Evapotranspiration - the Green Roof Engine

by Anna Zakrisson on Tuesday, January 15, 2019

stomatal pores on the underside of a leaf

8 min read

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Evapotranspiration is merely a combination of the words: evaporation (evapo-) and transpiration (-transpiration). Evapotranspiration determines green roof retention capacity, so it's a fundamental concept to understand.

So, what is evapotranspiration? What do these two terms mean? Let’s start with evaporation!

What is evaporation?

Evaporation occurs as a liquid becomes gas when it reaches its boiling point. As temperature increases, molecules move faster, and the ones close to the surface might overcome the vapor pressure and escape to become vapor.

In short, as the sun warms the roof surface, water molecules escape from the soil and plant surfaces on the roof and become gas.

Evaporation is a critical part of the global water cycle. Water evaporating from water bodies and terrestrial surfaces eventually form clouds and fall again as rain.

What drives evaporation?

The main driver for evaporation is temperature, but many other factors also play a role, like humidity. If the air is very humid, evaporation rates will decrease.

Air can only hold so many water molecules at a given temperature, and if the air is very humid, water molecules on the roof surface need even more energy to overcome the vapor pressure from all the water molecules above them, and evaporation will occur much more slowly.

The larger the concentration difference between the liquid and the air, the faster evaporation will occur.

Another important factor is wind. As water evaporates, it gets stuck in something called the boundary layer. This layer traps air due to laminar flow. Since the air is stuck and can’t mix with drier air above, it will become increasingly saturated with water and evaporation will decrease over time.

Windy conditions disperse the saturated boundary layer that covers roof surfaces. This reduces the effective water concentration close to the surface, allowing evaporation to proceed more quickly.

Surface area also affects evaporation. With larger surface areas, more molecules are close to the surface where they can escape the liquid and become gas.

Imagine a roof with vegetation and one without vegetation. Think of all the little crevices, leaves, stems, and hairs on the plants. The surface area is huge! The difference between the vegetated and the non-vegetated roof is enormous! This increase in surface area is value added by green roofs for stormwater protection and management.

Pressure also affects evaporation. The higher the pressure, the lower the evaporation rates. So, a green roof at a high altitude may have slightly higher evaporation rates compared with one at low altitudes (provided that all other parameters are kept constant).

Water molecules will continue escaping the liquid until an equilibrium has been reached. This happens when the same amount of water evaporates (escapes as gas) as condenses (becomes liquid, e.g. rain).

What is transpiration?

Transpiration is what we call the process of water loss from plants, mainly via their gas openings, called stomata or guard cells.

What drives transpiration?

A surprisingly small amount of water is needed for plant growth and metabolic processes; the rest is lost via transpiration or guttation. Guttation is the process whereby droplets are formed at the tips of straws, blades, and leaves during times of low transpiration.

The water that is lost represents 97-99.5% of the total water taken up by the plant.

The plants can act like pumps, speeding up the water cycle by pulling this excess water from the last storm out of the soil, leaving the green roof ready to absorb the next rainfall.

It is easy to see how the vegetation on a green roof can affect green roof retention significantly!

The water transport system

Water is taken up by the roots and then transported up the plant through a vascular system called xylem. There are two processes responsible for moving water through the plant and transpiration is the main one.

Plants absorb carbon dioxide and release oxygen through the opening and closing stomata/guard cells – little pores on the leaf surface. As the stomata open to allow gases in and out, water vapor is also lost. This lost water creates a pull in the vascular system and enables a mass-flow of water, minerals, and nutrients from the root to the shoot.

Like juice being sucked up a straw, each water molecule lost pulls on the next molecule behind it, stretching all the way through the vascular system. This pull drives the upward movement of water, minerals, nutrients from the roots to the shoots.

Transpiration in arid conditions

The loss of water through the stomata also has a cooling effect on the plants.

However, if days are hot and transpiration high, the plants may run out of water in the soil. This can be deadly for the plants as they simply dry up and die.

Some plants, such as sedum plants, have developed adaptations to survive arid and inhospitable environments such as deserts or tundra. These kinds of plants are very useful for green roofs, as the conditions on a green roof tend to be extreme. Green roof soil media tend to be very porous and dry out very fast.

Most “normal” (C3) plants open their stomata during the day and close them at night when photosynthesis comes to a halt.

Sedum plants are often used on extensive green roofs, and these plants have developed a strategy called crassulacean acid metabolism (CAM). These plants keep their stomata closed during the day and open them during the cool night in order to conserve water. Some of the sedums can even switch between CAM and C3 depending on the conditions. These plants are referred to as facultative CAM.

Another adaptation can be found in the so-called C4 plants (like grasses). We will discuss these plants in another article.

What affects transpiration rates?

As with simple evaporation rates, relative humidity, wind, and temperature are important for the same reasons. Water supply is also essential. Water-stressed plants often close their stomata to conserve water. Light can also affect how stomata open and close. This is something one should take into consideration when estimating transpiration rates in different countries at different latitudes.

There are varying strategies of plants to deal with water loss, e.g., CAM vs. C3, as we have already mentioned.

Leaf size is another critical factor, as is the number of leaves and number stomata per leaf. Some plants have leaf cuticles or tiny hairs called trichomes that also affect transpiration rates.

Green roofs are tricky. On the one hand, we want evapotranspiration to be very high to get rid of stormwater as quickly as possible and prepare to absorb the next storm. On the other hand, we want the plants to conserve water to not dry out in the thin soil media between storms.

We also do not want to have to irrigate green roofs to support plants. It is a challenging task to design a plant palette that minimizes the need for additional irrigation. LEED, in particular, is giving the green roof designer a tough time balancing the desire for native plants, in a very shallow soil profile, and yet at the same time forbid irrigation.

This shows the importance of using different roof systems for different climates and abandoning the current strategy of one-size-fits-all for soil and vegetation.

Root pressure

The second process driving water loss in plants is root pressure. This is a different process that contributes to water transport and loss in plants but is distinct from the mass flow created by water evaporating through the stomata.

Root pressure is created by osmotic processes in the roots. What are these “osmotic processes,” you may ask? Well, sugars and other solutes concentrate in the cells, which results in an osmotic potential gradient and a pull of water from the soil to the root cells.

The maximum pressure that can be achieved by this process is 0.6 megapascals, which can drive water to a maximum height of 6.87 m. So, for high trees, root pressure isn’t the main factor driving water loss.

However, for short plants, root pressure can be a significant force driving water transport, something that we should not forget when designing green roofs. Many plants are even able to accumulate nutrients and water despite being grown at 100 % humidity (e.g., sunflower plants). This suggests that for some species, root pressure may well be the primary process moving water through the plant, at least during certain conditions.

Root pressure is responsible for guttation, the creation of small droplets of water on leaf sheets and straws during the dark hours. Guttation should not be confused with dew formation, which is a different process entirely.

Sedums and transpiration rates

There is a lot of confusion in the green roof industry regarding plant choices and their subsequent evapotranspiration (ET) rates. It is said that Sedums do not evaporate enough water when comparing to other more perennial-like plants with larger leaves. This is partly correct but still incomplete.

There are 600 different Sedums in the same family. Some have tiny silvery needles which have extremely low evapotranspiration rates. Others have large perennial-type leaves that evapotranspire a lot. So the question to ask is what Sedums are we talking about?

In the US we mostly work with large-leaf Sedums, unlike Northern Europe where they use mostly needles. When these large-leaved Sedums have enough water, they ET the same as most other perennials. Nonetheless, when water becomes scarce, the Sedums switch to CAM to preserve water and deal with the prolonged drought, and the perennials need irrigation, or they will die.

Summary: evapotranspiration

Evapotranspiration is the driving factor to be considered for green roof design. Why? Water retention through soil profile design needs to match the evapotranspiration rates of the plants that are desired, on a particular location with a specific rain pattern and rain volume.

Evapotranspiration drives water requirements and hence irrigation scheduling and is the driver of retention. One truly cannot design a green roof without fully understanding the annual water balance and evapotranspiration rates of the plant palette in the making of green roof design. Needless to say, this is a task best left to the experts.

Please, don’t hesitate to contact our experts if you have any questions!

Reading tip: evapotranspiration

Cirkel, Dirk, et al. "Evaporation from (Blue-) Green Roofs: Assessing the Benefits of a Storage and Capillary Irrigation System Based on Measurements and Modeling." Water 10.9 (2018): 1253.

Audio article

This article is available as an audio version on all major podcast directories such as iTunes, Spotify and many more.

What is Evapotranspiration? Part1:

What is Evapotranspiration? Part2: