I love wetlands (surprise!). They
have the most interesting plants, the coolest soil, and the best birds. Wetlands have are so cool because they have
dynamic hydrology: water comes and leaves often according to increasingly unpredictable
schedules. Unpredictability is a pain in
the ass because it’s really hard to know the right pair of shoes and shirts to
wear into the field. One of my favorite
wetlands, affectionately known as BRU02, was bone dry for the first two years I
visited it, so I assumed that irrigation boots would be appropriate for my Year
3 visit. Then I
forgot my long-sleeved shirt, which is fine when wetlands are dry and bug-free. Unfortunately, on May 15, 2014 BRU02 was
flooded. This is how every minute of my
1.5 mile walk out to BRU02 went:
I've never seen so many birds there and they were all angry at me for waking their babies and leading away the mosquitoes they were hoping to eat. I was eaten alive slowly because I was also hauling around 100 pounds of water in my over-topped boots. Worst of all, once I got out to BRU02 the well I was looking to put a data logger in was gone! The only reason I walked all the way out there during prime bug season was to leave that data logger and some jag had pulled my monitoring well out of the ground. This unusual flooding pattern ruined my day and I couldn’t even capture it for analysis. Three trips later the well was reinstalled with a data logger and the data gathered there is what I’ll cover in this post.
Spoiler alert: in all my data analysis I haven’t been
able to predict what shoes you should wear to the marsh, so bring them all and select
according to the volume of water you’re comfortable hauling around.
I’ve tied myself in knots over my
2nd dissertation chapter worrying about whether I gathered enough of
the right data and figuring out how to display it. Every time I look at the data I decide I
should just go gather is all over again, then I remember that terrible day. Thankfully, a do-over is not an option. The data I gathered are super cool, though
adding the keyword ‘super cool’ to a publication probably won’t really sell it,
so I’ll be working through some ideas here.
Hydrology is all about the movement of water across the land – how
much is moving, how fast it flows, and where it is going. Measuring
flow gets real muddled in wetlands because… Well, take a look at the wetland
below and guess which direction water is moving:
A nice Great Salt Lake emergent wetland |
It’s going all these places:
When wetlands are flooded at the correct depth during the right time the plants grow real big, bugs proliferate, and soil
bacteria start changing soil chemistry [2].
The key finding from my dissertation is that impounded wetlands –
wetlands that have been diked and turned into shallow reservoirs – are in
better condition because they are flooded longer and deeper than un-impounded
wetlands. Impounded wetlands have enough
water to do the things wetlands do. That
took four years to figure out definitively, but it only took one year to figure
out I needed the details on how long and deeply my wetlands were flooded. During Year 1, my pilot project year, I
installed piezometers at 15 of my 50 wetland sites and found this:
What?!?! Is there a pattern hidden in there? What
parts of that four month period matter most? It was confusing and cool and
great justification for investing in piezometers for each site! A piezometer
is a well with holes or slots drilled into the bottom so that it fills as water
tables rise. Many piezometers are
equipped with data loggers called pressure
transducers that measure the weight of the water in the well and translate
that to a water depth. The water depth
measurement is recorded every hour and stored in the pressure transducers.
Piezometers and the people who work with them |
I wanted to answer two questions
with my piezometer data: 1) what kind of hydroperiod
do Great Salt Lake wetlands have, and 2) what hydroperiod attributes change
wetland condition? My piezometers got A
LOT of data, too much, so the first step was to condense hourly data into a single measurement for each day. Then I
calculated as many statistics to describe what the water was doing as I
could. For example:
- The deepest the water got and when
- The lowest the water table dropped and the number of times it fell
- Average and median water depth
- Frequency and magnitude of water table fluctuations
- The length of time a wetland was flooded, muddy, or dry
- Water depth at specific times of year
- Water regime according to other classifications
I felt pretty clever in some of
my calculations because I had a breakthrough with for-loops, but the results
were...urm… TBD. To answer Question 1: Great
Salt Lake emergent wetland hydroperiods are semi-permanent: water is deepest
in the late spring, driest in mid-summer, and flooded again in the fall. But there
is a lot of variability, from temporary to permanent flooding.
Combining the hydroperiod
information with the condition data I gathered to answer Question 2 is more
interesting. Getting reference condition
(the healthiest) wetlands requires a relatively narrow set of hydrologic conditions:
average water depth over the whole growing season should between 5 cm deep and 10 cm below the surface and wetlands should be saturated or shallowly flooded in late summer. The water should be drawn down a little, but
that low point shouldn’t fall below the reach of plant roots and only last a
couple weeks. While the zone of optimal
conditions is narrow, periodic deviation from that hydroperiod, like during a
dry year, won’t disrupt wetland condition.
Instead, many years of dry conditions or constant flooding are what hurt
the condition. And all that variability I saw – well, there are a whole bunch
of ways to be in non-reference condition: too wet, too dry, too flashy, too
stable….One big caveat – unimpounded wetlands were significantly drier than
impounded wetlands and in significantly lower condition – these things are
inextricably linked.
A few examples of GSL wetland hydroperiods |
Hydroperiods of the best (reference) sites and the worst (poor) |
I’ll continue working through
what the point of it all is. I'm struggling with staying just a few things about 164 hydroperiods that don't always remain consistent year to year and linking it clearly to the vegetation present (which doesn't always care about what I measured). If you've got any insights I'd love to hear them. In the meantime, here are some practical tips, should you be
thinking of using piezometers:
1. Logging the data – as long as you select the
correct date for automated pressure transducers to start gathering info, things
are stress free. Batteries remained ~70%
full after 3-4 years. Taking pressure
transducers out during the winter gave some piece of mind, but did miss a
potentially important window between the thaw and beginning of spring
runoff.
a. I used In-Situ Rugged TROLL 100 pressure
transducers (cost ~ $400) and installed 3 Rugged Baro-TROLL 100 barometers
approximately 20 miles away. Pressure
transducers started recording 12-18 hours after they were deployed in the
field. Pressure transducers were
deployed in May and gathered in September so I could upload data and check
batteries and skip gathering unusable ice data.
I measured water depth at the well when pressure transducers were
deployed and during any field visits; these measurements were used to
post-correct and validate data. All data
was uploaded using Win-Situ 5.0 software, corrections for barometric pressure
were made with the BaroMerge function and then with actual water depth data
using the Post Correction function.
2. Piezometer pieces – Wells were 5-feet long in
total, the top 4 feet was 2-inch diameter SCH-40 PVC, the bottom foot was
2-inch diameter 0.10 slotted environmental PVC (you can get it from drilling
supply companies); sections were glued together with a PVC coupling. I glued a fitted cap to the bottom and
drilled a ¼ inch drain hole. I used
3-inch diameter slip caps, stainless steel eyelet bolts, and four-feet of
paracord to suspend pressure transducers.
Caps were secured with 8-inch plastic zip ties. The caps and rope, while cheap and easy, were
suboptimal.
3. Installation – easier than expected, kind of. We
used a 3-inch auger to install wells to a depth of 3 feet (not possible with dense
clays, caliche layers, oolitic sand, or deep water); wells protruded about 2
feet from the ground. I recommend
installing wells deeper if a wetland is very dry or if there are cows. We placed wells in the deepest flooded area
that had dense, emergent vegetation; the deepest point was thought to be most
representative and the vegetation would prevent tampering.
4. Troubles – it was difficult to find 2-inch PVC
in dense Phragmites and hardstem bulrush (handheld GPS units have an accuracy
of 3-5 meters). Keep in touch with land
managers or hidden wells may be burned, bent by an ATV, or mowed over. Cows are always itchy and will scratch on the
sturdiest thing around, just build little fences around them because that
scratching will displace pressure transducers and mess up your data.
Some of the things that can go wrong with monitoring equipment |
Further Reading:
[1] US Environmental Protection Agency. 2008. Methods for Evaluating Wetland Condition #20: Wetland Hydrology. Prepared jointly by the U.S. EPA Health and Ecological Criteria Division and Wetlands Division.
[2] Brown, S. C., Smith, K., & Batzer, D. (1997). Macroinvertebrate responses to wetland restoration in northern New York. Environmental Entomology, 26(5), 1016-1024.
unpredictable schedules
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ReplyDeletethe most interesting plants, the coolest soil, and the best birds
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