Evapotranspiration – A Fancy Word For Corn Sweat

A Big Word Caused By Something On A Molecular Level

On a hot, sunny day in the middle of the Summer have you ever stopped and noticed how muggy it feels outside? Like instead of living in the Northern Plains, you’re living somewhere tropical but… there’s no ocean around. So what’s causing it to feel like that? Look around, chances are, especially if you’re in the Northern Plains, you’re surrounded by the culprit without even knowing it.

Each year, in the spring, thousands of farmers around the United States take to their fields and begin planting crops for the season. Chances are, their planting corn in hopes of having those stalks become “knee high by the Fourth of July” , come fall harvest a good crop and then start the process all over again after the winter months. During the heart of the corn’s growing season, once those stalks become knee high and their leaves fan out across the field, on any given day, a molecular process is going on which puts thousands of gallons of water into the atmosphere every single day.

Starting in their roots, plants like corn, transfer water from the ground, sometimes meters underground, up their stalks and pump them into their leaves. However, only a small percentage of that water is actually needed by the plant. The rest has to go somewhere which ends up being in the air around it, a process called transpiration. There are three types of transpirations that can occur with a plant – stomatal, lenticular and cuticular. Stomatal transpiration occurs when air through tiny openings on the bottom of the leaf. Lenticular transpiration occurs through small openings on the stalk of the plant while cuticular occurs on the topside of the leaf. It’s interesting to note, though, that each leaf is covered by a waxy layer of cuticule which actually acts to reduce the amount of water transpired since the cuticle layer is hydrophobic.

So why does it feel muggy outside if transpiration occurs on a molecular level? Plants, corn, weeds, flowers even trees, are transpiring every single day all the time, especially during growing seasons. For simplicity, we’re going to use the example of corn. It’s been found that, on average, a single corn plant transpires around 1.86 liters per day during its 110 days of growing. That equates to around  207.9 liters of water pumped back into the atmosphere over that time. On a given day, for simplicity, it’s been found that an acre of corn can pump anywhere from 11,000 – 15,000 liters of water back into the surrounding air. That’s a lot of water. A large oak tree can transpire nearly 40,000 gallons, that’s over 150,000 liters, of water per year. All that water in the surrounding air can increase the dew point as well as humidity locally. And, interestingly enough, a windy day only contributes to transpiration since fresh air is moved overhead and can absorb the water once again.

Here’s another interesting thing about corn sweat. If a large corn field, for instance, is planted next to a lake, the corn can actually act like an extension of the lake and here’s how. We already know that cornfields put plenty of moisture back into the atmosphere throughout the day but what you may not know is this, on a small scale, a small wind circulation develops due to differential heating during the day. Know as a land/sea breeze or land/lake breeze, depending on the body of water, a small circulation develops as the land heats up more than water, causes air to rise over the land then fall back over the water (it reverses at night since land cools faster than water). That large amount of water that the cornfield is putting back into the atmosphere actually acts like an extension of the water thus the lake/land breeze extends farther inland than it normally would! Didn’t think corn could do that did you?

Lake/Sea Breeze Circulation, via LakeErieWx.com

So what factors contribute to transpiration? Temperatures and the airmass overhead can affect the rate of transpiration; transpiration rates increase with clear skies and warm/hot temperatures and more water can be absorbed in drier airmasses versus already saturated air. It’s already been touched earlier but windy days help circulate air so saturated air is replaced with drier air. Ground water availability also contributes to how much moisture is then transpired back into the atmosphere. And, of course, the type of plant is a major factor; plants like cacti do not transpire as much since they grow in areas where water is scarce.

So the next time you step outside this summer and see that knee high corn waving in the wind, you’ll know it feels so humid outside! Because those leaves are working around the clock to transpire the water from the ground and pump it back into the air.

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