Climate, Weather and Water: A Quick Review From This Fall

 DEC 29, 2012    by ROSIE RECORDS

Mississippi River
Photo: gettyimages.com

The evening I’m writing this, our first real snow this winter has been on the ground for barely a day.  My desk (or rather, kitchen table) is in the watershed of the Cache La Poudre River, at the foothills of the Rocky Mountains of Colorado.  The Cache La Poudre is part of the South Platte River which drains to the Platte River, a tributary to the mighty Missouri. The snow is very welcome after a long drought year in the Cache La Poudre, but the drought is still playing out far downstream in the Missouri’s receiving waters, the Mississippi. The Mississipi’s water levels are so low this year that the U.S. Army Corps of Engineers and the Coast Guard are blasting river rock formations along a 15-mile stretch of river in Illinois to allow the passage of commercial barges. http://www.csmonitor.com/USA/2012/1218/Drought-s-winter-toll-Mississippi-barges-face-losses-while-US-blasts-river-video

The Mississippi’s predicament highlights the effects of weather extremes on water resources.  For me, it also shows how these effects can scale up—from our dry summer in a relatively small watershed near the Continental Divide, to low flows in a large river basin thousands of miles downstream.  And, to me, it suggests how much we have to learn about potential effects of climate change on water resources, and about the tools we use to make these inquiries. Here are some questions related to climate and water resources that are near the front of my mind.  I’ve added some references at the end so you can follow up on anything of interest.  Most ideas are combed from this fall’s literature and popular science news.

First, what are alternative tools for climate impact assessment?  In my research (and in much of the literature) we use general circulation models to parse out scenarios for climate impact studies, and consider the spread in GCM-based projections to represent (some) future climatic uncertainty.  A recent Eos article made me revisit this approach.  Wilby and others describe climate model projections as scoping a “minimum range of irreducible uncertainty” [Brown and Wilby, 2012].  They suggest several alternatives to this “top-down”, GCM-based approach, including “decision scaling” and “scenario neutral” approaches.  Both begin by identifying management problems, objectives and performance metrics first (for example, the UK’s objective of design to accommodate a 20% increase to peak river flows); use sensitivity analysis/stress tests to identify system response to climate; and assess risk using GCMs along with other climate data.  What tools do you use for climate impact assessment?  Please share.

Next question: What are potential effects of climate change on river form and function?  The Mississippi River this winter is an example of relatively straightforward, weather- and flow-related changes to a river’s use.  But changes in flows may also change the river’s form, through changes in the balance of sediment scour, deposition, vegetation establishment, shifts in meandering and braiding, etc.  Particularly in the semi-arid West, the channel-shaping flows in many rivers are high-magnitude, low-recurrence interval floods.  If future precipitation and floods become more extreme and/or of a more frequent recurrence interval—or, in snowmelt dominated systems, if peak spring flows decrease under earlier melt and less precipitation as snow—how could this change river morphology?

In fact, our understanding of climate change effects on geomorphology generally is limited, argue Knight and Harrison in a Nature Climate Change article from this fall [2012].  They suggest there is a large need for monitoring of geomorphologic changes in response to climate shifts—for example, of river morphology and sediment loads.  But looking at historic examples, it can be fairly difficult to pin geomorphologic changes on climate alterations alone—as opposed to say, changes in land use together with climatic/environmental change.

There are other issues, too: River forms and processes are a function of a wide variety of flows and the sequence in which they occur, along with the river and watershed’s other unique characteristics; sediment transport and changes in river morphology are difficult to predict, so linking climate-change related alterations in flow to altered sediment transport could be a challenge; also, there are problems with “scaling up” results from small study areas (e.g., experimental watersheds) to large river basin scales.

To address the first issue, could we borrow ideas from the environmental flow literature to better understand possible geomorphologic and environmental impacts of changes in climate and hydrology?  Holistic approaches to environmental flows use specific statistics to examine magnitude, timing, frequency, duration, and rate of change of flows that are environmentally “necessary” [Arthington et al., 2006]. Monitoring of geomorphologic climate sensitivity sounds like a great idea, but what specific data could be collected, and what would be a good sampling strategy to give a state- or national-level picture of changes, given that rivers are likely to respond uniquely to climate and other changes?

On a related note, how could changes in quantity, timing, and duration of streamflows affect riparian and floodplain areas?  Could climate-related changes in flows change river connectivity and habitat, agriculture and development?  For example, cottonwoods, poster child of western riparian forests, are famously reliant on a specific combination of high snowmelt flows to scour floodplains bare for new growth during the release of cottonwood seeds.  USGS researchers in Colorado are assessing how projected climate may change timing of both spring floods and the cottonwood life cycle—and whether or not these changes are likely to keep cottonwood and floodwaters in synch (check out more athttp://www.doi.gov/csc/northcentral/NCPP-Pilot-Project.cfm#Shafroth ).

What are your thoughts on all or none of the above?  Let me know.  Also, if you have a chance, look over some of these newer articles on (1) Apparent trends in decreased precipitation variability over land for the second half of the 20th century [Sun et al., 2012]; (2) Record decreases in the last four years in late spring Arctic snow cover (with a special thanks to Glenn Patterson at Colorado State University for finding and sharing the article) [Derksen and Brown, 2012]; and (3) Summary of projected changes in snowmelt-driven Western watersheds, using CMIP5 data [Diffenbaugh et al., 2012].

References

Arthington, A. H., S. E. Bunn, N. L. Poff, and R. J. Naiman (2006), The challenge of providing environmental flow rules to sustain river ecosystems, Ecological Applications16(4), 1311–1318.

Brown, C., and R. L. Wilby (2012), An alternate approach to assessing climate risks, Eos, Transactions, American Geophysical Union,92(41), 92–94.

Derksen, C., and R. Brown (2012), Spring snow cover extent reductions in the 2008–2012 period exceeding climate model projections, Geophysical Research Letters39(19), 1–6, doi:10.1029/2012GL053387.

Diffenbaugh, N. S., M. Scherer, and M. Ashfaq (2012), Response of snow-dependent hydrologic extremes to continued global warming,Nature Climate Change2(11), 1–6, doi:10.1038/nclimate1732.

Knight, J., and S. Harrison (2012), The impacts of climate change on terrestrial Earth surface systems, Nature Climate ChangeAdvance on(October), 1–6, doi:10.1038/NCLIMATE1660.

Sun, F., M. L. Roderick, and G. D. Farquhar (2012), Changes in the variability of global land precipitation, Geophysical Research Letters,39(19), 1–6, doi:10.1029/2012GL053369.

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