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June 19, 2023

Evapotranspiration: What is it?

At Smart Rain, we talk a lot about evapotranspiration. In fact, it's our secret sauce. Our product creation is all designed around computer algorithms that calculate evapotranspiration. But what is evapotranspiration? To put it simply, it's the process of water transferring through the earth’s surface, to the atmosphere. But that doesn't paint the full picture of why it's important for irrigation. To understand why it's important for your landscape, we need to go back to grade school science: to the water cycle.

The Hydrologic Cycle & Evapotranspiration

The hydrologic cycle is also known as the water cycle and is an integral part of evapotranspiration. This cycle explains the continuous flow of H₂O through the earth and atmosphere. It is a complex system where H₂O will go through all the three states of matter. The location of the H₂O, and the phase in the process, will determine what state the water is in.

All living things depend on water and this cycle. Thriving ecosystems are able to tap into this system, and the same goes for your landscape. When you water your landscape, what you are trying to do is artificially mimic the hydrological cycle for healthy plants(which is referred to as evapotranspiration). The first step to the hydrologic cycle is evaporation.

Picture of the word evapotranspiration


Evaporation is an essential part of the water cycle and is a key component of evapotranspiration. It occurs when liquid water turns into vapor and rises into the atmosphere, where it can condense as clouds or fall to earth as rain. On Earth, about 90 percent of all evaporation takes place over oceans, lakes, and rivers. However, evaporation can also occur from plants, soil, and other surfaces.

When the sun's energy is absorbed by liquid water, it causes molecules to move faster and break apart into tiny droplets that evaporate into vapor form and move up into the atmosphere. As evaporation and evapotranspiration occurs, more heat is released which helps circulate the atmosphere and wind. Evaporation helps regulate global temperatures, as well as provides clean air for us to breathe. H₂O will evaporate at 212°F (100°C), which is very hot. So how does water evaporate in the cold?

Particles in colder temperatures have portions of H₂O molecules that have enough energy to break away from the other molecules to evaporate. An important thing to note with your landscape: the rate of evaporation will increase when temperatures are higher resulting in evapotranspiration. This is why you shouldn't water during the hot hours of the day.


In evapotranspiration, condensation is responsible for creating clouds and precipitation. As air masses rise due to convection currents or topography, they cool and moisture contained within them condenses into droplets of liquid water that form clouds. As air masses rise due to convection currents or topography, they cool and moisture contained within them condenses into droplets of liquid water that form clouds.

The transportation of H₂O relies heavily on condensation and that is why it plays a critical role in the hydrological cycle. It allows H₂O to return to the earth after evaporation. Often, when you go outside in the morning, you may have frost or tiny droplets of water on your car, and consequently, it's an easy way to visually see evapotranspiration. When the air cools in the middle of the night, the change in temperature will cause the H₂O that is in the air to change from a gas to a liquid or solid. That is a very simple example that shows you condensation in action in your daily life.  


We briefly touched on precipitation when we talked about condensation. This is the process is defined by changing the water in the air, to water that falls to the earth. Precipitation includes rain, snow, hail, sleet etc., which fall from the atmosphere to the Earth's surface. This moisture replenishes streams and rivers, recharges aquifers and allows vegetation to grow. In evapotranspiration, this process happens on a much smaller lever.

As condensation is in the sky through the form of clouds (which are tiny droplets of water floating in the air), the weather will cool and clouds will become “heavier” or in other words, they will change water’s physical state to a liquid or solid. If it changes to a liquid, water will fall to the earth in the form of rain. If it changes to a solid, it will come into the form of hail or snow.  


This next part of evapotranspiration isn’t very spoken about, but it is very important to your landscape and irrigation. Interception is the process where water droplets from precipitation are caught and held on the surface of vegetation or other objects before it reaches the ground. This interception can happen through leaves, twigs, needles, branches, stems or even trunks of trees and shrubs. The amount of intercepted water depends on several factors, such as the size and shape of vegetation, humidity, air temperature and wind speed.

Interception can help reduce runoff in a particular area and helps replenish soil moisture levels. The plants in your landscape play a direct role in this. Interception is often a key source of water for many plants in your landscape. Sprinkler systems are trying to mimic a small part evapotranspiration by artificially precipitating and the plants intercept that moisture.  


Movement of water from the ground surface to the soil is known as infiltration. This process is driven by gravity and occurs when precipitation or irrigation water seeps through cracks and crevices in the surface soil, saturating it with moisture.

The rate of infiltration can vary depending on a number of factors, such as soil type, soil structure, slope, and land cover. Infiltration is an important part of evapotranspiration because it allows water to be stored in the soil and later used for plant growth or released slowly into nearby streams and rivers.

This process helps regulate stream flow by controlling the amount of water entering the streams and rivers, reducing flooding and drought. Additionally, infiltration helps filter pollutants from the water before it enters larger bodies of water, improving water quality. 


This is the movement of water around Earth. It refers to the movement of water through soils, sand, and other permeable substrates. It is part of the process by which precipitation infiltrates into the ground and becomes groundwater that eventually flows out as surface runoff.

Percolation replenishes groundwater, which supplies much of the worlds drinking water. In addition, percolation contributes to the health of ecosystems by transferring nutrients between the surface and subsurface layers of soils. This is a simple explanation of a very complicated process.  This doesn't have much to do with evapotranspiration, but is essential to understanding the hydrologic cycle.


If you notice in the word evapotranspiration, you’ll see two words that relate to stages in the water cycle. Those stages are evaporation and transpiration. We already spoke about transpiration, but now is the time to talk about the last step of the cycle which is transpiration. This is similar to evaporation, but just ever so slightly different.

Transpiration refers specifically to the water that evaporates from the plants. Transpiration occurs when water moves up through a plant's roots and stem, then evaporates into the air from its leaves. The amount of transpiration that takes place varies depending on environmental conditions. Thus, we see evapotranspiration.

For example, higher temperatures and more wind speed can cause plants to transpire more rapidly. Without transpiration, evapotranspiration would be incomplete, and water would not be able to move between the land, atmosphere, and oceans. 

Conclusion on Evapotranspiration

With this information given, it’s clear that what we are trying to do when we water our plants is to create a small hydrologic cycle in our landscapes through evapotranspiration. The Earth regulates the water by itself and runs all on its own without the help any humans. Since we are artificially watering our landscapes, we need an artificial regulator that can help us know how much we need to water our landscape. That is what Smart Rain does. Smart Rain monitors the natural processes of evapotranspiration to give your lawn the exact amount of water needed. When taking care of your landscape, keep these things in mind. Monitoring the hydrologic cycle will be your ticket to a healthy landscape.

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