calculation so that the results are reliable and accurate. The parameter of evapotranspiration (ET) is perhaps the most important of all. Underestimating this value can lead to water stress in the crop and a consequent loss of yield. Conversely, overestimating ET can result in high material and installation costs.
It is therefore necessary to correctly determine the evapotranspiration value, since all other calculations in the design phase are based on it. Now, What is evapotranspiration?
The FAO defines ET as “the combination of two separate processes by which water is lost from the soil surface through evaporation and, separately, through crop transpiration”.
These are two separate processes, but they are closely linked. Let’s look at each one individually:
EVAPORATION
You’ve probably noticed the steam that forms when you heat water in a saucepan to cook. That steam is the result of water evaporation, but what exactly is happening there?
Let’s start from the beginning. Water can exist in three different states: solid, liquid, and gas. These correspond to ice, water, and steam. The energy level determines the state in which water particles exist. Thus, the solid state has the lowest energy level, while steam has the highest.
So, when we pour water into a pot to cook rice and heat it, the energy level of the water gradually increases until it reaches the threshold at which it changes states, going from liquid to gas: water vapor. This is what is known as the process of evaporation. The steam escapes from the pot and disperses into the air.
Water can evaporate from any surface, such as seas, oceans, and rivers—and even the roof of a house. Returning to the previous example of the kitchen, if we replace the pot with farmland and the heat source with solar radiation, we see evaporation occurring from the ground into the atmosphere. This process occurs naturally everywhere.
Depending on the energy of solar radiation (along with other climatic factors), water in the form of vapor rises from the ground’s surface at different rates. This indicates that the evaporation process depends on local weather conditions, which can vary from day to day.
The evapotranspiration rate varies daily and is directly dependent on solar radiation and other weather conditions.
On the other hand, soil type affects the rate of evaporation from the soil surface, since, depending on the soil’s texture, the particles retain water within their pores with greater or lesser “force.” Similarly, if we install some type of cover on the ground—whether natural or artificial—the direct exposure of the soil surface to the sun is reduced, which also reduces evaporation.
Soil texture, moisture content, and coverage also affect the rate of evaporation.
In the photo above, we can see a series of photovoltaic panels installed on a floating frame in an irrigation reservoir. Instead of causing the water to evaporate, the solar energy striking the surface is used to generate electricity. This reduces water loss due to evaporation and also provides energy from a renewable and sustainable source.
SWEATING
Let’s imagine a healthy plant growing in our garden. Its roots extend deep into the soil, where they absorb nutrients and water. This solution of water and nutrients rises from the roots through the plant’s entire structure to the leaves. It is primarily in the leaves where this water and the dissolved nutrients are used to generate the energy needed to continue growing and developing the plant’s vegetative organs. The water not consumed in this process is released through a kind of opening on the underside of the leaves (stomata). This volume of water, which has been transferred from the leaf to the atmosphere, is known as transpiration.
Generally, the more developed the plant is, the greater the volume of water and nutrients it will require, which will result in increased transpiration. However, if there is not enough moisture in the soil available to the plant, the stomata will be inactive and the transpiration process will be interrupted.
The transpiration rate varies from day to day, as it depends directly on local weather conditions and the plant’s growth stage and health.
EVAPOTRANSPIRATION
So, if we add the amount of water that evaporates directly from the soil to the amount transpired by the plant, we get the evapotranspiration rate.
These two parameters—evaporation and transpiration—are closely linked when it comes to Irrigation. The irrigation system used will determine the proportion of each of these parameters, since a sprinkler irrigation system is completely different from a localized irrigation system.
Did you know that only 1–5% of the water absorbed by plants is used for their own metabolism, while the remaining 95–99% is released into the atmosphere (through transpiration)?
DRIP IRRIGATION
Irrigation is typically applied during the period of lowest rainfall and highest water demand by plants, coinciding with the time when the moisture content in the upper part of the soil profile is negligible for the purposes of calculating evaporation.
In the drip irrigation systems, unlike flood irrigation, furrow irrigation, or sprinkler irrigation, only a portion of the area occupied by the crop is moistened, concentrating the water application in the area of highest root density. This facilitates water absorption by the roots and reduces water evaporation in the uncultivated surface area, concentrating Irrigation in the application zone.
If we can reduce the evaporation rate and, at the same time, implement precise Irrigation practices, we can reap significant benefits by optimizing available resources.
SUB-SURFACE DRIP IRRIGATION (SDI)
One of the advantages offered by the SUB-SURFACE DRIP IRRIGATION (RGS) is the application of water in such a way that moisture does not reach the soil surface, thereby preventing water loss through evaporation. For the operation of this irrigation system to be proper, it must be designed, installed, and operated by personnel specialized in this field.
If we face water shortages and/or the water quality is not suitable for irrigation, RGS systems can be a good solution. The emitter dripline pipes are buried at a specific depth—depending on the crop, planting system, and soil type—and once water is supplied, it is applied directly to the plant’s active root zone. This prevents moisture from appearing on the soil surface. The irrigation application efficiency—not only for water but also for nutrients—achieved with the RGS system is the highest of all, thereby reducing operating costs as well.
