cc >> Tom Zinnen: Welcome,
everybody, to Wednesday Nite at the Lab.
I'm Tom Zinnen and on behalf of the Biotechnology Center where I
work and Cooperative Extension, UW Extension and the Wisconsin
Alumni Association and Wisconsin Public Television, thanks for
coming to Wednesday Nite at the Lab.
We do this every Wednesday night, 50 times a year.
Tonight I'm delighted to have Steven Loheide be the presenter.
He's going to talk about his work in hydroecology and
ecohydrology in the alpine meadows in the Sierra Nevada.
He's an assistant professor in the Department of Civil and
Environmental Engineering here at UW Madison.
He received his bachelor's in environmental chemistry and
geology from the University of Northern Iowa in 1999,
his master's in geology from Indiana University in 2001,
and his PhD in hydrogeology from Stanford University in 2006.
Steve's research focuses on the interactions between ecological
and hydrological processes in natural and built systems.
His approaches use a combination of field data, remote sensing
and numerical modeling to understand the feedbacks between
vegetation patterning, otherwise known as plants and how they
grow, vegetative water use, soil moisture availability,
groundwater and stream aquifer interactions.
His work is focused on improving the scientific basis for stream,
food plain, meadow and wetland restoration efforts under the
current and future climatic conditions.
So if any of you have interests in stream restoration, I think
his work is going to be very interesting to hear.
Please join me in welcoming Steve Loheide to Wednesday Nite
at the Lab. [APPLAUSE]
Steve Loheide: Thank you for that introduction, Tom,
and thank you for giving me the opportunity to speak here
at Wednesday Nite at the Lab. I'd like to take this
opportunity to talk a little bit about hydrologic processes and
how the movement of water through the landscape, over the
land surface, and through the subsurface affect ecological
processes. In this case ecological
processes means vegetation patterning and vegetation
composition within high elevation meadows.
This is Tuolumne Meadows in the Sierra Nevada of California.
By some of the smiles on people's faces, I can tell that
some of you have been there, and this place holds a fond place in
your heart as it does in mine. And what I'll really be focusing
on are these interactions between hydrology and ecology.
So the first part I will talk about meadow hydrology at two
different scales. One at the watershed scale, the
very large scale in which stream flow is generated.
And then at a very fine scale, at the actual movement of
groundwater within the meadows. After we go through the basics
of hydrological processes, then I'll talk about the meadow
vegetation communities that we see in Tuolumne Meadows and I'll
start to link those with the hydrologic conditions in the
meadow. And then once we've developed
this hydroecologic framework, an understanding how hydro are
linked, then we'll apply that to understand how meadows become
degraded and potentially how we could start to restore these
meadows. And so Tuolumne Meadows is
perhaps the most well loved and iconic of high elevation meadows
in Yosemite National Park. It's in this location in the
Sierra Nevadas of California, and this is an outline of the
meadow boundary. So the meadow proper is this
treeless and flat region within the mountains.
I'll be talking about a lot about the main channel of the
Tuolumne, running from right to left through the meadow, as well
as about three tributaries, Delaney, Budd and Unicorn Creek.
I'll also be making reference to Pothole Dome.
That's this granitic outcrop here.
And that's where this photograph was taken, kind of looking out
over the meadow. And I'll also be talking about
this hill slope here which is a north-facing, and therefore more
shady, less solar exposure hill slope and forested, heavily
forested. So the hydrology of the Sierras
is much different than that in Wisconsin.
About 80% of the precipitation falls during the winter months
in the form of snow. And that accumulates quite a
deep snow pack in the spring when that begins to melt.
And then we have the peak melt in May or June, and that creates
meadow flooding. You can see that much of this
meadow is now inundated with water.
And then as we enter the growing season, the stream flow begins
to recede and water levels, both within the channel, we can see
the channel right here, kind of a higher level and then a lower
level, and within the connected groundwater system begin to
drop. And once the water levels drop
sufficiently, water becomes limiting for the plants and they
senesce for the season. Here's a little time lapse video
of that snow melt from Pothole Dome.
And we can see here at the beginning of April we just start
to have the melt. You can see the snow pack
diminishing here in the foreground.
And in the background the meadow then begins to melt and flood.
We get a couple other small snow events, and then after that
becomes the growing season. So that's really time zero for
the processes that we're interested in.
The snow melt is the major hydrologic event every year in
the Sierra Nevada. And so we need to be able to
understand something about that snow melt.
And so we're not working on this project by ourselves, but we
also have collaborators, Jessica Lundquist's lab at the
University of Washington, and the work that they do is really
involved in the larger watershed scale modeling.
And so they do distributed hydrologic modeling in which
they break down the watershed into a grid.
And really what they're looking at is how the water is processed
in each of these grid cells. How precipitation that lands on
this parcel of land either infiltrates into the subsurface
or runs off and flows down the hill slope.
And if it does infiltrate it can be stored in the soils and then
used by plants, taken up by plants and used for
evapotranspiration or ET, or it can move through that soil
down the hill slope. And any water that's moving down
the hill slope, either on the surface or in the subsurface,
will eventually encounter a stream and start to generate
stream flow. And so stream flow will be
generated in each of these small valleys.
How do they measure whether it goes through the grid or
flows at different levels? How do they determine that?
Well, part of the water is measured on the land surface.
You can see overland flow when it occurs.
This is run off that might occur on a hill slope.
And the part that falls into the land surface eventually enters
the groundwater system. So we can monitor the
groundwater, and we'll be talking about a little bit of
that later in the talk. But right now we're focusing a
little bit more on the surface water.
And this is the Tuolumne watershed.
Here is the meadow that we're interested in, Tuolumne Meadows.
And this is about 250 square kilometers.
And it's very mountainous. We can see here we're looking
here up the Tuolumne towards these large mountains.
They're still within the watershed, this region kind of
over here. And this stream discharge that
we reproduce, the modeled version, is in red, and what
we've measured, or sorry the measured is in red and the model
is in the solid black line, reproduces that annual flooding
cycle. And so each one of these peaks
represents the snow melt, and we can expect that every year.
Some years it's quite large like 2006, and other years, like
2007, it's relatively small. And we also know that in the dry
growing season in the summer months we're going to have very
low stream flow. There's going to be very limited
rainfall and very low water levels in the stream as we can
see in this picture here. Then we're also interested in
really the detailed processes within the meadow.
So we go from this very large scale hundreds of square
kilometers to just our small meadow ecosystem.
And within the meadow what we're really interested in is the
water table depth. And as we'll see, the depth to
the water table has a very large influence on the plants.
This is a groundwater dependent ecosystem.
When that water is limited in the summer, the plants really
rely on the shallow groundwater. And so the water table, shown
here as the distinction between the blue saturated zone and the
unsaturated soils above, is shown sloping down towards the
surface water. The water table intersects in
the stream bank, the surface water.
And so in this case it slopes down toward the surface water.
That would indicate groundwater flow from the aquifer, from
these sediments, towards the stream.
If it had sloped the other way, we would have water moving from
the surface water, from the stream, into the aquifer.
I'm not going to go into a lot of the details about our
modeling of this system, but it's very well known how water
moves within a groundwater system and we do finite element
modeling in which we can predict what depth the water table will
be at all locations using a mesh like this and at all times.
The tricky part is knowing what inputs we have of water into the
system, how water is entering from the boundaries.
These are known as our boundary conditions for the model.
And so we do have a lot of interactions with those larger
scale watershed processes. And when we're trying to, with
our groundwater flow model, just simulate the movement of
groundwater through this meadow aquifer we have to consider a
lot of other factors. One are stream aquifer
interactions. So we already saw the annual
pattern of the snow melt, the high spring water levels and
flooding, and then that summer recession.
Well as that stream level rises and falls, that affects this
interconnected water table. And so as the stream levels
rise, that can cause water to flow into the aquifer, and as
they fall, water can flow from the aquifer back into the
stream. So we have an exchange through
the stream bed. We also have precipitation, it's
relatively minor in the Sierras, that could be falling directly
on the meadow adding an extra source of water for our aquifer.
Or, more importantly, we have the snow melt processes which
adds a big, big pulse of water during the snow melt season.
Then we also have evapotranspiration in which
plants are pulling water out of the saturated zone and
discharging it into the atmosphere.
And then we also have a connection to the hill slope
where water is moving down through the soils on the hill
slope, maybe at the soil on the rock interface, and feeding the
meadow from the boundary. The first one of these processes
that I want to focus a little bit on are the stream aquifer
interactions that occur. And so what we did was we went
ahead and put a pressure transducer, actually a couple of
them, in the channel, and that allows us to measure the height
of water above that. And what we saw is that the
level of the water in the stream varies quite a bit.
This portion in here represents the spring snow melt, and this
is that recession of stream flow later in the growing season.
But we also see this very regular periodic fluctuation of
the stream flow, stream water level.
It turns out that that has a 24-hour period, and if you are a
hiker in snowy regions, you are probably very familiar with
this. You know you might cross a
little stream in the morning as you're going out on a hike, and
then when you come back in the late afternoon that little
stream is no longer a little stream.
It's something that might be dangerous to cross, and that's
because of the increased runoff because of the snow melt during
the day. And so we see this very regular
pattern which stream levels rise and fall during a 24-hour
period. Now those fluctuations are the
biggest during the peak snow melt, but I'm going to be
focusing here on this portion of the record in July, a two-week
period, in which those fluctuations look very, very
small. But if we blow them up, we can
see that they still have this very regular pattern.
There's only about a two-centimeter amplitude to that
fluctuation. And not only did we measure that
in the channel but we also put wells, or piezometers, in which
we could measure that in the groundwater system.
And one of them, well A, is closer to the stream, and then
we get progressively further away.
And what we see is we have the largest fluctuations in the
channel itself. And those fluctuations become
smaller, somewhat more muted, and they're actually slightly
lagged a bit as well as we move further away from the aquifer.
So we can see that this signal, these daily variations in stream
levels are propagating into the aquifer.
And we're able to model that, and we get a pretty good
representation of those fluctuations and the muting of
those fluctuations as we get further and further into the
aquifer. And that gives us some
confidence that we're really able to understand what's
causing these hydrologic fluctuations and that we're able
to model the groundwater flow which transmits those to these
wells further in the meadow from the stream.
Here's a picture of the stream, and you can imagine that
subsurface aquifer extending in this direction.
And that's the same graph that we have above.
So here is the stream itself, and so this is moving up and
down. And this is the water table at
different positions. And we can see that the water
table at different snapshots in time is moving up and down, and
that signal is being propagated inward.
So it's kind of like the stream level that is are going up and
down are someone holding a jump rope and moving it up and down,
and someone else is holding the other end still.
And so we see the water table rising and falling from the
stream end. And what this means is that a of
water is actually moving in and out of this aquifer on a daily
basis. And that has some important
implications because as water moves through sandy sediments it
can be filtered. In fact that's how a process
very similar how a water treatment plant works is they
sand filter to remove any particles.
And so a process like this may have some water quality
implications. The stream and aquifer is not
only connected on the main stem of the Tuolumne River but also
on some of these tributaries, these smaller tributaries that
enter. And these tributaries are
ephemeral streams. They do not run all season long
but dry up towards the end of the season.
And so Unicorn, for instance, dries at about this point in
time, and we can see that once Unicorn dries up, the wells
where we monitor the water table begin to drop quite quickly
after that. So the stream had been holding
up the water table, and then once the stream dries the water
table starts to drop very quickly.
And that same process here happened at Budd Creek where you
see the water table being relatively high, and then all
the sudden we get a much different drop in the water
table once Budd Creek went dry. So what we realized then was
that we have to pay a lot of attention to understanding when
these small channels are going to go dry because that's going
to really affect what the water table response is, which as
we'll see later is very tightly coupled to what the vegetation
looks like on the surface. The second part that I want to
focus on is precipitation and meadow snow melt.
So the adding of water from the top of the meadow system.
And here is a graph showing how much water would be entering the
meadow from the top due to snow melt.
And we can see that starts kind of in mid-March and may go
through the beginning of June. We can also see those kind of
daily fluctuations in the melt. And this is all simulated with
our model. We're also interested in the
hill slope flux, how much water is moving through the hill
slope, either on the land surface or in the soils, and
entering the meadow at the margin.
And here I want to focus on two different hill slopes.
One, that Pothole Dome, that granitic rock outcrop where we
took the pictures from. And then this forested hill
slope, this northern facing forested hill slope that is
quite shady. And so on Pothole Dome, which
has very good exposure, we have melts beginning in mid-April and
going through the early part of June.
Whereas on that southern hill slope, this green boundary, we
have hill slope flux that's beginning much later, maybe
three weeks later, and is continuing much longer into the
beginning of the summer. So these have quite different
timings, and we weren't surprised that they had
different timings but we were a little surprised at how big and
important that difference was. And so what we wanted to do is
test the importance of this in our model.
And so we ran our groundwater model in two different ways.
One in which we considered these differences in the timing of
hill slope fluxes in our orange boundary, the Pothole Dome
boundary, and our green boundary, the southern hill
slope boundary. And then compared that to a case
where we just averaged it and said maybe we'll just use the
average of all the boundaries and put that into them.
And what we found out was if we plotted the difference between
those simulations, if we really considered the individual
timings we would create, we would simulate water levels that
were about 30 centimeters higher here at the peak of snow melt
from Pothole Dome, and 30 centimeters lower near this
southern hill slope boundary. And on the contrary, we would
have that opposite effect if we would have simulated that way at
the peak of the southern hill slope boundary.
So that tells us it really does matter.
We can be off by almost a foot if we don't include this spatial
variability in hill slope fluxes.
That was a little disconcerting to us.
We were hoping that like everyone else we could just use
the average, but we found out know we actually really have to
care about those details and we have to be confident in our
ability to simulate fluxes from individual hill slopes.
The next hydrologic process that I want to talk about is
evapotranspiration. So that's the removal of water
from our meadow aquifer by plants.
And to understand some of this, we installed soil moisture
sensors in the soil. We have five of them at
different depths. And the first three of those
were put in this very fine grain soil here at the top.
And the bottom two were in a much coarser grain sand and
gravel layer. And what we saw was that as the
water table, which we know starts at the land surface when
the meadow is completely flooded, drops below each of
these sensors, we see quite a different response.
In these shallow ones, the water table probably dropped below
them sometime at the very, very early part of June, maybe even
the later part of May. And even after the water table
dropped below those, much of that soil moisture was retained.
It stayed in the soil. That's because this fine grained
soil acts as a sponge and holds it in place even though the
water table itself is dropping below each of these sensors.
That's quite different from this gravel area which acts more like
marbles. And as the water table drops
through the gravels, here we can see it crossing one of the
sensors, then once the water table drops below it, the soil
moisture drops very, very quickly.
No water is retained by those gravel, virtually no water.
And so we developed this conceptual model of the
stratigraphy or the layering of sediments within the meadow.
And we have this fine grain sediment, the sponge layer, on
the top. And on the bottom we have this
coarse grain sand and gravel or our marble layer on the bottom.
And this is actually quite typical of a lot of flood plains
because these gravels are deposited as a channel migrates
across the flood plain. So that's the stream bed, and it
moves across the flood plain. Then as you get towards the edge
of that stream you start to get some sands deposited in point
bar deposits, and then eventually you get over-bank
deposits that cover that up through time.
So you can see at this location you're having depositions of
sand, then on the top some of these finer grain materials
which are represented by those dashed lines.
So that's why we see this two-layered model with the fine
grain material on top. In looking at the plant water
use or the evapotranspiration we decided that in this environment
we had to maybe look at it in a little different way.
We know that these are groundwater dependent
ecosystems. They only occur when you have a
shallow water table. And so the plants are getting
some benefit from that shallow water table that enables them to
survive in this climate that has a very dry growing season.
And so we define this term the groundwater subsidy to plants as
any additional water that's available for root water uptake
resulting from those shallow water table conditions.
And so this kind of is meant to illustrate what I'm talking
about. So even if we didn't have a
shallow water table, represented by this blue line, we would have
some evapotranspiration. There would be some water that's
retained within that soil column that the plants can use during
the growing season, and they would also be able to use some
water that would fall during those dry summer months if any
occur. But when they have that shallow
water table, they have a lot more water available to them,
and so they're total evapotranspiration is going to
be greater. And we're determining that,
we're calling that extra water that's available the groundwater
subsidy. And we want to look at how that
groundwater subsidy changes based on what water table regime
we have. How the water table falls from
when it's at the land surface when it's flooded to later in
the season. So we can consider cases where
it drops very quickly through the growing season or cases
where it drops very slowly, and then we can also consider
different water table maximum positions, how far it actually
drops during that process. This is just an example here
with it dropping to three meters.
So to define this groundwater subsidy just a little bit more,
we can simulate this with our models what the transpiration
is. This is for a case with a think
soil layer. So here's our thick soil layer.
It's two meters thick. We can see this is soil moisture
here. There's a lot of soil moisture
in that layer. And since we have this as the
free drainage case, we have very little soil water retained in
the marble layer in our gravels. Whereas, oops, let me just
explain this first. So we can see that even in that
case, because we have so much water retained in that soil
layer, we're able to meet all of the transpiration demand here at
the early part of the season, and water only becomes limited
at maybe day 40 where we start to see reduced transpiration.
If, in this second case, we have a water table now, that water
table drops very quickly, we can see that that's creating
saturated conditions below this level.
And because of that we have a little bit more water available
for the plants. That means it's a little bit
longer before the plants start to have a reduction in their
transpiration. And the difference between that
green curve and that black curve represents the groundwater
subsidy. That's plotted here separately.
That's that extra water, about 2.5 centimeters over the growing
season, that is available to the plants because of this water
table regime. And if the water table drops a
little bit slower, we have a little bit greater subsidy.
It takes a little bit longer before we start to see water
limited conditions. And if it drops very slow, then
we have an even larger groundwater subsidy, about
8.7 centimeters. We can also do this for
different thicknesses of our sponge layer on the top.
And so here we see we have a much thinner sponge layer, so a
much smaller reservoir of water for plants.
And so as a result, when the plants use up water from that
reservoir we see a very sharp reduction in transpiration.
And the other consequence of that is that for these different
ground water regimes, these different water table regimes,
we can have the potential for a much larger groundwater subsidy.
And so these systems with thin soils really have a lot to gain
from having shallow water available to them.
When we looked at this for a whole bunch of cases, this is
all cases where the water table is dropping very fast but it's
dropping to different final positions and for different soil
thicknesses, we can see the general pattern that the
shallower the water table ends up at the end of the season, the
greater the groundwater subsidy. So that's kind of what we
expected, but we were able to put some numbers on that.
And you might say, well, what affects the depth to which the
water table ends up at the end of the year?
And one of the main controls on that is stream incision.
So in this case the water table may only drop at the end of the
year to the bottom of this channel, but if this channel has
become incised, it may drop much, much deeper.
We look here, oops, at different rates of water table decline,
and we can see that as the water table drops slower and slower we
get to from these blue colors to more of these warmer colors.
We get larger groundwater subsidies.
So the slower the water table drops, the greater our
groundwater subsidy. And we also see something else
that's really interesting. For a given change, in the water
table end, some parts of, some areas of these graphs are much
more sensitive. So right here if we were to have
about 75 centimeters of incision, we would have a huge
change in our groundwater subsidy going from this orange
all the way to the green. So that would be an area that's
highly vulnerable to hydrologic change to incision.
That would be a meadow that we would be really concerned that
if we altered the channel we're going to see a major effect on
the type of vegetation that we see.
Whereas in other parts of these, for instance here, if we had
that same amount of incision we'd see a relatively minor
change in the groundwater subsidy.
So that's an area that's more resilient.
Something that even though we'd see a large change in the water
table regime, we wouldn't really see that much of a change on how
it affects the plants. And so as a result we can see
that as the water table regime goes from fast draining to slow
draining, we increase the ecosystem resilience of these
meadows, and as the soils get thicker and they have a larger
reservoir to store soil water, we see more resilience of these
ecosystems. So the summary of meadow
hydrology, and I know these graphs are all very small, is
that we have, our able to simulate meadow melt, that's the
area where we see the first pulse of water kind of entering
the meadow aquifers. We also then start to see water
coming off of Pothole Dome, these granitic domes in the
watershed. And then after that, at much
later times, we start to see hill slope fluxes coming from
the forested and north facing hill slopes.
So each of these has a different timing in which they're feeding
water to the meadows in this region.
We also have an influence from the Tuolumne River itself as the
water levels in the river go up and down, both on the annual
cycles with the annual floods and on diurnal cycles with the
snow melt. And then we have ET, which is
relatively constant during the season unless water starts to
become limiting. And if, and only if, we know all
of these different processes affecting the boundaries of our
groundwater model can we actually simulate groundwater
flow through the meadow. And so we tested that with some
measurements, and here on this plot we have distance away from
the edge of the meadow. So the stream is right about
here. And each of these dots
represents measurements of the water table that we made at
different times. And we can see that those agree
fairly well. We're often able to simulate the
water table within about 10 or 15 centimeters of where it
actually is. So this tells us that we were
able to get it right spatially, and then at some of those wells
we also monitored it with pressure transducers, and we're
also able to get the temporal changes in water level, the
seasonality of the water table rise and fall at several
different wells. So that closes off the meadow
hydrology portion of this, and now I want to talk a little bit
about the vegetation communities we see and connect that with
some of the hydrologic processes that we've already started to
discuss. So the wettest of the
communities we see is called the wet meadow vegetation community.
And it's dominated by these mountain willow and blister
sedge plant communities. Those are really kind of key
species for identifying wet meadows.
In the moist meadow community, really the most important
species is this short-haired weed grass.
You can see it really dominates many parts, like this picture
here shows, of the meadow. And then we have the dry meadow
community. And that dry meadow community
might have this threadleaf sedge or some sagebrush or even some
invasion of lodgepole pine saplings.
So really much dryer sorts of communities starting to appear
in this part of the meadow. And the National Park Service
has gone out and measured the composition of the vegetation at
all of these individual plots, and we went through and plotted
them up so that the wet community showed, species that
fell in the wet community are plotted as blue.
So we can see a lot of blue species, a lot of the wet meadow
vegetation occurring in this region of the meadow and up
through here by Budd Creek. The moist community is found
predominately in this upper terrace of the meadow, up here
at the edge of the meadow, and then over in this terrace near
Unicorn. And then the dry vegetation
community really isn't found that extensively throughout the
meadow at all, but the little bit of it that we do see is
largely up here in this upper terrace near Delaney and some in
this region here. This is the road, the park road.
If you've driven on Highway 120 through the park, you've driven
right on that road there. So now we are able to simulate
what's called the water table hydrograph, the variations in
the water table position. If this is the land surface and
this is the water table as it changes throughout the year.
And we have that at all locations within the meadow
including all of those locations where the National Park Service
measured the vegetation composition.
And so what we thought is, can we in any way boil down all the
information that's held within this water table hydrograph into
what really matters to the plants?
And so we thought about that and thought, well, one thing that is
kind of stressful for the plants is when the water table becomes
really close to the land surface and the entire root zone becomes
saturated, that causes water-logged conditions and
water-logged conditions make it very, very hard to get oxygen
into the roots. And so we can have
oxygen-limited conditions that can inhibit plant growth.
And so we decided there's probably some threshold.
When the water table is above a certain threshold, that starts
to cause stress for the plants. And we said, well, if we could
find that threshold, we could measure how long and how severe
that sort of oxygen stress is by taking the area between the
water table hydrograph and that threshold.
And so that number that we would get representing that area
represents oxygen stress to the plants.
There's also some stress that would be caused in this
groundwater dependent ecosystem when the water table gets too
low, when it's not really accessible anymore to the
plants. And so we have then defined
a similar characteristic, the water stress level, that tells
us when it's too dry for the plants.
And we found that if we consider a threshold for oxygen stress of
25 centimeters and a water stress of 75 centimeters, those
were the most predictive. Those gave us the best ability
to predict vegetation composition.
So we're going to use those thresholds through the rest of
this talk. So here is that water stress
value and the oxygen stress value, and for the dry meadow
community we see that we have the most probability to see
those dry vegetation only when we have very high water stress.
So that's what we would expect is that we're going to see those
only when we have a lot of water stress.
That's what they're adapted to is making the most of limited
water supplies. The moist community is found
kind of right here in the middle where we see some oxygen stress,
but importantly there's a threshold, a very strong
threshold actually, where if we have more oxygen stress than 20
we're really going to see virtually none of this moist
meadow species in those regions. And then we see another large
region of this parameter space where we are likely to see some
moist meadow vegetation but not nearly as much as we'd see in
kind of the optimal hydrologic niche, this kind of middle
ground for the moist meadow community.
And then for the wet meadow community we see kind of this
stronger gradient that the more water stress there is, sorry the
more oxygen stress there is, the higher the water tables, the
more wet meadow vegetation we're going to find in that given
location. And so now we have all the
pieces that we really need to develop a hydroecologic
framework. We've modeled the watershed
scale processes. That gives us stream flow in the
main Tuolumne. And with a little bit of
hydraulic modeling we can then convert those stream flows to
water levels within the channel. And then we also have, from our
watershed scale model, the snow melt fluxes, those hill flow
fluxes, and the stream discharge that we use to get stream level.
And so all of those pieces can then be fed into our groundwater
flow model. So the boundary conditions for
our groundwater system. And with that we're then able to
predict water table depth at all places and at all times with our
model. The Park Service had that
vegetation data set for us, and so we were able to then compare
what vegetation communities we observed to some characteristics
of the groundwater regime how deep the water table was.
And with that we created those hydrologic niches that told us
how likely we were to find a given vegetation community based
on a water stress and oxygen stress variable that we defined.
And then we can apply that relationship of the hydrologic
niche and our groundwater flow data, and we can predict
vegetation distributions everywhere in the meadow for
each of our three communities. And we can do that not only for
current conditions but we can start to look at asking
questions. What if the runoff regime
changes? What if there's climate change
and we have a different hydrology in this system, how is
that going to affect vegetation? Or we can also ask questions
about what if we restore the Tuolumne River in some way?
What if we try to make it a little bit more narrow, how is
that going to affect the hydrology and the vegetation?
So now we have really a tool that managers could use to help
enact some change on this ecosystem.
Here are the distributions of meadow vegetation that we
simulated. And we can see, first of all,
that we get a lot of those fine scale features in the vegetation
patterning that we can see when we're on the ground and that we
can see kind of from that picture looking down from
Pothole Dome over the meadow. So we reproduced a lot of those
small features, but we also reproduced a lot of the larger
scale variabilities. So this area here is where we
find the most wet vegetation, and that's what the National
Park Service data set also showed.
Here for the moist vegetation community we really see a little
bit of it everywhere, but the highest probability of finding
it is up here on this dryer terrace, as well as kind of at
the edge of the meadow throughout the side, the
southern boundary. And then the dry meadow
vegetation we really don't find that in the meadow very often,
but where we do start to see it is somewhat on this upper
terrace here as well as towards the edge of the meadow,
particularly in this region. And so that's where we had a
little bit higher probability of finding it based on our model
results. So the last part of this talk
is now kind of getting into exciting questions of
restoration of meadows and understanding processes by which
meadows become degraded. Estimates have indicated that
98% of meadows in the Sierra Nevada are degraded to some
extent. So it's a pretty pervasive
problem in the Sierra Nevadas. So first I want to talk just a
little bit about pristine meadow function.
And so in a pristine meadow, the elevation of the stream water
surface is very close to the elevation of the meadow flood
plain. And so during that spring snow
melt, water inundates the meadow, and often these meadows
can look like large lakes during the snow melt period.
And that provides some storage for water on the meadow.
That can reduce down stream flooding.
That recharges this meadow aquifer and creates a very
shallow water table, and that shallow water table supports the
native sedges and rushes and wet meadow vegetation that these
communities are known for, these meadows are known for.
In a degraded meadow, often the channel has become incised.
And that channel incision can occur from overgrazing of these
lands either by sheep or by cattle.
And the physical action by the hooves can cause erosion of the
stream bed, the cattle tend to congregate in the channel
itself, and that can cause down cutting and the channel cuts
through these sediments. Or we can also have logging of
these watersheds and that causes a flashier hydrologic response.
So if you clear a lot of the forests, then when it does rain
there's a lot more runoff, that creates higher stream flows.
During rain and snow events and even during the snow melt those
higher stream flows cause higher velocities, higher velocities
cause more erosion and that can also cause down cutting of these
channels. But either way, once this
channel is down cut and we have an incised channel, then we have
this draining to a lower level. So the water table has dropped
from where it was in our pristine meadow to a much lower
elevation. And when the water table is
deeper, then we have a transition from the wet meadow
vegetation to sagebrush and dryland grasses.
Here we can see a degraded meadow with a lot of sagebrush
present. So if we look at Tuolumne
Meadows and the vegetation patterning along just a given
transect, we see kind of what we expect to see.
This meadow is only slightly degraded, but it's really in
pretty good condition. So we see the wet meadow
vegetation here near the river. So this is part B of this
transect going from the river up towards the road.
So by the river up towards the road.
We see the wet meadow vegetation near the river, and then we get
into a little bit dryer communities as we go up onto
this terrace. So I would call that normal
vegetation pattern. Wetter communities being found
near the channel. That's exactly opposite what we
find in other meadows. And so in this meadow, in the
northern Sierra Nevadas, we see the driest vegetation
communities immediately adjacent to the channel.
So here's the channel. This kind of lighter colored
vegetation is sagebrush. And that's this xeric or dry
vegetation community. And we see the wet meadow
community, the mesic community, here towards the margin of the
meadow. And so what causes reverse
vegetation patterning in some meadows but not others?
Well, our first clue into what was causing this came from some
near infrared imagery that we collected.
And in near infrared imagery healthy wet meadow vegetation
appears this really bright, somewhat obnoxious, pink color.
Whereas dryer xeric vegetation, sagebrush, appears this kind of
grayish color. And so the difference shows up
very, very clearly. And what we saw when we looked
at these three meadows and several others was that the
swath of xeric vegetation near the channel was narrow in some
meadows and got wider in other meadows.
And what we found that was related to was the depth of
stream incision. So there is increasing
degradation and increasing stream incision as we move down
in these three photos. And that xeric vegetation
community comes at the expense of the wet vegetation community
which we would have expected to find pretty much across the
meadow under pristine conditions.
And so that was our first clue. When we ran through a
groundwater model and simulated different amounts of incision,
we found that as we had more incision of the channel, the
channel kind of runs through the center part of this meadow, we
had a much deeper water table near the channel.
And when we tied that to the vegetation communities, that
predicted this widening swath of xeric vegetation as we had
increased incision. And so what was quite
counterintuitive, initially, was having this dry vegetation
community near the channel now made a lot of sense in that as
the stream incised it pulled the water table down, and it pulled
it down most strongly immediately adjacent to the
channel. And so the location that had the
greatest depth to the water table here is actually
immediately next to the channel, and that's where we saw the
driest vegetation communities, and that's what causes that
reverse vegetation patterning. Here I just want to touch on
some of the restoration techniques.
Here we have a pristine channel. It's a relatively narrow
channel, and in addition to incision, another common form of
degradation of meadows is channel widening.
And that's probably what we've experienced a little bit of in
Tuolumne Meadows is widening of the channel.
And so as that channel becomes wider, it has a greater capacity
to transmit flow. And so for any given stream
discharge we're going to have a lower water level in the
channel. That means we'll also have a
lower water table in the meadow system.
So what can we do about that? Well one thing we could to is
plant willows. Those willows provide a little
bit of resistance. They help capture sediments, and
they help to narrow in this channel.
So they may raise water levels just a little bit.
So that might have a positive effect.
One thing we could do, probably not in Tuolumne Meadows but in
other lower elevations where beavers were native, we could
introduce beavers. And it's turned out that this is
actually quite effective. The beavers build little dams
and they change the flow and they raise the water table over
sometimes extensive parts of the flood plain and the vegetation
community responds accordingly. Gravel augmentation has also
been proposed. In this technique you just add
gravel so you kind of raise the stream bed a little bit so that
you raise the water level. Addition of woody debris that's
more resistant to flow may back up water just a little bit and
increase water levels a little bit.
Addition of rip rap weirs or veins.
This is kind of a combination of the beaver dams and the gravel
augmentation. Instead of adding gravel
everywhere, you only add it at kind of intervals of maybe 20 to
50 feet. And so you have kind of little
gravel dams essentially along the stream.
People have actually tried that technique in some parts of the
Sierras. And then also channel
recreation. And so completely destroy the
existing channel and try to create a new channel at some
other portion. This is really only a technique
that you would really try under very degraded ecosystems.
So the current work that we're doing is starting to look at
what the effects climate change are.
And so here are our stream discharge records that are
simulated for current conditions.
This is a decade long average of snow melt discharge, starting
from the current decade and going to the mid part of this
century to the late part of this century.
And what we see as we go from the current decade to the later
part of this century, we see a huge shift in the timing of the
snow melt. So as temperatures warm, the
snow melt occurs earlier and earlier, and in this picture we
can see that it will be on the order of a month earlier.
And so why that's important for the plants is that's the time at
which we start to see the recession of the water table in
the meadow ecosystem. And so if that starts a month
earlier, we're likely to see a lot dryer conditions by the end
of the summer, and the plants will have to tolerate those for
a much longer portion of the growing season.
So they'll be experiencing a lot more water stress than they do
under current conditions. And so here we see a nice sunset
in the Tuolumne watershed, indicates that we're wrapping up
the talk and bringing us to the conclusions.
And so what we've learned through all of this is that
evapotranspiration, hill slope fluxes, snow melt and stream
interactions all affect the spatial and temporal groundwater
regimes of a meadow. So we have to really carefully
consider all of these processes if we hope to be able to
simulate the water table regime accurately.
We also have found that meadows are groundwater dependent
ecosystems, and they rely on a shallow water table to support
the wet meadow vegetation communities.
And as a result, the spatial patterning we see of vegetation
communities on the surface is really a reflection of the
subsurface groundwater processes that are in play.
And then finally, the restoration and conservation of
meadow resources is critical. They provide a lot of ecosystem
services, they provide a lot of habitat, and they're well-loved
by a lot of people. There is no one size fits all
solution in terms of meadow restoration.
And strategies must be tailored to site specific conditions.
And I think that this hydrologic framework that we've been
developing gives us a tool to add a scientific basis towards
restoration practices, to really choose between one restoration
strategy and another. So I'd like to acknowledge our
funding. The National Science Foundation,
the Environmental Sustainability Program, the Yosemite National
Park for letting us do this research, for letting us hide
all of our equipment within Tuolumne Meadows out of sight of
the visitors, and to Chris Lowry who was the postdoc who did much
of the modeling on this project, and my collaborator Jessica
Lundquist at the University of Washington.
And then all of these other folks, most of whom are at the
National Park. So thank you.
[APPLAUSE]