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everybody, to Wednesday Nite >> Tom Zinnen: Welcome,
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
coming to Wednesday Nite at the Public TeleviLab., thanks for
We do this every Wednesday
Tonight I'm delighted to have Steven Loheide be the presenter.
He's going to talk about his
ecohydrology in the alpine work in hydroecology and
He's an assistant professor in meadows in the Sierra Nevada.
Environmental Engineering here the Department of Civil and
He received his bachelor's at UW Madison.
geology from the University inof Northern Iowa in 1999,and
his master's in geology from Indiana University in 2001,
and his PhD in hydrogeology from
Steve's research focuses on the interactions between ecological
and hydrological processes in natural and built systems.
His approaches use a combination
and numerical modeling to understand the feedbacks between
vegetation patterning, otherwise
grow, vegetative water use, soil moisture availability,y
groundwater and stream aquifer
His work is focused on improving interactions.
food plain, meadow and wetland the scientific basis for stream,
current and future climatic restoraticonditions.under the
So if any of you have interests in stream restoration, I think
his work is going to be very
Please join me in welcoming Steve Loheide to Wednesday Nite
at the Lab. [APPLAUSE]
Steve Loheide: Thank you
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
how the movement of water through the landscape, over the
land surface, and through the subsurface affect ecological
processes.
processes means vegetation In this case ecological
composition within high pattelevation meadows.ion
This is Tuolumne Meadows in the Sierra Nevada of California.
By some of the smiles on
some of you have been there, and people's faces, I can tell that
your heart as it does in mine. this place holds a fond place in
on are these interactions Abetween hydrology and ecology.g
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
And then at a very fine scale, flow is generated.
groundwater within the meadows. After we go through the basics
of hydrological processes, then I'll talk about the meadow
vegetation communities that we
start to link those with the see in Tuolumne Meadows and I'll
meadow. And then once we've developed
this hydroecologic framework, an understanding how hydro are
linked, then we'll apply that to
degraded and potentially how we ucould start to restore these
meadows.
perhaps the most well loved and iconic of high elevation meadows
in Yosemite National Park.
Sierra Nevadas of California, and this is an outline of the
meadow boundary.
treeless and flat region within So the the mountains.is this
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
That's this granitic outcrop Pothole Dome.
And that's where this photograph here.
over the meadow. was taken, kind of looking out
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
About 80% of the precipitation Wisconsin.
in the form of snow. falls during the winter months
deep snow pack in the spring And that accumulates quite a
And then we have the peak melt when that begins to melt.
meadow flooding. in May or June, and that creates
meadow is now inundated with You can see that much of this
And then as we enter the growing water.
to recede and water levels, both season, the stream flow begins
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
to have the melt. begYou can see the snow packtart
diminishing here in the foreground.
And in the background the meadow
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
The snow melt is the major interested in.
the Sierra Nevada. hydrologic event every year in
understand something about that And so wesnow melt.e able to
And so we're not working on this
also have collaborators, project by ourselves, but we
University of Washington, and the work that they do is really
involved in the larger watershed
And so they do distributed scale modeling.
they break down the watershed hydrologinto a grid.in which
And really what they're looking at is how the water is processed
in each of these grid cells.
this parcel of land either How precipitation that lands on
or runs off and flows down the infiltrates into the subsurface
And if it does infiltrate it can hill slope.
used by plants, taken up by be stoplants and used ford then
evapotranspiration or ET, or it can move through that soil
down the hill slope.
the hill slope, either on the Asurface or in the subsurface,wn
will eventually encounter a stream and start to generate
stream flow.
generated in each of these small And so stream flow will be
How do they measure whether it goes through the grid or
flows at different levels?
Well, part of the water is How do they determine that?
You can see overland flow when measured on the land surface.
This is run off that might occur it occurs.
And the part that falls into the on a hill slope.
the groundwater system. landSo we can monitor thenters
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
And this is the Tuolumne water.
Here is the meadow that we're interested in, Tuolumne Meadows.
And this is about 250 square kilometers.
And it's very mountainous.
here up the Tuolumne towards We can see here we're looking
They're still within the these large mountains.
over here. watershed, this region kind of
we reproduce, the modeled Aversion, is in red, and whatt
we've measured, or sorry the
is in the solid black line, reproduces that annual floodingl
cycle.
represents the snow melt, and we And so each one of these peaks
Some years it's quite large like 2006, and other years, like
2007, it's relatively small.
growing season in the summer And we also know that in the dry
low stream flow. months we're going to have very
rainfall and very low water Tlevels in the stream as we cand
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
And within the meadow what we're meadow ecosystem.
water table depth. really interested in is the
the water table has a very large And as we'll see, the depth to
This is a groundwater dependent influenecosystem.plants.
When that water is limited in the summer, the plants really
rely on the shallow groundwater.
here as the distinction between the blue saturated zone and the
unsaturated soils above, is shown sloping down towards the
surface water.
the stream bank, the surface The water table intersects in
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
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
modeling of this system, but of the details about our
moves within a groundwater it's very well known how water
modeling in which we can predict system and we do finite element
be at all locations using a mesh what depth the water table will
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
scale watershed processes. interactions with those larger
our groundwater flow model, just And when we're trying to, with
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
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
stream. So we have an exchange through
the stream bed. We also have precipitation, it's
relatively minor in the Sierras,
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
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
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.
ahead and put a pressure And so what we did was we went
them, in the channel, and that transducer, actually a couple of
of water above that. allows us to measure the height
level of the water in the stream And what we saw is that the
This portion in here represents the spring snow melt, and this
is that recession of stream flow
But we also see this very regular periodic fluctuation of
the stream flow, stream water level.
It turns out that that has a
hiker in snowy regions, you are 24probably very familiar withe a
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
stream is no longer a little late afternoon that little
It's something that might be stream.
because of the increased runoff because of the snow melt during
the day.
pattern which stream levels rise And so we see this very regular
period. Now those fluctuations are the
biggest during the peak snow
focusing here on this portion of melt, but I'm going to be
period, in which those the record in July, a two-week
small. fluctuations look very, very
see that they still have this But very regular pattern.e can
There's only about a
fluctuation. two-centimeter amplitude to that
in the channel but we also put And not only did we measure that
we could measure that in the wells, or piezometers, in which
And one of them, well A, is closer to the stream, and then
we get progressively further
And what we see is we have the away.
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
aquifer. further and further into the
confidence that we're really And that gives us some
causing these hydrologic able to understand what's
to model the groundwater flow fwhich transmits those to thesee
wells further in the meadow from
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
So here is the stream itself, and so this is moving up and
down. And this is the water table at
different positions.
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.
implications because as water moves through sandy sediments it
can be filtered.
very similar how a water In fact that's how a process
sand filter to remove any treatment particles.s is they
And so a process like this may
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.
ephemeral streams. And these tributaries are
but dry up towards the end of They do nothe season.eason long
And so Unicorn, for instance, dries at about this point in
time, and we can see that once
where we monitor the water table Unicorn dries up, the wells
after that. So the stream had been holding
up the water table, and then
table starts to drop very once the strquickly.s the water
And that same process here
see the water table being hrelatively high, and then allou
the sudden we get a much
table once Budd Creek went dry. So what we realized then was
that we have to pay a lot of
these small channels are going attention to understanding when
to really affect what the water ttable response is, which asng
we'll see later is very tightly coupled to what the vegetation
looks like on the surface.
focus on is precipitation and The semeadow snow melt.want to
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.
daily fluctuations in the melt. And this is all simulated with
our model.
hill slope flux, how much water Wis moving through the hille
slope, either on the land surface or in the soils, and
entering the meadow at the
And here I want to focus on two different hill slopes.
One, that Pothole Dome, that
took the pictures from. gAnd then this forested hillwe
slope, this northern facing forested hill slope that is
quite shady.
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.
timings, and we weren't So surprised that they hadent
different timings but we were a little surprised at how big and
important that difference was.
test the importance of this in And so what we wanted to do is
And so we ran our groundwater model in two different ways.
One in which we considered these
hill slope fluxes in our orange dboundary, the Pothole Domef
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
from Pothole Dome, and 30 hcentimeters lower near thist
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
variability in hill slope if we don't fluxes. this spatial
That was a little disconcerting to us.
We were hoping that like
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
evapotranspiration. So that's the removal of water
from our meadow aquifer by
And to understand some of this, we installed soil moisture
sensors in the soil.
different depths. And the first three of those
were put in this very fine grain
And the bottom two were in a soil here at the top.
gravel layer. And what we saw was that as the
water table, which we know
the meadow is completely starts at the land surface when
these sensors, we see quite a floodifferent response.ch of
In these shallow ones, the water
them sometime at the very, very early part of June, maybe even
the later part of May.
dropped below those, much of And even after the water table
It stayed in the soil. that soil moisture was retained.
soil acts as a sponge and holds Thit in place even though thened
water table itself is dropping
That's quite different from this below each of these sensors.
marbles. gravel area which acts more like
through the gravels, here we can And as the water table drops
sensors, then once the water see it crossing one of the
moisture drops very, very table dropsquickly.t, the soil
No water is retained by those gravel, virtually no water.
And so we developed this
stratigraphy or the layering of sediments within the meadow.
And we have this fine grain sediment, the sponge layer, on
the top.
coarse grain sand and gravel or our marble layer on the bottom.
And this is actually quite
because these gravels are deposited as a channel migratess
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
deposits that cover that up eventuathrough time.ver-bank
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
So that's why we see this two-layered model with the fine
grain material on top.
use or the evapotranspiration we In looking at the plant water
we had to maybe look at it in a decidlittle different way.onment
We know that these are
ecosystems. They only occur when you have a
shallow water table.
some benefit from that shallow And so the plants are getting
survive in this climate that has water table that enables them to
And so we define this term the a very dry growing season.
any additional water that's available for root water uptakes
resulting from those shallow
And so this kind of is meant to illustrate what I'm talking
about.
shallow water table, represented So even if we didn't have a
some evapotranspiration. by this blue line, we would have
retained within that soil column There would be some water that's
the growing season, and they that the plants can use during
water that would fall during would also be able to use some
occur. those dry summer months if any
water table, they have a lot But when they have that shallow
and so they're total more water available to them,
be greater. eAnd we're determining that,to
we're calling that extra water
subsidy. that's available the groundwater
groundwater subsidy changes based on what water table regime
we have.
when it's at the land surface when it's flooded to later inm
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.
with it dropping to three This is jumeters.xample here
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.
here. WThere's a lot of soil moisturee
in that layer.
free drainage case, we have very And since we have this as the
the marble layer in our gravels. little soil water retained in
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 early part of the season, tand water only becomes limitedt
at maybe day 40 where we start
If, in this second case, we have to see reduced transpiration.
table drops very quickly, we can a water table now, that water
saturated conditions below this see that level. creating
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
transpiration. to have a reduction in their
green curve and that black curve And the difference between that
subsidy. represents the groundwater
That's that extra water, about That's plotted here separately.
season, that is available to the 2.5 centimeters over the growing
table regime. And if the water table drops a
little bit slower, we have a
It takes a little bit longer little bit greater subsidy.
limited conditions. And if it drops very slow, then
we have an even larger
8.7 centimeters. groundwater subsidy, about
different thicknesses of our We can also do this for
And so here we see we have a sponge layer on the top.
much smaller reservoir of water much thinnfor plants.ayer, so a
And so as a result, when the
reservoir we see a very sharp preduction in transpiration.t
And the other consequence of
ground water regimes, these tdifferent water table regimes,t
we can have the potential for a much larger groundwater subsidy.
And so these systems with thin
from having shallow water soils ravailable to them.o gain
When we looked at this for a
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
greater the groundwater subsidy. up at the end of the season, the
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
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
become incised, it may drop channelmuch, much deeper.nel has
We look here, oops, at different
and we can see that as the water rates of water table decline,
get to from these blue colors to table drops slower and slower we
We get larger groundwater more of tsubsidies.r colors.
So the slower the water table drops, the greater our
groundwater subsidy.
that's really interesting. And we also see something else
table end, some parts of, some For a given change, in the water
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
if we altered the channel we're going to see a major effect on
the type of vegetation that we
Whereas in other parts of these, see.
that same amount of incision for instance here, if we had
change in the groundwater we'd see asubsidy.ely minor
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
it affects the plants. see that much of a change on how
that as the water table regime And so as a result we can see
draining, we increase the gecosystem resilience of thesew
meadows, and as the soils get thicker and they have a larger
reservoir to store soil water,
ecosystems. we sSo the summary of meadowese
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.
coming off of Pothole Dome, Wethese granitic domes in theer
watershed.
later times, we start to see And then after that, at much
the forested and north facing hill slophill slopes.ing from
So each of these has a different
water to the meadows in this timing in which they're feeding
We also have an influence from region.
water levels in the river go up thand down, both on the annualhe
cycles with the annual floods and on diurnal cycles with the
snow melt.
relatively constant during the And then we have ET, which is
become limiting. season unless water starts to
of these different processes And if, and only if, we know all
groundwater model can we affecting the boundaries of our
flow through the meadow. actually simulate groundwater
measurements, and here on this And so we tested that with some
the edge of the meadow. plSo the stream is right aboutm
here. And each of these dots
represents measurements of the
different times. water table that we made at
fairly well. And we can see that those agree
water table within about 10 or We're often able to simulate the
actually is. 15 centimeters of where it
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
different wells. rise and fall at several
hydrology portion of this, and So that closes off the meadow
about the vegetation communities now I want to talk a little bit
some of the hydrologic processes we see and connect that with
discuss. that we've already started to
communities we see is called the So the wettest of the
And it's dominated by these wemountain willow and blisterty.
sedge plant communities.
species for identifying wet Those are meadows.ind of key
In the moist meadow community,
species is this short-haired really the most important
You can see it really dominates many parts, like this picture
here shows, of the meadow.
community. And then we have the dry meadow
might have this threadleaf sedge And that dry meadow community
invasion of lodgepole pine or some sagebrush or even some
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,
them up so that the wet acommunity 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
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
Unicorn. then over in this terrace near
community really isn't found And then the dry vegetation
meadow at all, but the little thbit of it that we do see ishe
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
measured the vegetation where the composition.k Service
And so what we thought is, can
information that's held within we in any way boil down all the
what really matters to the this water table hydrograph into
And so we thought about that and plants?
kind of stressful for the plants thought, well, one thing that is
really close to the land surface is when the water table becomes
saturated, that causes and the entire root zone becomes
water-logged conditions make it very, very hard to get oxygen
into the roots. And so we can have
oxygen-limited conditions that
And so we decided there's cprobably 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
water table hydrograph and that taking ththreshold.ween the
And so that number that we would
represents oxygen stress to the get repreplants. that area
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
And we found that if we consider plants.
25 centimeters and a water a threshold for oxygen stress of
were the most predictive. sThose gave us the best ability
to predict vegetation composition.
So we're going to use those thresholds through the rest of
this talk.
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
So that's what we would expect we have very high water stress.
only when we have a lot of water is that we're going to see those
That's what they're adapted to stress.
water supplies. iThe moist community is found
kind of right here in the middle
but importantly there's a wherthreshold, a very strongess,
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.
region of this parameter space And then we see another large
moist meadow vegetation but not wnearly as much as we'd see ine
kind of the optimal hydrologic
ground for the moist meadow niche, tcommunity.f middle
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
more wet meadow vegetation we're higher the water tables, the
location. gAnd so now we have all the
pieces that we really need to develop a hydroecologic
framework.
scale processes. We've modeled the watershed
main Tuolumne. That gives us stream flow in the
hydraulic modeling we can then And with a little bit of
water levels within the channel. convert those stream flows to
watershed scale model, the snow Anmelt fluxes, those hill flowr
fluxes, and the stream discharge
And so all of those pieces can that we use to get stream level.
flow model. tSo the boundary conditions forr
our groundwater system. And with that we're then able to
predict water table depth at all
model. places and at all times with our
vegetation data set for us, and so we were able to then compare
what vegetation communities we
of the groundwater regime how obsdeep the water table was.tics
And with that we created those
how likely we were to find a hydrologic niches that told us
on a water stress and oxygen given vegetation community based
And then we can apply that stress variable that we defined.
niche and our groundwater flow relationship of the hydrologic
vegetation distributions data, and we can predict
each of our three communities. everywhere in the meadow for
current conditions but we can And start to look at asking for
questions.
changes? What if the runoff regime
and we have a different hydrology in this system, how is
that going to affect vegetation?
about what if we restore the Or we can also ask questions
What if we try to make it a little bit more narrow, how is
that going to affect the
So now we have really a tool hydrology and the vegetation?
enact some change on this that manageecosystem.se to help
Here are the distributions of meadow vegetation that we
simulated.
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.
find the most wet vegetation, Sand that's what the National
Park Service data set also
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
southern boundary. throughout the side, the
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.
little bit higher probability of And so that's where we had a
results. finding it based on our model
is now kind of getting into So texciting questions oftalk
restoration of meadows and
meadows become degraded. understanding processes by which
98% of meadows in the Sierra Estimates have indicated that
extent. NSo it's a pretty pervasive
problem in the Sierra Nevadas.
little bit about pristine meadow So first I want to talk just a
And so in a pristine meadow, the function.
surface is very close to the elevation of the stream water
plain. elevation of the meadow flood
melt, water inundates the meadow, and often these meadows
can look like large lakes during
And that provides some storage the snow melt period.
That can reduce down stream for watflooding. meadow.
That recharges this meadow
shallow water table, and that shallow water table supports the
native sedges and rushes and wet
communities are known for, these meadow vegetation that these
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
stream bed, the cattle tend to hooves can cause erosion of the
itself, and that can cause down cutting and the channel cuts
through these sediments.
these watersheds and that causes Or we can also have logging of
So if you clear a lot of the a flashier hydrologic response.
there's a lot more runoff, that focreates higher stream flows.n
During rain and snow events and even during the snow melt those
higher stream flows cause higher
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
elevation. pristine meadow to a much lower
deeper, then we have a And when the water table is
vegetation to sagebrush and transitdryland grasses. meadow
Here we can see a degraded
present. meadow with a lot of sagebrush
Meadows and the vegetation So if we look at Tuolumne
transect, we see kind of what we patterning along just a given
This meadow is only slightly expect to see.
pretty good condition. degraded, but it's really in
vegetation here near the river. So this is part B of this
transect going from the river up
So by the river up towards the towards the road.
We see the wet meadow vegetation road.
into a little bit dryer necommunities as we go up ontot
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.
northern Sierra Nevadas, we see And the driest vegetation the
communities immediately adjacent
So here's the channel. to the channel.
vegetation is sagebrush. This kind of lighter colored
vegetation community. AAnd we see the wet meadowry
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
near infrared imagery that we was causingcollected. from some
And in near infrared imagery
appears this really bright, somewhat obnoxious, pink color.
Whereas dryer xeric vegetation, sagebrush, appears this kind of
grayish color.
very, very clearly. And what we saw when we looked
at these three meadows and
swath of xeric vegetation near several others was that the
meadows and got wider in other the channel was narrow in some
And what we found that was meadows.
stream incision. related to was the depth of
degradation and increasing So there is increasing
in these three photos. stream incision as we move down
community comes at the expense And that xeric vegetation
which we would have expected to offind pretty much across thety
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
channel kind of runs through the incision of the channel, the
had a much deeper water table centernear the channel.dow, we
And when we tied that to the
predicted this widening swath of vegetation communities, that
increased incision. xeAnd so what was quitehad
counterintuitive, initially, was
community near the channel now having this dry vegetation
the stream incised it pulled the made a lot of sense in that as
it down most strongly waimmediately adjacent to theed
channel.
greatest depth to the water And stable here is actuallyd the
immediately next to the channel,
driest vegetation communities, and that's where we saw the
reverse vegetation patterning. and that's what causes that
some of the restoration Here I jtechniques. touch on
Here we have a pristine channel.
channel, and in addition to incision, another common form of
degradation of meadows is channel widening.
And that's probably what we've
Tuolumne Meadows is widening of experiencthe channel.bit of in
And so as that channel becomes
to transmit flow. wiAnd so for any given streamity
discharge we're going to have a lower water level in the
channel.
lower water table in the meadow That meanssystem.also have a
So what can we do about that? Well one thing we could to is
plant willows.
bit of resistance. Those willows provide a little
they help to narrow in this They help cachannel.diments, and
So they may raise water levels
So that might have a positive just effect.e bit.
One thing we could do, probably not in Tuolumne Meadows but in
other lower elevations where
introduce beavers. beavers were native, we could
actually quite effective. AThe beavers build little damsis
and they change the flow and
sometimes extensive parts of the they raise the water table over
community responds accordingly. fGravel augmentation has alson
been proposed.
gravel so you kind of raise the stream bed a little bit so that
you raise the water level.
more resistant to flow may back Aup water just a little bit and
increase water levels a little
Addition of rip rap weirs or veins.
This is kind of a combination of
augmentation. theInstead of adding gravelvel
everywhere, you only add it at
50 feet. kind of intervals of maybe 20 to
gravel dams essentially along And so yothe stream. of little
People have actually tried that technique in some parts of the
Sierras.
recreation. And so completely destroy the
existing channel and try to create a new channel at some
other portion.
that you would really try under Thivery degraded ecosystems.que
So the current work that we're
what the effects climate change doing is staare.g to look at
And so here are our stream
simulated for current discharconditions.that are
This is a decade long average of
from the current decade and going to the mid part of this
century to the late part of this
And what we see as we go from the current decade to the later
part of this century, we see a
snow melt. huge shift in the timing of the
snow melt occurs earlier and So as temperatures warm, the
can see that it will be on the earlier, and in this picture we
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
of the summer, and the plants will have to tolerate those for
a much longer portion of the
So they'll be experiencing a lot growing season.
under current conditions. And so here we see a nice sunset
in the Tuolumne watershed,
the talk and bringing us to the indicates conclusions.rapping up
And so what we've learned through all of this is that
evapotranspiration, hill slope
interactions all affect the spatial and temporal groundwater
regimes of a meadow.
consider all of these processes So if we hope to be able tolly
simulate the water table regime accurately.
We also have found that meadows are groundwater dependent
ecosystems, and they rely on a
the wet meadow vegetation shallow wcommunities.o support
And as a result, the spatial
communities on the surface is patreally a reflection of theon
subsurface groundwater processes
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
developing gives us a tool to add a scientific basis towards
restoration practices, to really
strategy and another. So I'd like to acknowledge our
funding.
the Environmental Sustainability The National Science Foundation,
Park for letting us do this research, for letting us hidel
all of our equipment within Tuolumne Meadows out of sight of
the visitors, and to Chris Lowry
of the modeling on this project, who was the postdoc who did much
Lundquist at the University of and my collaborator Jessica
And then all of these other folks, most of whom are at the
National Park.
[APPLAUSE]