Wisconsin Public Television University Place Wednesday Nite at the Lab  Steven Loheide, "Hydroecology in the Sierra Nevada"  January 26, 2011 UP545    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]