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Posts from the ‘SC-1 leaf porometer’ Category

Irrigation and Climate Impacts to the Water-Energy Balance of the WI Central Sands (Part I)

Due to controversy over the growing number of high capacity wells in the Wisconsin Central Sands, University of Wisconsin PhD student, Mallika Nocco, is researching how agricultural land use, irrigation, and climate change impact the region’s water-energy balance.  She and her team have uncovered some surprising results.

Fisher women leans in for a kiss with a class 1 trout she caught

A class 1 trout stream has sufficient natural reproduction to sustain populations of wild trout at or near carry capacity.

Water Use Debate

There are class 1 trout streams in the Central Sands region, and some people worry that the increasing number of high capacity wells used for agriculture will reduce the water levels in those streams.  “Lake Huron has lost about 11 feet of water since 2000,” says one resident of the Central Sands area, “and water levels are continuing to drop.” In 2008, the small well he used to pump drinking water went dry, and he blames the high capacity wells.” (Aljazeera America)  On the other side of the debate, agriculture irrigated by these wells is extremely valuable to the state, and growers have taken quite a bit of time to understand the water cycle and their role in it. You can read about their water management goals and accomplishments here.

Updating Former Research

Irrigated agriculture wasn’t prevalent or profitable in the Wisconsin Central Sands until groundwater irrigation with high capacity wells became feasible in the 1950s.  Since then, this relatively small ecological region has gone from 60 high capacity wells in 1960 to over 2,500 today.

Mallika Nocco is studying potential groundwater recharge from irrigated cropping systems that use the wells, hoping to understand if the irrigation water is lost or returned to the groundwater.  She says, “Until now, we’ve been relying on models validated by two lysimeters in the 1970s. Champ Tanner (one of the fathers of environmental biophysics) designed the weighing lysimeters, and they were very accurate, but we wanted to do a larger scale study with multiple crops to get a handle on interannual variability and to improve our understanding of recharge in the region so we can do a better job of managing irrigation and groundwater.”

Lysimeter installation into a dirt and a field

Lysimeter installation into actively managed fields presented challenges that the research team had to overcome.

Measuring Recharge

Nocco used twenty-five drain gauge lysimeters to capture vadose zone flux under potato and maize cropping systems.  She monitored soil water (and temperature) flux by stratifying water content sensors from the soil surface to a depth of 1.4 meters.  She also estimated evapotranspiration (ET) using a porometer to measure stomatal conductance, in addition to obtaining micrometeorology, leaf area index, and gas exchange measurements.

Nocco and her team had to put their sensors in to avoid cultivation, so they extended the drain gauge PVC that comes up to the soil surface and removed it any time there was major fieldwork, whether it was tillage or planting, so that the area over the lysimeter got the same treatment as the rest of the agricultural fields.

Below the Root Zone

Nocco says getting the lysimeters below the root zone was a challenge.  Next week, read about how she solved that challenge, how she used a GPS system to find the lysimeters within a half-inch of accuracy, and about her surprising conclusions.

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Gore-Tex, House Wrap, and Stomatal Conductance

Want to develop an appreciation for Gore-Tex?  All you need is five minutes in a rubber raincoat.  But how do you know whether the North Face knock-off you’ve just purchased in China for a ridiculously low price is Gore-Tex or rubber?  If you’re a METER researcher, you dash back to your hotel room and clamp a porometer onto the fabric.

The leaf porometer was designed to measure stomatal conductance in leaves.  It’s typically used by canopy researchers to relate stomatal resistance to canopy attributes like water use, water balance, and uptake rates of herbicides, ozone, and pollutants.  Yet, from the beginning, Dr. Gaylon Campbell, the porometer’s designer, saw the possibilities:  “Give this to someone with only a passing interest in research, a ten-year-old kid for example, and they’ll go around the garden and come back with some really interesting observations,” he said.  “There are lots of questions about what loses water and what doesn’t that you can answer with this instrument.”

Researcher Clamping a LEAF POROMETER onto a Leaf in a Forest

SC1 Leaf Porometer

Dr. Campbell was probably thinking the questions would be about organic material—but it hasn’t always turned out that way.  By putting a wet paper towel on one side of an inorganic material and clamping the towel and the material into the porometer head, you can measure how well water vapor diffuses through the material.  Using this strategy, the researcher in China discovered that his raincoat was pretty much impermeable (unlike real Gore-Tex, which is a good vapor conductor).  Spotting the fake North Face coat is now a favorite part of METER’s canopy seminar.  And the coat is not the only leafless item that has been tested. “People will clamp the porometer on just about anything,” Doug Cobos, a METER research scientist, admitted. He himself grabbed it when a local contractor brought in a sample of some supposedly unique house wrap.

Siding is supposed to protect a house from the elements, but most building codes now require that houses be wrapped under the siding.  House wraps provide a secondary defense against liquid water and increase energy efficiency by preventing drafts.  As with raincoats, high-performance house wrap needs to repel water and stop wind while remaining permeable to water vapor.

A House Under Construction with Trees Around it

House under construction with protective wrap under the siding.

The practice of applying a sheathing of tar paper under siding is a hundred years old, but in the last fifteen years, high tech house wraps made from polypropylene in combination with a push towards energy efficiency have made the house wrap market big and competitive.  Upstart wraps try to gain market share through innovation and the one brought in by the local contractor came along with an outlandish claim.  According to the manufacturer’s rep, this plastic wrap would allow water vapor to diffuse out while preventing any from diffusing in.  Some builders might have scratched their heads and moved on.  Our local man decided to check it out.  He brought a sample of the mystical wrap to Decagon.  Out came the porometer and a quick scientific study of house wrap was born.

Dr. Cobos tested industry standard Tyvek house wrap along with the great one-way pretender.  The results?  “The vapor conductance of the new material was basically the same, regardless of which side of the material faced wet filter paper,” Dr. Cobos said.  “And, in fact, the material didn’t diffuse well at all.  Its conductance was similar to cheap perforated plastic.  It didn’t come close to the performance of Tyvek.” Ultimately, the newfangled wrap was retested by the manufacturer and taken off the market.

Probably the porometer’s best and highest use is still in canopy research, but it still gets pulled out to measure whatever seems interesting, organic and inorganic alike.  That dovetails with Dr. Campbell’s vision of it as a tool for routine use in canopy studies—and everywhere else.  Can you use it on yourself?  “Oh sure,” says Dr. Campbell.  “People clamp the porometer on their fingers all the time.  That’s a quick way to see if it’s working.”  He grins.  “Maybe you could use it as a lie detector on your kids.”

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Burn victim research leads to new method for measuring stomatal conductance

Measuring the stomatal conductance of a leaf should be a pretty straightforward problem.  The conductance is just the flux density of water vapor divided by the concentration difference between the leaf and its surroundings.  Common approaches to this problem involve either flowing air of known vapor concentration over the leaf and measuring how much water vapor is picked up, or sealing a cup of known capacity to the leaf surface and measuring how quickly the vapor concentration in the cup increases.  Both of these, though simple in concept, require quite a bit of expensive equipment to pull off.  We wanted a simpler approach.  We put a humidity sensor in a small tube, the end of which could be pressed against the leaf.  As vapor diffused through the tube the humidity in the tube increased.  The conductance of the tube is easily calculated.  It is the diffusivity for water vapor divided by the tube length.  The leaf conductance could be computed from the tube length, the humidity in the tube and the ambient humidity.  That worked, but it turned out that ambient humidity variations introduced too much error, so we later added a second humidity sensor toward the distill end of the tube. Our approach was very simple, and works well, but it wasn’t a new idea.

stomatal conductance

Cross section of METER’s Leaf Porometer

I read of a similar device in a conference proceedings (I don’t recall the name of the conference)  in 1977 when I was on sabbatical at University of Nottingham in England.  The device wasn’t for leaves.  It was developed by a medical researcher to assess severity of burn injuries, and for use on neonatal infants.  The skin of a non-sweating human is pretty impermeable to water.  A typical conductance is around 5 mmol m-2 s-1.  This is about half the value for a leaf with stomates closed, and about two orders of magnitude lower than leaves with open stomates.  Burned skin, however, is much more permeable, and the permeability is related to the severity of the burn.  A device that could measure the permeability of skin would therefore give information on the severity of the burn.  The researcher built an apparatus, similar to our porometer, with two closely spaced humidity sensors in a diffusion tube.  As I recall, it was somewhat successful, but I’m not aware of it ever having been commercialized or used much after that. The application for infants is also interesting.  Full-term babies have low skin conductances.  I haven’t seen measurements, but assume they are similar to adult conductances.  The skin of premature infants, though, has a much higher conductance.  I don’t know typical conductance values, but do know that, without intervention, the conductance can be so high that evaporative water loss from the baby will reduce body temperature to dangerously low levels, even in an incubator. I don’t know if later work has been done to measure skin conductance, but it is interesting that the first applications of the technology we now use in our porometer was for measuring conductance of the human epidermis, not the epidermis of leaves.

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