It’s amazing what you can see when you compress time.
Back in October, I posted some early results from a timelapse photography project at our Niobrara Valley Preserve. That project is helping document the recovery of the property from a wildfire and to see other changes that our eyes would otherwise miss. We’re hoping to learn just as much from a similar, though smaller, timelapse project along a restored wetland/stream complex in the Platte River Prairies. Last week, I got my hands on the images from that project and have been looking through them to see what we can learn.
So far, one of the most fascinating things I’ve seen comes from a series of images from July 2012 – right after the camera was installed. As I scanned through the photos, I realized that one little wetland pool kept changing size. Its water level (exposed groundwater) was going up and down, making the pool bigger and smaller. Looking more closely, I realized that it was happening in a regular daily pattern. I’ve put a selection of images from a three day timeframe into a slideshow below. If you let your cursor hover over the slideshow window, you can click the arrows to move through the images more quickly. Watch the water level in the pool in the center of the photo – it starts out full in the morning, empties as the day goes along, and then is full again the next morning. This same pattern repeated itself over and over throughout the summer.
I shared what I was seeing with John Heaston, The Nature Conservancy’s Platte River Program Director (in Cozad, Nebraska), who said the pattern is a great illustration of the effects of evapotranspiration – the process through which water moves from the surface of the earth to the atmosphere. Evapotranspiration is a combination of direct evaporation of exposed water and plant transpiration – the movement of water out of a plant’s stomata (pores in their leaves).
Plants try to regulate how much water they lose through their stomata by opening and closing those pores, but transpiration is also strongly affected by light, heat, humidity, and wind. During hot, dry and windy days, plants pull a lot of water in through their roots and then lose it into the atmosphere as they try to cool themselves. In dry soils, plants may shut their stomata altogether during the hottest part of the day in order to conserve moisture. In wetlands, however, plants have abundant water, so they can continue transpiration through the heat of the afternoon. Add that high rate of transpiration to the evaporation of standing water during those same sunny afternoons, and you have evapotranspiration in high gear.
What’s exciting to me about these timelapse images is the opportunity to watch evapotranspiration happen. The plants in the wetland are pulling so much water from the ground during the day, the local level of groundwater drops by at least a few inches. During the night, transpiration slows dramatically, and that groundwater level recovers. Interestingly, the stream to the left of the little pool doesn’t appear to change during the day. While it doesn’t appear to have a strong flow, there is presumably enough water coming from upstream to negate any losses from plant transpiration, so the level of water stays stable through the day.
Watching the repeating pattern of dropping and rising water levels in a wetland is fascinating. It’s as if we’re watching the earth breathe – which, in some ways, we are. It’s also a great example of the power of timelapse photography. By condensing time, we can see patterns we would otherwise miss.
To see more examples of timelapse photography, check out the Platte Basin Timelapse Project’s website. They have one of their cameras on this same wetland (separate from the one that produced the images shown above) and you can see a couple years’ worth of images in a few minutes.
The other interesting aspect of both sets of timelapse imagery from this wetland is that 2012 was the driest year on record for this area. Most streams, rivers, and wetlands went dry last summer, including most of the stream that runs through our property. However, throughout the entire summer, the wetlands and stream in the stretch shown by these cameras maintained a fairly steady water level. It’s one of the reasons we’ve spent so much effort trying to restore the site – we feel like we need to take full advantage of the unique resource there. We’re not sure why the groundwater is so strong in that particular place, but in dry years, the fish, birds, invertebrates, and plants sure appreciate it. So do we.
Thanks to John Heaston and Tala Awada for technical help with this post. Any errors are mine, not theirs. In addition, thanks to Michael Forsberg and Jeff Dale of Moonshell Media for partnering with us on the set-up of the timelapse project(s), and to Steven Speicher for his help and advice.
Very interesting! Just a note, your hotlink to the Platte Basin Timelapse website does not work, but you can link to it with the subsequent “on this same wetland” hotlink. Thanks!
Thanks Malia – I think I’ve got the link fixed now.
Still broken from here (Dec. 17). Produces a ‘Page Not Found’. Could be a proxy server issue, I suppose. Dunno.
Hi Chris, Your time laspe reminds me of coastal plain ponds. The water flowing through the sand causes the level of those ponds to fluctuate drastically. I noticed the water fluctuations from year to year, but not on a daily basis. It would be interesting if you also had a time lapse that showed the same date from consecutive years.
I wanted to also mention that the complexity of water flow into coastal plain ponds results in their water levels not tracking recent precipitation trends. This means the pond can be filled to a high level when there is a drought or alternatively the pond could be at a low level even after a rain event causes flooding in nearby areas.
This time lapse is really nice – As you say, like watching the wetland breathe (or drink, perhaps).
This, however, seemed a touch cognitively dissonant,”as they try to cool themselves” – Hmmm.
Technically, it’s more a matter of the plant staying hydrated rather than thermoregulation, and “try” is anthropomorphic. The process is essentially passive, built into the structure of the plants’ vascular system.
Thanks James. Sorry for the anthropomorphic language. As you say, it’s not a conscious process by the plants. In terms of cooling vs. hydrating – my impression (from people who know more than I) is that it’s both. I could have made that more clear, though. Regardless, I’m glad you enjoyed the imagery.
Hi James, I think Chris is right about the thermoregulation being the paramount purpose of these plants drawing up water. If hydration was the driving goal then they would draw up the same amount of water on cool days as hot days. I do not believe this is the case. Also, if you believe the movement of sap in plants is passive, then you’ve never cut grape vines when their new leaves are actively growing in spring. Grape vine stems pump like a hose. Whether or not the plant can ‘try’ is semantics. A definition of try is “To make an effort to do or accomplish (something); attempt.” Even if it is not conscious, I think plants try very hard.
Themoregulation, to the extent that it occurs, seems to me a by-product. Indeed, transpiration rates slow considerably at lower air temperatures (yes, the plant body is cooler), but also at lower levels of windiness (in this case, less cooling by air movement, so in essence the plant is warmer). This is because water columns from the soil to the stomata through the vascular system, are maintained by capillary action and other lesser forces. The rate of upward movement of this column is determined by rate of evaporation of water at its upper terminus. The primary effect of this would seem to me to be hydration of the plant’s above-ground tissues.
Hi James, I thought of an experiment that can determine whether hydration or thermoregulation is the reason why plants up take water. On a very hot day this summer we should both take a piece of cellophane and wrap it around a plant. The cellophane will prevent any water from evaporating. The plant will therefore be hydrated to the maximum extent, but receive no cooling from evaporation. If the plant’s tissues are not damaged by the buildup of heat then thermoregulation must not be the selection criterion that has caused plants to take up water.
I have also read the theory that capillary action is the mechanism by which water flows to the leaves. I do not believe this theory could work in most cases. I think active pumping of water is involved. Water is often a limiting factor in plant growth. In the great majority of cases, it would make more sense for a plant to pump hot water from the leaves back into the ground for cooling than lose the water to evaporation.
We might both do well to consult an up to date plant physiology text.
That is amazing, don’t you just love science. It turns us all into investigators and detectives. Love it!
Bob Henszey has reams of data showing this phenomenon from his groundwater monitoring project at the Trust, Chris. On a typical summer day, the water table would fall an inch or two (driven by, as you said, transpiration by plants) then partially recover during the night. This partial recovery is driven by recharge from the river. Whenever the river went dry, the water table would fall with no recovery at night until it got below the level where plants could reach it. It tended to remain at a pretty stable level after that (assuming you weren’t near an irrigation well of course), until water returned to the river. And, as you know, precipitation has a huge effect. Each inch of rain will raise the water table about a foot.
Thanks for adding that context, Kent. Just for the benefit of people not familiar with the Platte system, Kent’s statement about the recharge being due to the river is because the river and groundwater are connected. A flowing river is a pretty good indication that groundwater is still fairly high because the river helps recharge that groundwater (and vice versa). When the river is dry (as it often is during dry summers and heavy irrigation time) groundwater is also low, and thus can’t recharge wetlands such as the one featured in the timelapse images. During extended periods of low groundwater and a dry channel, even wetland plants might not be able to reach their roots to moist soil regardless of time of day.
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The colour of the houses roof changes between 1 and 3 pm July 16th