Evapotranspiration Rates Determine Green Roof Retention

by Dr. Anna Zakrisson on Sunday, May 26, 2019

Retention and evapotranspiration are always THE SAME VOLUMES!

Retention is water captured by the green roof that only LEAVES AS VAPOR through the process of evapotranspiration. Evapotranspiration is the process of water escaping as vapor through evaporation + plant transpiration.
Thus, you want to increase retention by 5 Gallons, you need to increase evapotranspiration by 5 Gallons.

Let's unpack this:

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What is evaporation?

A green roof sucks up water like a sponge, and just like a sponge holds water, water is also held on the roof. Yes, of course, some water may also become runoff, but today, we will focus on that part of the water that never runs off: the retained water:

If a wet sponge sits on an impervious surface in the sun, the only way for the water to disappear is for it to evaporate. On a green roof, it’s the same: evaporation is the loss of water from any old surface such as leaves and stems, soil particles or debris.

If it’s hot and windy, the roof dries faster than if it’s cool and still.

Wind speed matters as it stirs up the so-called boundary layer allowing water molecules to leave the surface as vapor.

…but a green roof can do more. Enter vegetation!

The word evapotranspiration is a combination of the terms: transpiration and evaporation. Evaporation we have already mentioned, but what is transpiration?

What is transpiration?

During transpiration, water is lost through small holes on the plant leaves that are called stomata. These openings are responsible for the plant’s gas exchange.

Recall from high-school biology that, contrary to us humans, plants need water, carbon dioxide, and light to make sugars and produce oxygen in the process. When the stomata are open and allowing carbon dioxide entering the leaf and oxygen leaving, water is also lost in the process.

Water is sucked up from the soil by the roots, like through a milkshake straw, and travel by mass flow to the shoots, where it’s lost to the air as vapor.

A huge portion of the water taken up by the plant is lost this way, often over 95%. It may seem like a wasteful process, but this is also how the plant transports nutrients and other important compounds through their structures. After all, they don’t have blood, lungs, and a heart like we do.

We should also not forget the cooling effect of the transpired and evaporated water. This cooling effect is substantial and is one of the major benefits of having a green roof as it leads to significant energy savings.

Evapotranspiration from green roofs thus also help mitigate heat island effects in urban areas - an ever-growing problem in larger cities.

Retention and evapotranspiration

How much water can a green roof retain? Well, it depends on how wet it already is. If it’s already dripping wet, not much… Plants have to work hard to remove that water before the roof has retained the capacity to store more rain.

Thus, the retention capacity of a green roof is completely determined on how fast water can be removed from the roof via evapotranspiration.

Hence, if you know the annual evapotranspiration rates you know the roof’s retention capacity.

Matching vegetation with your local climate

It’s vital that local precipitation rates match the evapotranspiration rates of the green roof vegetation. If this is not carefully considered, you might end up with a roof that is sucked dry in no time and is in constant need of expensive irrigation.

This is why it’s so important to carefully consider the green roof vegetation. Know your plants and know their evapotranspiration rates.

Also, for these reasons, it may not be a great idea to put unresearched local vegetation on a roof as often the evapotranspiration rates are unknown. Though this vegetation has evolved to the local climate, these plants may not be well adapted to the harsh green roof environment.

However, if the local plants have been researched and are known to tolerate the harsh green roof conditions, they could be a good choice.

Green roofs experience extreme temperature fluctuations and are extremely exposed to wind and weather. Hence, it’s often a safe bet to go for well-researched plants such as sedum mats with a successful track record of many decades than wild cards that we know little about.

This is especially true if your budget is not extensive and can't easily cover a full roof plant replacement.

However, we should never stop researching and improving. In a few years’ time, the industry might have developed plant selections that are specialized to local climates in ways we can only dream of. One thing is for sure, we have exciting times ahead!

If you have any questions, suggestions, or feedback, don’t hesitate to contact us at info[at]purple-roof.com or through our contact form.

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Reading-tip on the link between green roof/living roof vegetation and hydrological performance:

Stovin, Virginia, et al. "The influence of substrate and vegetation configuration on green roof hydrological performance." Ecological Engineering85 (2015): 159-172.