A nice British video explaining the connection between rivers and groundwater. I can’t get the embed to work, so you’ll have to click through to watch: http://www.groundwateruk.org/How-Rivers-Work-Role-of-Groundwater.aspx This is why I say I study rivers AND groundwater – if you want to understand how water moves through a watershed, you’ve got to …
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At the 2011 GSA Meeting in Minneapolis next week, I’ll be presenting the following talk in the session “Monitoring and Understanding Our Landscape for the Long Term through Small Catchment Studies I: A Tribute to the Career of Owen P. Bricker.” My talk is in Minneapolis Convention Center: Room M100FG, on Wednesday, 12 October 2011 at 9:30 am.
Spatial variability in groundwater-stream interactions in first-order North Carolina Piedmont streams
JEFFERSON, Anne J. and MOORE, Cameron, Dept. of Geography and Earth Sciences, University of North Carolina at Charlotte,
Groundwater upwelling and hyporheic exchange are spatially variable in three first-order Piedmont streams, resulting in variable discharge, water chemistry and temperature. Stream gradient, valley confinement, and woody debris jams appear to be the major controls on the distribution and size of upwelling zones. Temperature and specific conductance values at 25 m intervals on 18 dates revealed distinct zones of groundwater-stream interaction, confirmed by discharge and piezometer measurements. Baseflow accumulates unevenly along the streams, with upper reaches in confined valleys generally gaining discharge more rapidly than lower reaches. Elevated calcium concentrations occur in groundwater upwelling zones, such as in a 50 m reach in which baseflow triples. Near their mouths, where the streams reach a river floodplain, baseflow quantity and chemistry may be influenced by a larger groundwater system. At a smaller scale, spatial variability in stream chemistry and streambed hydraulic gradients are dominantly controlled by the size and position of woody debris jams. Fine sediment wedges extend 5-15 m upstream of the 0.25-1 m high jams, and strong down-welling hydraulic gradients occur in these areas. Upwelling of water with higher specific conductance and moderated temperatures occurs downstream of the jams. Nitrate concentrations decreased up to 50% immediately below large woody debris jams, while ammonium concentrations tended to be highest there.
Rapid urbanization in the Carolina Piedmont is drastically altering headwater catchments, but well-documented reference watersheds are lacking. The measurements described above are from three first-order streams in forested watersheds, with permanent protection by a land conservancy. Their hydrology and water chemistry demonstrates the rich spatial variability of Piedmont headwater streams, and we hope that long-term study of these sites provides useful understanding for stream restoration and watershed management.
In July 2011, Anne was a plenary speaker at the Chapman Conference on The Galápagos as a Laboratory for the Earth Sciences in Puerto Ayora, Galapágos. Anne was tasked with reviewing the state-of-knowledge of volcanic island hydrology and identifying pressing questions for future research in this 40 minute talk. The following is the abstract which she submitted when she began the task.
Top down or bottom up? Volcanic history, climate, and the hydrologic evolution of volcanic landscapes
Volcanic landscapes are well suited for observing changes in hydrologic processes over time, because they can be absolutely dated and island chains segregate surfaces of differing age. The hydrology of mafic volcanic landscapes evolves from recently emplaced lava flows with no surface drainage, toward extensive stream networks and deeply dissected topography. Groundwater, a significant component of the hydrologic system in young landscapes, may become less abundant over time. Drainage density, topography, and stream and groundwater discharge provide readily quantifiable measures of hydrologic and landscape evolution on volcanic chronosequences. In the Oregon Cascades, for example, the surface drainage network is created and becomes deeply incised over the same million-year timescale at which springs disappear from the landscape. But chronosequence studies are of limited value if they are not closely tied to the processes setting the initial conditions and driving hydrologic evolution over time.
Landscape dissection occurs primarily by erosion from overland flow, which is absent or limited in young, mafic landscapes. Thus, volcano hydrology requires conceptual models that explain landscape evolution in terms of processes which affect partitioning of water between surface and subsurface flows. Multiple conceptual models have been proposed to explain hydrologic partitioning and evolution of volcanic landscapes, invoking both bottom up (e.g., hydrothermal alteration) and top down processes (e.g., soil development). I suggest that hydrologic characteristics of volcanic islands and arcs are a function of two factors: volcanic history and climate. We have only begun to characterize the relative importance of these two drivers in setting the hydrologic characteristics of volcanic landscapes of varying age and geologic and climatic settings.
Detailed studies of individual volcanoes have identified dikes and sills as barriers to groundwater and lava flow contacts as preferential zones of groundwater movement. Erosion between eruptive episodes and deposits from multiple eruptive centers can complicate spatial patterns of groundwater flow, and hydrothermal alteration can reduce permeability, decreasing deep groundwater circulation over time. Size and abundance of tephra may be a major geologic determinant of groundwater/surface water partitioning, while flank collapse can introduce knickpoints that drive landscape dissection. The combination of these volcanic controls will set initial conditions for the hydrology and drive bottom up evolutionary processes.
Climatic forcing drives many top down processes, but understanding the relative effectiveness of these processes in propelling hydrologic evolution requires broader cross-site comparisons. The extent of weathering may be a major control on whether water infiltrates vertically or moves laterally, and we know weathering rates increase until precipitation exceeds evapotranspiration. Weathering by plant roots initially increases porosity, but accumulation of weathered materials, such as clays in soils, can reduce near-surface permeability and promote overland flow. Similarly, eolian or glacial inputs may create low permeability covers on volcanic landscapes.
Cross-posted at Highly Allochthonous
I’ve got a shiny new Emriver Em2 river processes simulator (i.e., stream table), thanks to departmental equipment funds and enthusiastic colleagues. I’ve been on sabbatical this semester and away from campus, so I haven’t had a chance to play with it yet, but it is enticing me to return. I’ll be teaching Fluvial Processes fall semester, so I’m sure that my students and I will get plenty of chances to explore all of the nifty ways in which we can demonstrate and experiment with fluvial geomorphology. I’m also playing with ideas for using the Emriver model in my hydrogeology class in the spring. I think it will be a perfect way to demonstrate ideas of hyporheic flow, seepage erosion, and break through curves in tracer tests. I think my colleagues are planning to use it in sedimentology, geomorphology and hydrology classes, and one colleague may take it with him when he does outreach activities. I’m sure we will come up with even more uses for it once we get started.
My appetite for experiment with the stream table was whetted by a recent visit to Carbondale, Illinois and the base of operations for Little River Research and Design (LRRD). Steve Gough is the owner of LRRD, the mastermind behind the Emriver models, and a genuinely fantastically nice person. Motivated by the idea that hands on education about stream processes is the best way to instill respect for and promote protection of streams and rivers, Steve has poured himself into making the best stream table on the market, and making it affordable enough to for people like me to get their hands on.
Personally, I’d always been somewhat underwhelmed by teaching- and demonstration-grade stream tables before seeing the Emriver ones. Partly it was because I’d seen and read about big research flumes, like those at the St. Anthony Falls Lab and Johns Hopkins. But another part of it was that every time I had a chance to play with a home-built stream table I was frustrated by what it couldn’t do. Principally, most stream tables don’t do a very good job of reproducing the meandering behavior of lowland streams. This has even been an area of active and high profile research in the fluvial geomorphology community. Steve’s use of low density plastic beads instead of quartz sand solves that problem pretty nicely, though there’s definitely still some braiding going on.
In addition to the 2-m long Em2 model that I have, LRRD also makes an extremely cool and versatile 4-m long model Em4. With beads colored by size, you can see (and measure) the sorting and selective transport of sediments. You can tilt the table laterally – simulating differential uplift/subsidence across the basin. There’s even a groundwater feed and extraction system! This model is pretty much as cool as I can imagine – at least short of the big research flumes mentioned above.
I can personally attest that this stream table model has the versatility to entrance both a PhD and a preschooler for more than two hours…and the preschooler wanted to go back the next day! Below I’ve added some shots of the Em4 in action. What geomorphic processes do you see?
Major congratulations to two Watershed Hydrogeology Lab graduate students who have finished writing their MS theses and will defend them next week. Ralph McGee and Cameron Moore both started in our MS in Earth Science program in August 2009, and less than two years later they have each completed impressive MS projects on headwater streams in Redlair Forest of the North Carolina Piedmont.
Ralph McGee will present his research on “Hydrogeomorphic processes influencing ephemeral streams in forested watersheds of the southeastern Piedmont U.S.A.” on Thursday, May 12th at 10:00 am in McEniry Hall, room 111 on the UNC Charlotte campus.
The unofficial title for Ralph’s work is “Tiny Torrents Tell Tall Tales.” Watch the video below to see why.
Cameron Moore will present his research on “Surface/Groundwater Interactions and Sediment Characteristics of Headwater Streams in the Piedmont of North Carolina” on Friday, May 13th at 9:00 am in McEniry Hall, room 111 on the UNC Charlotte campus.
When Cameron started working on this project, I had thought that the story would focus on how fractured bedrock contributed to groundwater upwelling in the streams, but it turns out the small debris jams (like the one below) are the dominant driver of groundwater/stream interactions and spatial variability of channel morphology.
Faculty, students, and the public are encouraged to attend the presentations and ask Ralph and Cameron any questions they may have.
Cross-posted at Highly Allochthonous
Permeability (the ease with which a fluid moves through a material) is the ultimate goal of many hydrogeologic investigations, because without that information it is impossible to quantify subsurface water and heat flow rates or understand contaminant transport. Yet permeability is notoriously difficult to quantify, both at the local-scale and the landscape-scale. Permeability varies over 13 orders of magnitude across rock and sediment types, because of differences in pore sizes, geometry, and connectedness. Loose gravel could have permeability as high as 10-7 m2, but unfractured igenous and metamorphic rocks could be as low as 10-20 m2. The diagram below is an example of the sort of relationship between rock type and permeability shown near the beginning of every major hydrogeology textbook.
Most of the time, hydrogeologists are happy to just to get permeability to within an order of magnitude or two. Knowing permeability is not just useful for those interested in in water supply problems and transport of contaminants. For scientists who model watersheds or land-atmosphere interactions in climate models, being able to easily estimate landscape-scale permeability would be incredibly helpful.
In a new paper in Geophysical Research Letters, scientists from Canada, Germany, the Netherlands, and the US have just done a big favor for those scientists. Gleeson et al. (2010) compiled the first regional-scale maps of permeability for the North American continent and the terrestrial globe. They are interested in permeability in the uppermost 100 m of the subsurface, but below the water table, where all pore spaces are saturated with water. They defined regional-scale as greater than 5 km, because they wanted to avoid influences by things like individual fractures. Using previously published hydrogeologic models, in which permeability was calibrated against groundwater flow, tracers, or heat fluxes, Gleeson and colleagues identified permeability values for 230 hydrogeologic units, grouping them into seven “hydrolithologic” categories, by rock type.
The scientists compared the permeability values from the models to the expected permeabilities for each rock type based on smaller-scale measurements (like those used to make the graph above), and they found reasonably good correspondence. They also examined whether permeability values within each hydrolithologic category were correlated with the scale of the model used to generate them. They found that permeability was scale-independent above 5 km, except in carbonates, where large karst features may result in changing permeability with increasing area.
Using pre-existing geologic maps for North America and the world, Gleeson and colleagues divided the Earth into their hydrolithologic categories. For each category, they calculated the geometric mean of the modeled permeability values, and applied that mean permeability to all of the map units in that category. The resultant maps show the distribution of permeability across the land surface.
The global map uses a single geology dataset, so there are no weird boundaries in the data, but it is of coarse resolution. The North American map (shown above) is at much finer resolution (75 km2 mean polygon area, with 262,111 polygons), but it has a few odd edges that correspond to state and national borders. The authors point to these boundary problems in their discussion of caveats, along with the problems associated with permafrost, deep unsaturated zones in arid areas, and deep weathering in the tropics. In addition, the use of a single permeability value for each category will necessarily lump together some terrains with similar rock types but differing geologic histories and resultant permeabilities (e.g., the High and Western Cascades in Oregon).
The work of Gleeson and colleagues represents an important first step in translating regional-scale geologic data into permeability fields. These maps will be useful for continental-scale and larger earth system models and for data sparse regions. Their methodology also raises some interesting possibilities for subdividing the hydrolithologic categories in areas where there are more hydrogeologic model data available, but where there hasn’t been comprehensive hydrogeologic modeling. Finally, their finding that regional-scale model values are in accord with the ranges reported in every hydrogeology textbook is a significant confirmation of the fall-back position of many students of hydrogeology: “If you have no data from wells in your field area, use a textbook to estimate permeability from the rock type.”
Gleeson, T., Smith, L., Moosdorf, N., Hartmann, J., Dürr, H., Manning, A., van Beek, L., & Jellinek, A. (2011). Mapping permeability over the surface of the Earth Geophysical Research Letters, 38 (2) DOI: 10.1029/2010GL045565
In a few weeks, I’ll be giving the following talk at the American Geophysical Union Fall Meeting in a session on Groundwater/Surface Water Interactions: Dynamics and Patterns Across Spatial and Temporal Scales. My talk will be in Moscone West 3014 at 11:05 am on Wednesday, December 15th, 2010.
Spatial heterogeneity in isotopic signatures of baseflow in small watersheds: implications for understanding watershed hydrology
A. J. Jefferson
Time series of stable isotopes of oxygen and hydrogen in stream water are widely used to characterize watershed transit times and flowpaths, but synoptic sampling of multiple locations within a watershed can also provide useful information about heterogeneity of stream water sources and groundwater-surface water interactions that may affect interpretations of watershed hydrology. Here I present results of same-day baseflow sampling campaigns in low-relief, 0.1 to 100 km2 watersheds. More than half of less than 5 km2 forested and urban watersheds sampled in this study had variability in ?2H exceeding 2‰ and ?18O variability exceeding 1‰, substantially larger than the analytical uncertainty. In some cases, the heterogeneity was extreme, with ?2H varying by >10‰ over 150 m in one stream. Some isotopic perturbations occur in conjunction with stream conductivity and temperature changes, and such zones likely reflect localized contributions from fractured crystalline bedrock. In the urban 100 km2 watershed, mainstem baseflow isotopes were relatively homogeneous, but ?2H varied by more than 10‰ across tributaries, suggesting that subwatersheds are fed by water with different sources or transit times. Some urban streams were isotopically similar to the municipal water supply, suggesting that water main leakage and wastewater discharge may be locally significant contributors to baseflow. The isotopic heterogeneity of small streams and watersheds suggests that an understanding of groundwater-stream interactions is needed to correctly interpret isotope-based inferences about watershed transit times and flowpaths.
How does a landscape go from looking like this…
to looking like this?
Find out in my new paper in Earth Surface Processes and Landforms.
Hint: Using a chronosequence of watersheds in the Oregon Cascades, we argue that the rates and processes of landscape evolution are driven by whether the water sinks into the lava flows and moves slowly toward springs with steady hydrographs or whether the water moves quickly through the shallow subsurface and creates streams with flashy hydrographs. Further, we suggest that this water routing is controlled by an elusive landscape-scale permeability which decreases over time as processes like chemical weathering create soil and clog up pores in the rock. And as a bonus, because of the high initial permeability of basaltic landscapes, the formation of stream networks and the dissection of the landscape appears to take far longer than in places with less permeable lithologies.
Jefferson, A., Grant, G., Lewis, S., & Lancaster, S. (2010). Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA Earth Surface Processes and Landforms, 35 (7), 803-816 DOI: 10.1002/esp.1976
Cross posted at Highly Allochthonous
The Yellowstone caldera is home to thousands of geothermal springs and 75% of the world’s geysers, with kilometers-deep groundwater flow systems that tap magmatic heat sources. As that hot groundwater rises toward the surface, it interacts with shallower, cooler groundwater to produce multi-phase mixing, boiling, and a huge array of different hydrothermal features. While the deep, geothermal water is sexy and merits both the tourist and scientific attention given to it, there’s a largely untold story in the shallow groundwater, where huge volumes of cold water may advect more heat than the hydrothermal features.
Grand Prismatic Spring. (Photo by Alaskan Dude on Flickr.)
Yellowstone is a rhyolitic caldera that has produced 6000 cubic kilometers of ash flow tuffs, rhyolites, and basalts that form a poorly-characterized, heterogeneous fractured rock aquifer, hosting both hot/deep and cold/shallow flow systems. The Yellowstone volcanics lie on top of the Rocky Mountain Cordillera, which itself is a complex hydrogeologic system, ranging from low permeability metamorphic rocks to high permeability limestones.
In a paper in the Journal of Hydrology, Gardner and colleagues (2010) use stream hydrographs and groundwater residence times to characterize the cold, shallow groundwater of the greater Yellowstone area. Stream hydrographs, or the time series of stream discharge, are useful indicators of groundwater dynamics, because in between rain or snowmelt events, streamwater is outflowing groundwater. The recession behavior of a hydrograph during periods between storms can be used to estimate aquifer volumes. In the Yellowstone region, the annual hydrograph is strongly dominated by the snowmelt peak, and Gardner et al. used the mean daily discharge record from 39 streams to characterize the recession behavior of streams on different lithologies. What they found was that streams flowing in watersheds dominated by volcanic rocks have much less variable hydrographs than those on other rock types. The figure below uses data from the USGS to illustrate these differences, which are in line with studies in the Oregon Cascades* and elsewhere which suggest that young volcanic rocks produce groundwater-fed streams with muted hydrographs.
Daily discharge for the Firehole River (USGS gage #06036905) and Soda Butte Creek (USGS gage #06187950) for the 2006-2007 water years, expressed on a unit area basis.
Using a nifty technique to separate the recessions into components attributable to snowmelt versus groundwater, Gardner et al. were able to calculate a ratio of the groundwater discharge to the total discharge of each stream and to calculate the hydraulic diffusivity, which is a ratio of permeability (how easily a fluid moves through a rock) compared to the amount of water stored in the system. If hydraulic diffusivity is low, the flow in the stream decreases slowly over time, like the Firestone River in the figure above. But hydraulic diffusivity can be low either because of low permeability or large aquifer storage volumes, so being able to tease apart those two components is key to understanding the hydrograph behavior. Gardner et al. did this by looking at the ratio of groundwater discharge to maximum discharge and using that as an index of aquifer storage. Based on these ratios, Gardner et al. separated the streams in the Yellowstone area into three groups (runoff-dominated, intermediate, and groundwater-dominated) with contrasting hydrogeologic properties.
Geologic map of a portion of the Yellowstone Plateau, with approximate locations of stream gages of interest noted. Modified from Christiansen (2001, USGS Prof. Pap. 729-G).
Soda Butte and Teton Creeks are runoff dominated, with low groundwater storage and middling recession behavior. Since there is little groundwater storage, in order for hydraulic diffusivity to be low, then permeability must also be low. Sure enough, Teton Creek lies on top of Precambrian gneiss and granite, and unfractured metamorphic and intrusive igneous rocks like these have the lowest possible permeabilities. The Soda Butte Creek watershed comprises Eocene Absaroka volcanics, and older volcanic rocks like these can be quite weathered to clays and relatively impermeable.
The intermediate watersheds of Tower Creek and Cache Creek have significant ratios of groundwater discharge to maximum discharge, but their hydrographs recede rapidly over the summer. This means that they have high permeabilities relative to their aquifer storage volume. The Tower Creek watershed has Eocene tuffs and glacial valleys with alluvial fill, and Cache Creek watershed has Paleozoic carbonates. These materials are known for their high permeabilities, and the low storage volumes can be explained if those layers thinly overly less conductive materials.
The Firehole River, Gibbon River, and Snake River above Jackson Lake are groundwater-dominated, with very high permeabilities but even larger aquifer storage volumes. All of those streams drain primarily Quaternary Yellowstone volcanics, and this hydrologic behavior is in keeping with other young volcanic terrains.
Not content to stop with this hydrogeologic classification of the Yellowstone area, Gardner et al. collected water samples from small, cold springs to analyze CFC and tritium concentrations, which are useful tracers of groundwater travel times. For the springs they sampled, they found an average travel time (from recharge to discharge) of ~30 years. Using those CFC-derived groundwater transit times and some back-of-the-envelope estimates of aquifer geometry, Gardner et al. estimate that the Quaternary Yellowstone volcanics have a permeability of 10-11 to 10-13 m2, which is in line with estimates of young volcanics elsewhere. They also estimated that the aquifer depth represented by these small springs was ~70 m, but speculated that deeper flowpaths might have been discharging directly into the streams, out of reach of their CFC and tritium sampling abilities.
Finally, Gardner et al. note that the cold springs they studied are actually not as cold as they should be. In fact, they appear to be what are coming to be called “slightly thermal” springs. Groundwater recharge temperature is commonly assumed to be approximately mean annual temperature, and in the Norris Geyser Basin area, that’s around 4-5 ° C. But the cold springs in the area are around 10 ° C. Using this temperature difference and a handy equation from Manga and Kirchner (2004), Gardner et al. are able to calculate the heat flux advected by these cool springs. Their value of ~3800 W/m2 for the springs around Norris is about 10% of the heat flux from the Norris and Gibbon Geyser Basins themselves. That number becomes even more astonishing when you consider the relative scales of the cool versus the thermal groundwater systems. Geyser basins cover ~10 km2 of the Yellowstone Plateau, whereas cool groundwater drains under the entire ~1000 km2 plateau, and could be discharging far more heat than those showy thermal springs and geysers themselves.
So if you happen to go to Yellowstone this summer, in between gawking at Old Faithful, Artist Paint Pots, and Mammoth Hot Springs, take a few moments to appreciate the waters of the less dramatic cool rivers and streams. Their waters too are profoundly shaped by the geologic history of Yellowstone, and they are taking an awful lot of heat.
Payton Gardner, W., Susong, D., Kip Solomon, D., & Heasler, H. (2010). Snowmelt hydrograph interpretation: Revealing watershed scale hydrologic characteristics of the Yellowstone volcanic plateau Journal of Hydrology, 383 (3-4), 209-222 DOI: 10.1016/j.jhydrol.2009.12.037
Cross-posted at Highly Allochthonous
The theme for the next edition of the geoblogosphere’s Accretionary Wedge carnival is along the lines of “what are you doing now?” Recently as I was whining to my Highly Allochthonous co-blogger about how busy my teaching was keeping me, and how I wouldn’t have time to write anything for the Wedge, Chris suggested that I exhume some navel-gazing writing I’d done a while ago and simply post that. And in slightly modified form, now I have.
So, what do I do? The major theme of my research is analyzing how geologic, topographic, and land use variability controls hydrologic response, climate sensitivity, and geomorphic evolution of watersheds, by partitioning water between surface and ground water. The goal of my research is to improve reach- to landscape-scale prediction of hydrologic and geomorphic response to human activities and climate change. My work includes contributions from field studies, stable isotope analyses, time series analyses, geographic information systems, and hydrological modeling. My process-based research projects allow me to investigate complex interactions between hydrology, geomorphology, geology, and biology that occur on real landscapes, to test conceptual models about catchment functioning, and to show whether predictive models are getting the right answers for the right reasons. My current and past research has allowed me to investigate landscapes as diverse as the Cascades Range volcanic arc, the Appalachian Mountains and Piedmont of the southeastern United States, the Canadian Arctic Archipelago, and the Upper Mississippi River watershed.
My on-going and developing research program focuses on three areas:
- Watershed influences on hydrologic response to climate variability and change;
- Controls on and effects of partitioning flowpaths between surface water and groundwater; and
- Influence of hydrologic regimes on landscape evolution and fluvial geomorphology
If you really want the long version of my research interests, venture onward. But don’t say I didn’t warn you.
Watershed influences on hydrologic response to climate variability and change
On-going climate change is predicted to have dramatic effects on the spatial distribution and timing of water resource availability. I use historical datasets, hydrologic modeling, and GIS analysis to examine how watershed characteristics can mediate hydrologic sensitivity to climate variability and change. Currently, I focus on climate sensitivity in watersheds with seasonal and transient snow and on down-scaling hydrologic impacts of climate change to smaller watersheds.
Watersheds with seasonal and transient snow: A long-held mantra is that watersheds with extensive groundwater are buffered from climate change effects, but in a pair of papers set in the Oregon Cascades, my collaborators and I showed the opposite to be true. Minimum streamflows in watersheds with abundant groundwater are more sensitive to loss of winter snowpack than in watersheds with little groundwater (Jefferson et al., 2008, Tague et al., 2008). Glaciers are another water reservoir often thought to buffer climate change impacts, and in a paper in review, we show that projected glacier loss from Mt. Hood will have significant impacts on water resources in the agricultural region downstream.
I have also been examining hydroclimate trends relative to hypsometry (elevation distribution) of watersheds in the maritime Pacific Northwest. Almost all work investigating hydrologic effects of climate change in the mountainous western United States focuses on areas with seasonal snowpacks, but in the maritime Northwest, most watersheds receive a mixture of winter rain and snow. My research investigates how much high-elevation watershed area is necessary for historical climate warming to be statistically detectable in streamflow records. Preliminary results were presented at the American Geophysical Union meeting in 2008, and I’m working on a paper with more complete results. Extending this work into the modeling domain, I am currently advising a graduate student using SnowModel to investigate the sensitivity of snowmelt production to projected warming in the Oregon Cascades, Colorado’s Fraser Experimental Forest, and Alaska’s North Slope, in collaboration with Glen Liston (Colorado State University).
Down-scaling climate impacts to watersheds and headwater streams: Most studies of hydrologic impacts of climate change have focused on regional scale projections or large watersheds. Relatively little work has been done to understand how hydrologic and geomorphic impacts will be felt in mesoscale catchments or headwater stream systems, yet most of the channel network (and aquatic habitat) exists in these small streams. In August 2009, I submitted a proposal to a Department of Energy early career program to investigate the effect of climate change on hydrology of the eastern seaboard of the US. This work proposed to contrast North Carolina’s South Fork Catawba River and New Hampshire’s Pemigewassett River and their headwater tributaries through a combination of modeling and field observations of the sensitivity of headwater stream networks to hydroclimatic variability. While the project was not funded, I am using the reviews to strengthen the proposal, and I plan to submit a revised proposal to NSF’s CAREER program in July. I have a graduate student already working on calibrating the RHESSys hydroecological model for the South Fork of the Catawba River.
Controls on flowpath partitioning between surface water and groundwater and the effects on stream hydrology, geomorphology and water quality
Many watershed models used in research and management applications make simplifying assumptions that partition water based on soil type and homogeneous porous bedrock. These assumptions are not reflective of reality in many parts of the world, including the fractured rocks of North Carolina’s Piedmont and Blue Ridge provinces. I am interested in understanding how water is partitioned between groundwater and surface water in heterogeneous environments, and what effect this partitioning has on stream hydrology, geomorphology, and water quality. My interest in the controls on flowpath partitioning began during my work in the Oregon Cascades Range, where I showed that lava flow geometry controlled groundwater flowpaths and the expression of springs (Jefferson et al., 2006). Currently, I am using fractured rock environments and urbanizing areas as places to explore the effects of heterogeneous permeability.
Fractured rock: The Piedmont and Blue Ridge provinces of the eastern United States are underlain by crystalline rocks, where groundwater is largely limited to discrete fractures. Groundwater-surface water interactions on fractured bedrock are largely unexplored, particularly at the scale of small headwater streams. I am interested in how groundwater upwelling from bedrock fractures and hyporheic flow influence the hydrology and water quality of headwater streams. A small grant facilitated data collection in three headwater streams which is forming the thesis for one of my graduate students, has precipitated a collaborative project with hydrogeologists from the North Carolina Division of Water Quality, and will serve as preliminary data for a proposal to NSF Hydrologic Sciences in June 2010.
Urban watersheds: Urbanization alters the partitioning of flowpaths between surface water and groundwater, by creating impervious surfaces that block recharge and installing leaky water and sewer lines that import water from beyond watershed boundaries. Also, the nature of the drainage network is transformed by the addition of stormwater sewers and detention basins. In September 2009, my collaborators and I submitted a proposal to NSF Environmental Engineering to look at how stormwater best management practices (BMPs) mitigate the effects of urbanization on headwater stream ecosystem services. While we weren’t funded, we were strongly encouraged to resubmit and did so in March 2010. We are also submitting a proposal to the National Center for Earth Surface Dynamics (NCED) visitor program to use the Outdoor Stream Lab at the University of Minnesota to investigate the interplay between stormwater releases and in-stream structures.
Influence of hydrologic regimes on landscape evolution and fluvial geomorphology
The movement of water on and through the landscape results in weathering, erosion, transport, and deposition of sediment. In turn, that sediment constrains the future routing of water. I am interested in how the hydrologic regime of a watershed affects the evolution of topography and fluvial geomorphology. My work in this area has examined million-year scales of landscape evolution in high permeability terrains, century-scale evolution of regulated rivers, and the form and function of headwater channels.
Evolution of high permeability terrains: The youngest portions of Oregon’s High Cascades have almost no surface water, because all water infiltrates into high permeability lava flows. Yet on older parts of the landscape, streams are abundant and have effectively eroded through the volcanic topography. In a paper in Earth Surface Processes and Landforms (Jefferson et al., 2010), I showed that this landscape evolution was accompanied and facilitated by a hydrologic evolution from geomorphically-ineffective stable, groundwater-fed hydrographs to flashy, runoff-dominated hydrographs. This coevolutionary sequence suggests that permeability may be an important control on the geomorphic character of a watershed.
Human and hydrologic influences on large river channels: Almost all large rivers in the developed world are profoundly affected by dams, which can alter the hydrologic regime by suppressing floods, supplementing low flows, and raising water levels in reservoirs. On the Upper Mississippi River, in the 70 years since dam construction, some parts of the river have lost islands, and with them habitat diversity, while in other areas new islands are emerging. In 2008-2009, I had a small grant that facilitated the examination of some islands with a unique, unpublished long-term topography dataset and its correlation with hydrologic patterns and human activities. This project became the thesis research of one of my graduate students, who will be defending his M.S. in May 2010.
Headwater channel form and function: Although headwater streams constitute 50-70% of stream length, the geomorphic processes that control their form and function are poorly understood. Most recent research on geomorphology of headwater streams has focused on streams in very steep landscapes, where debris flows and other mass wasting processes can have significant effects on channel geometry. In the Carolina Piedmont, gentle relief allows me to investigate the formation and function headwater channel networks, isolated from mass wasting processes. One of my graduate students has collected an extensive sediment size distribution dataset which shows that, at watershed areas <3 km2, downstream coarsening of sediment is more prevalent than the downstream fining widely observed in larger channels. Another graduate student is collecting data on channel head locations and flow recurrence and sediment transport in ephemeral channels in order to sort out the relative influences of topography, geology, and legacy land use effects on the uppermost reaches of headwater streams. Both of these projects have already resulted in presentations at GSA meetings.
Whew. So that’s what I do, between teaching some field-intensive courses and raising a preschooler. But, what am I? Hydrologist? Geomorphologist? Hydrophillic geologist? Or something else entirely?