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Stormwater control measures modify event-based stream temperature dynamics in urbanized headwaters

Next week, the Watershed Hydrology Lab will be well represented at the CUAHSI 2014 Biennial Colloquium. We’ll be presenting four posters, so here come the abstracts…

Stormwater control measures modify event-based stream temperature dynamics in urbanized headwaters

Grace Garner1, Anne Jefferson2*, Sara McMillan3, Colin Bell4 and David M. Hannah1
1School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
2Department of Geology, Kent State University, Kent, OH, 44240, USA
3Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, Charlotte, NC, 28223, USA
4Department of Infrastructure and Environmental System, University of North Carolina at Charlotte, Charlotte, NC, 28223, USA

Urbanization is a widespread and growing cause of hydrological changes and ecological impairment in headwater streams. Stream temperature is an important control on physical, chemical and ecological processes, and is an often neglected water quality variable, such that the effects of urban land use and stormwater management on stream temperature are poorly constrained. Our work aims to identify the influence of stormwater control measures (SCMs) of differing design and location within the watershed on the event-based temperature response of urban streams to precipitation in the North Carolina Piedmont, in order to improve prediction and management of urban impacts. Stream temperature was measured within SCMs, and upstream and downstream of them in two streams between June and September 2012 and 2013. Approximately 60 precipitation events occurred during that period. To unambiguously identify temperature increases resulting from precipitation, surges were identified as a rise in water temperature of ?0.2°C between the hours of 15:30 and 5:30, when the diurnal temperature cycle is either decreasing or static on days without precipitation. Surges up to 5°C were identified in response to precipitation events, with surges occurring both upstream and downstream of the SCM under some conditions. Surges were also recorded within the SCMs, confirming that temperature surges are the result of heated urban runoff. Classification tree modeling was used to evaluate the influence of hydrometeorological drivers on the generation and magnitude of temperature surges. In both streams, event precipitation, antecedent precipitation, and air temperature range were identified as the drivers of whether or not a surge was observed and how large the surge was, though the order and thresholds of these variables differed between the two sites. In a stream with an off-line, pond SCM, the presence of the pond in the lower 10% of the watershed did not affect the magnitude of temperature surges within the stream, but the pond itself had a wider range of surge magnitudes than did the stream. In a watershed with a large in-line pond, and a downstream contributing wetland SCM receiving flow from 40% of the watershed, the wetland increased both the frequency and magnitude of temperature surges observed in the stream. Our results suggest dynamic hydrometeorological conditions, SCM design, and position within a watershed all influence whether stormwater management reduces or enhances temperature surges observed within urban headwater streams, and that these factors should be considered in the recommendations for urban stormwater management systems.

AGU 2011 abstract: Understanding channel network extent in the North Carolina Piedmont in the context of legacy land use, flow generation processes, and landscape dissection

The following talk will be presented by Anne at the 2011 AGU fall meeting on Wednesday, December 7th from 9 to 9:15 am in the session “EP31G. Predictive Understanding of Coupled Interactions Among Water, Life, and Landforms II.” It will be in rooms 2022-2024, and the abstract acceptance said something about video on demand.

Understanding channel network extent in the North Carolina Piedmont in the context of legacy land use, flow generation processes, and landscape dissection

Anne J. Jefferson and Ralph W. McGee
Department of Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte, NC

Nearly all land in the eastern US Piedmont region was cleared for intensive agriculture following European settlement, but some areas have been afforested over the last century. In these areas, an extensive ephemeral stream network drains into perennial headwater streams. In order to understand the present-day functioning of the ephemeral network in afforested watersheds, we mapped 102 channel head positions at 6 sites and monitored 6 channels at 2 sites in North Carolina’s Piedmont. The ephemeral channels are activated by subsurface flow from high intensity precipitation with wet or dry soils, or long duration precipitation with wet soils. Overland flow does not occur upslope of channel heads in forested watersheds, but it is observed in present-day pastures and fields.

Channel head contributing areas range from 0.1 – 3.0 ha, with local slopes that average 0.13 (range: 0.04 – 0.36). The relationship between slope and area at the channel heads has the form c = AS1.1, with an exponent much lower than the commonly reported exponent of ~2 that is associated with subsurface or saturation overland flow. Instead, the lower exponent may reflect the legacy of 18th-19th century of intense land use and degraded cover, which may have produced turbulent overland flow upslope of channels. Though established by relict land use conditions, we suggest that this network extent is maintained by the frequent activation of the channels through subsurface flow under forest cover. Further, channel heads are located within or downslope of colluvial hollows suggesting that gullying from historical land use is not the most extensive channel network experienced by the Piedmont over the course its landscape evolution, and that the dissection of the landscape may be the result of a precipitation and land cover regime much different from the modern one.

Gully down to bedrock, Morrow Mountains State Park, North Carolina (photo by A. Jefferson)

Gully down to bedrock, Morrow Mountains State Park, North Carolina (photo by A. Jefferson)

GSA 2011 abstract: Spatial variability in groundwater-stream interactions in first-order North Carolina Piedmont streams

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.

Debris jam and sediment in a first order Deep Creek at Redlair. Photo by Cameron Moore.

Debris jam and sediment in a first order Deep Creek at Redlair. Photo by Cameron Moore.

Ralph McGee and Cameron Moore will graduate next week!

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.
[youtube=http://www.youtube.com/watch?v=PjINxXuy5Aw&w=640&h=390]

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.

Debris jam in Deep Creek

Looking upstream at a debris jam in Deep Creek


Faculty, students, and the public are encouraged to attend the presentations and ask Ralph and Cameron any questions they may have.

Heat in the Southeast

Cross-posted at Highly Allochthonous
Here in Charlotte we had a hot summer. We barely escaped the dubious distinction of hottest summer on record, with an average temperature of 81.1° F (27.3 ° C) between 1 June and 31 August. The record had been set in 1993, when Charlotte recorded an average temperature of 81.5° F (27.5 ° C). In terms of record breaking heat, we actually fared better than many parts of the east coast, where temperature records from New York City to Greenville-Spartanburg, South Carolina were broken. Below there’s a nice map from NOAA of how far average temperatures deviated from the 30-year climate normal period (here, 1966-1996).

U.S. surface temperature departure from average (°C), June 1 to August 31, 2010, from NOAA/ESRL Physical Sciences Division, Boulder Colorado

U.S. surface temperature departure from average (°C), June 1 to August 31, 2010, from NOAA/ESRL Physical Sciences Division, Boulder Colorado

Of course those average temperature records belie the minima and maxima experienced by each place over the course of those three summer months, so there’s another statistic that I’m finding even more interesting: the number of days where maximum temperatures exceeded 90° F (32.2 ° C). I think of it as Anne’s index of intolerable heat, especially when combined with the Southeast’s oppressive humidity. In Charlotte, between 1 June and 31 August, we had 67 days that exceeded 90° F. That means that 73% of days this summer were intolerably hot (at least for me). Also, that’s only counting the days in the climatological summer. We had 90+° F degree heat in early April, some in May, and we’ve already had some in September, with more in the forecast this week. I suspect that by the time the year is out, our total days above 90° F will be something around 80, if not more.

The long-term predictions for the index of intolerable heat look grim for Charlotte and the rest of the southeast. The image below shows historical and modeled days with peak temperatures exceeding 90° F. By the end of the century, at least under a high emissions scenario, 80+ days of intolerable heat will be considered a cool summer in North Carolina. We’re heading towards 120 days or more of hot, hot weather, a doubling of our historical average. In parts of Florida and Texas, more than half the year will be hotter than 90° F. Yuck. Glad I won’t be around here then.

Historical and predicted days with peak temperatures above 90 degrees Fahrenheit

These temperature trends are not just bad news for people who like to play (or do field work) outside in the summer, but are too wimpy to drop bucketloads of sweat. Hotter average temperatures and more days with ridiculous heat have real health consequences. On hot days, the chances go up that people playing outside end up with heat exhaustion or life-threatening heat stroke. People without air conditioned homes or workplaces, people too poor to pay tremendous energy bills for air conditioning, or people who just happen to have their AC break do not even need to play outside to be at risk of heat related illness or death. About 700 people already die each year from heat-related causes, and the elderly are a disproportionate share of the victims. Those with cardiovascular disease are also at substantially increased risk of heat-related mortality.

And it’s not the heat alone that spells bad news for the Southeast. With hotter temperatures come increasing rates of photochemical reactions…such as the production of ground-level ozone from nitrous oxides and volatile organic compounds released by car exhaust, power plants, and natural sources. The chemistryof photochemical ozone production is pretty complex and we don’t have a fantastic handle on how coming climate changes will impact the percent of hot days with sun versus clouds, but if the number of hot sunny days increases, it is likely that ozone production will increase too. Ozone brings its own host of adverse health effects, particularly respiratory problems, so even if you don’t mind the heat, running around outside on hot, sunny days can be a bad idea. Once again, children, the elderly, and those with asthma and other respiratory problems are most at risk on high ozone days. Such days, labeled as orange alerts, occur sporadically thoughout the summer already. In Charlotte, we’ve had 13 days with air quality in the orange category since May 1 this year. On those days, people at risk are encouraged to avoid outdoor exercise, and daycare centers limit the time kids spent playing outside. Some days, the air quality is bad enough (red alert) that even healthy adults are encouraged to avoid to outdoor exercise. That’s happened once this year in Charlotte.

As Charlotte and other parts of the southeast move towards one-third of their days in the intolerably hot range, with the probable added bonus of worse air pollution, it will be interesting to watch the societal shifts in attitudes toward the climate. Will Southerners get serious about reducing emissions from cars? Will Charlotteans end their love affair with sprawl in order to improve air quality? Will the Southeast be depopulated of Yankee transplants like me, who finally decide that they can’t take the heat? Or will we just stay inside and crank up the air conditioning units and complain about the weather?

Call for abstracts: NC WRRI Annual Conference

Note: If any of my students are interested in attending and/or presenting, just let me know and we will make it happen.

WRRI Annual Conference Call for Presentations Abstracts http://www.ncsu.edu/wrri/conference/cfa.html

The Water Resources Research Institute of The University of North Carolina (WRRI) requests abstracts for oral and poster presentations for the 12th Annual Conference: The State of Water Resources in North Carolina, to be held March 30-31, 2010, at the Jane S. McKimmon Center in Raleigh, North Carolina.

We are soliciting for oral and poster presentations and organized sessions related to the following topics:
* State of North Carolina water resources
* Drinking water management
* Erosion and sedimentation control
* Groundwater issues
* Surface water quality
* Stormwater management
* Wastewater management
* Water availability, use and climate interaction
* Water law and policy
* Water quality and ecosystem function
* Water reuse
* Other water-related topics

The Opening Session on March 30th and WRRI Luncheon on March 31st will focus on the theme: State of Water Resources in North Carolina. Many of the technical session themes will be based on abstracts received for oral presentations.

Oral and Poster Presentations:

Please submit abstracts of 300 words or less via the abstract submission form by Monday, February 15, 2010. For information on the required presentation abstract content and guidelines please go to: http://www.ncsu.edu/wrri/conference/cfa.html. All oral and poster presenters will be notified by email of the status of their abstract submission.

Undergraduate and graduate students are encouraged to submit abstracts for either oral or poster presentations. The North Carolina Water Resources Association (http://www.ncsu.edu/ncwra/) is sponsoring a student poster contest during the conference. First place will receive $200, 2nd place will receive $150 and 3rd place will receive $100.

Special Technical Sessions:

WRRI is accepting suggestions for special technical sessions/panel discussions. The sessions will be 90 minutes in length. Please submit your suggestion via email to Dr. David Genereux, genereux@ncsu.edu, and Kelly Porter, kelly_porter@ncsu.edu, with the following information:

* Chair name (individual who will oversee and moderate session)
* Title of session
* Speaker names or panel discussion participants
* Abstract describing the content of the session or a series of abstracts for each suggested speaker

Questions? Contact Kelly Porter at 919-513-1152 or kelly_porter@ncsu.edu.

GSA Abstract: Sediment size distributions in forested headwater streams of the North Carolina Piedmont

The Watershed Hydrogeology Lab is going to be busy at this year’s Geological Society of America annual meeting in Portland, Oregon in October. We’ve submitted four abstracts for the meeting, I’ll be co-convening a session, and I’ll be helping lead a pre-meeting field trip.

New lab member Cameron Moore has been busy working all summer long at our Redlair field sites, and he’s become an expert at Wolman pebble counts. We think it’s pretty exciting to have such a high density of data in a small area in small streams. Here’s his abstract:

Sediment size distributions in forested headwater streams of the North Carolina Piedmont

Cameron Moore and Anne Jefferson, Department of Geography and Earth Sciences, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223, fax: 704-687-5966, phone: (704) 687-5973

Headwater streams constitute more than 70% of total stream channel length in North America, yet geomorphic controls on such streams are still poorly understood in many regions. For example, close coupling between hillslope, debris flow, and fluvial processes in headwater streams may counter general downstream-fining trends. The goal of this study was to perform an intensive analysis of sediment size distributions in moderate-relief headwater streams, something rarely performed at such a small (<5 km2) scale. Twenty-seven Wolman pebble counts were conducted on 13 first to third-order streams within 3 km of each other in Gaston County, North Carolina.  The watersheds are forested and represent relatively undisturbed conditions for the Carolina Piedmont. The underlying rock types are felsic metavolcanic rocks and quartz-sericite schist, and each stream drains only one lithology. Ongoing data analysis will relate sediment size distributions to watershed area and channel slope. Median (D50) grain size in all the reaches where Wolman pebble counts were performed ranged from 12 – 46 mm, and averaged 26.9 mm overall. Of the ten streams where multiple counts were conducted, six display a distinct trend toward downstream coarsening. Grain sizes in the lower reaches of two streams may be influenced by a possible backwater effects from the South Fork of the Catawba River. Uniformity coefficients ranged from 1.89 – 9.00, showing a pattern of increasingly well-sorted bed material in the downstream direction in eight of the ten streams sampled multiple times. The data also show that woody debris jams lead to accumulations of poorly sorted sediment with low D50 values relative to the mean D50 value.  Between-stream variability approaches the magnitude of longitudinal variability in any single stream. This suggests that extremely local geomorphic history exerts a strong influence on headwater stream sediments.

Where rocks, water, and history intertwine

[Cross-posted at Highly Allochthonous]

“Ten thousand rocks and grassy islets meet the traveler’s eye, ten thousand murmuring streams meander through them. During low water the cattle delight to graze upon the islets…at such times they furnish a curious spectacle in the midst of a mighty river.”

So wrote architect Robert Mills in 1826, describing an outcrop of ~550 million year old diorite in the Catawba River south of Rock Hill, South Carolina. The Catawba River is one of the principal rivers of the Carolinas, with an annual average flow of 4018 cubic feet per second (114 m3/s) just upstream of our diorite outcrop. The outcrop is about 2.6 km long, and changes the single-thread river into a substantially wider, multi-thread anabranching river (Figures 1 and 2). This cattle-friendly piece of rock then represents a major obstacle to flow of the river, and that has ensured it a place in the region’s history.

Figure 1. Aerial view of the Catawba River in the vicinity of the diorite outcrop (left) and immediately downstream (right). Both photos are at the same scale and captured from Google Earth.

Figure 1. Aerial view of the Catawba River in the vicinity of the diorite outcrop (left) and immediately downstream (right). Both photos are at the same scale and captured from Google Earth.

Figure 2. Close-up of diorite outcrops along the shore of the Catawba River at Landsford Canal State Park. The vegetation on the left bank of the channel is an island on a larger diorite protrusion. Flow at USGS gage # 02147020 was 3840 cubic feet per second (108 m3/s) on the date this picture was taken.

Figure 2. Close-up of diorite outcrops along the shore of the Catawba River at Landsford Canal State Park. The vegetation on the left bank of the channel is an island on a larger diorite protrusion. Flow at USGS gage # 02147020 was 3840 cubic feet per second (108 m3/s) on the date this picture was taken.

The diorite outcrop made an easy place for early travelers to ford and cross the river. In the 1700s, Thomas Land owned the land around the outcrop, and the area today is known as Landsford. But the rock that made the river easy to cross, also made it hard for boats traveling downstream, particularly because of the rapids upstream of the ford proper. And that’s where Robert Mills comes in to the story. Mills was the architect of the Department of Treasury, U.S. Post Office, and the Washington Monument, and he was also the architect of a solution to the transporation dilemma presented by the diorite at Landsford.

In 1823, a canal opened to allow boats to bypass the rapids and ford. Robert Mills was the architect that designed the canal and the locks.  All in all, the 1.5 mile (2.4 km) canal, with two locks, circumvented 32 feet (9.75 m) of elevation loss by being dug in at the upstream end and elevated at the downstream end. The canal was lined with clay to prevent dewatering to the alluvial soils. The canal allowed boats 7 feet (2.14 m) wide, 60 feet (18.29 m) long, and carrying up to 50 bales of cotton to be pulled by mules or horses. The canal was built during the height of the American canal craze, and by 1846 it was out of use, replaced by railroads. Today the canal is mostly dry and has been substantially eroded at its downstream end. In places on the banks, there are large trees that couldn’t have been there when the boats were being pulled through the canal. The stonework in the locks is diorite from a nearby quarry and is well-preserved.

Figure 3. The upstream end of Landsford Canal.

Figure 3. The upstream end of Landsford Canal.

The diorite outcrop and the canal are preserved in Landsford Canal State Park, and you can walk on the mule path along the canal, or along a trail on the banks of the Catawba River. In late May and early June, the park draws thousands of visitors to see spider lilies blooming from amidst the rocky islets. I’m planning to head back in a few weeks and check it out, but I’d really like to explore the area from a kayak. Information in this post was gleaned from interpretive signs at the State Park and from Exploring the Geology of the Carolina by Kevin G. Stewart and Mary-Russell Roberson (UNC Press).

Snowfall map from 1-2 March 2009

The National Weather Service has produced a pretty map of snowfall totals from the storm a few weeks ago.  Mecklenburg County (Charlotte) got around 4″, which is a hair more than I measured at home on Monday morning (~3.5″ plus an ice layer). At our field site in Gaston County, the land owner told me he got ~5″ of snow, and that’s what the map shows as well.

A few semantics about climate variability and change

Last week, the Southeastern United States received several inches of snow. This late season snowfall was certainly a novelty, though not an unprecedented occurrence. But it did stir up conversations among local residents, especially when the week ended with ~25 degree Celsius (75 Fahrenheit) sunshine. The weather’s fickleness also got me thinking about climate variability and climate change and how easily we can slip up and confuse the two. I even see scientists (who should know better) conflating variability and change, so below I offer a short, illustrated tutorial on the differences.

Hydrometeorological variables are things like precipitation, streamflow, groundwater levels, temperature, and humidity and are often expressed as annual or seasonal averages. The average value of one of those variables over 30 years is called a climatological normal. Below, I’ve illustrated a hypothetical climate variable as it varies of a 30 year period. These normals are redefined every 10 years, so right now we are using 1971-2000 as our normal period.

An example (hypothetical) climate variable through time

Figure 1. A hypothetical climate variable through time

The average value of the variable is 0.5, and the squiggles above and below the mean represent climate variability. I’ll define climate variability as the oscillations around a mean state. (An aside: it’s fairly common to see a few years in a row that are below the mean or above the mean, in a phenomenon known as serial correlation, where the value of a variable is influenced by the values that precede it. As an example, if you have a severe drought one year, even if it rains more than normal the next year, streamflow may stay quite low as groundwater is replenished. This is what is happening in the southeast now after our 2007 drought.)

Variability then is all about the oscillations, but it doesn’t tell you anything about what’s happening with the mean. Below, I’ve illustrated the same time series shifted progressively by 0.003 per time step. Here the mean is changing, while the variability stays the same.

A (hypothetical) climate variable in blue is trending by 0.003 per year (with the non-trending time series in gray for comparison)

Figure 2. A hypothetical climate variable in blue is trending by 0.003 per year (with the non-trending time series in gray for comparison)

As in the illustration above, variables like average temperature and sea surface temperature are experiencing changes in their mean values. So, climate change can take the form of a trend in the mean value of a variable over time. A climatological variable experiencing change in the mean would not have the same “normal” values from one climate normal period to the next.

But climate change can also affect the variability of a variable, as illustrated below. Here the mean is not changing, but I’ve made below-mean points successively lower by 0.0067 per time step and above mean points are successively higher by 0.00347 per time step.

A hypothetical climate variable (blue) showing an increase in variability with time (gray line is the variable with unchanging mean and variability)

Figure 3. A hypothetical climate variable (blue) showing an increase in variability with time (gray line is the variable with unchanging mean and variability)

This sort of change is the sort of change we might see in precipitation in some areas. For example, the Southeastern United States is predicted to have more intense summer rainfall and more intense droughts, and retrospective trend studies suggest that this may already be the case. Even though the mean precipitation is not changing, the Southeastern United States is still experiencing a climate change effect manifested in a change in climate variability.

Finally, climate change can take the form of a trend in the mean and a trend in the variability, as shown below.

A hypothetical climate variable with changing mean and variability (gray solid line indicates variable with unchanging mean and variability, gray dotted line has a changing mean without changing variability)

Figure 4. A hypothetical climate variable with changing mean and variability (gray solid line indicates variable with unchanging mean and variability, gray dotted line has a changing mean without changing variability)

This final pattern may be the case for streamflow in some regions. Mean streamflow could decrease because of increasing evapotranspirative losses in a warmer climate, and streamflow variability could increase because of changes in precipitation and drought intensity. This sort of complicated pattern may occur for other climatological variables as well.

So what does this mean for “freak” late winter snowstorms in the southeastern United States? Climate change trending towards warmer temperatures makes frozen precipitation less likely (Figure 2), but given the variability inherent in meteorological systems (Figure 1), I wouldn’t rule it out entirely. But the snowshoes in my garage are still feeling a bit neglected.