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earth and environmental sciences

NATURAL SELECTION

Submitted by jdp on Tue, 12/02/2014 - 02:38 am

 

Natural selection is most familiar with respect to Darwinian evolution. However, though some biologists will argue that selection acts only on genes, this is a very narrow and restricted view. Selection operates on a variety of environmental phenomena, and at a variety of scales. In hydrology and geomorphology, the principle of gradient selection dictates that the most efficient flow paths are preferred over less efficient ones, and that these paths tend to be reinforced. That’s why water flows organize themselves into channels (more efficient than diffuse flows), and channels into networks. The principle of resistance selection in geomorphology is simply that more resistant features will persist while less resistant ones will be removed more quickly. Thus geomorphic processes select for certain forms and features and against others. Among others, Gerald Nanson, Rowl Twidale, and Luna Leopold have written on selection in geomorphology, and Henry Lin, among others, in hydrology.

 

Principle of gradient selection at work--Board Camp Creek, Arkansas

ANASTAMOSING CHANNELS

Submitted by jdp on Fri, 11/07/2014 - 06:33 am

Recently published in Earth Surface Processes & Landforms: Anastamosing Channels in the Lower Neches River Valley, Texas. The abstract is below: 

 

Active and semi-active anastomosing Holocene channels upstream of the delta in the lower valley of the meandering Neches River in southeast Texas represent several morphologically distinct and hydrologically independent channel systems. These appear to have a common origin as multi-thread crevasse channels strongly influenced by antecedent morphology. Levee breaching leads to steeper cross-valley flows toward floodplain basins associated with Pleistocene meander scars, creating multi-thread channels that persist due to additional tributary contributions and ground water inputs. Results are consistent with the notion of plural systems where main channels, tributaries, and sub-channels may have different morphologies and hydrogeomorphic functions. The adjacent Trinity and Sabine Rivers have similar environmental controls, yet the Trinity lacks evidence of extensive anastomosing channels on its floodplain, and those of the Sabine appear to be of different origin. The paper highlights the effects of geographical and historical contingency and hydrological idiosyncrasy.

 

TELECONNECTIVITY

Submitted by jdp on Sat, 11/01/2014 - 12:04 pm

 

Last month the climatologist Justin Maxwell from Indiana University gave an interesting talk at our department about drought-busting tropical cyclones. In his talk, and in conversations before and after with our physical geography crew, he had some interesting things to say about climate teleconnections involving mainly sea surface temperature and pressure patterns such as ENSO, NAO, etc. If teleconnections and the various acronyms are unfamiliar, check out the National Climatic Data Center’s teleconnections page: http://www.ncdc.noaa.gov/teleconnections/

PLACE SIMILARITY

Submitted by jdp on Wed, 10/22/2014 - 04:13 pm

I've thought, written, and talked a lot about the need to incorporate geographical and historical contingency--that is, idiosyncratic characteristics of place and history--in geosciences, in addition to (not instead of!) general or universal laws. I've also emphasized the fact that places and environmental systems have elements of uniqueness. This leads to the issue of how to measure or assess place similarity (or the similarity of different, e.g., landscapes, ecosystems, plant communities, soils, etc.). This is a way of thinking about this problem, dressed up with some formal mathematical symbolism. Though I'm personally pretty informal, I'm a big believer in formal statements in science, as it makes arguments at least partly independent of linguistic skills (or lack thereof). 

 

 

 

SCALE RATIOS

Submitted by jdp on Sun, 10/12/2014 - 06:10 pm

 

In fluid dynamics the Reynolds Number is the ratio of inertial to viscous forces, and is used to distinguish laminar from turbulent flow. Peter Haff (2007) applied this logic to develop a landscape Reynolds number, and also suggested how other generalized “Reynolds numbers” can be constructed as ratios of large-scale to small-scale diffusivities to measure the efficiencies of complex processes that affect the surface. As far as I know, there has been little follow-up of this suggestion, but the premise seems to me quite promising at an even more general level, to produce dimensionless indices reflecting the ratio of larger to smaller scale sets of processes or relationships. The attached file gives a couple of examples. 

 

THE SEMANTICS OF RESILIENCE

Submitted by jdp on Fri, 10/10/2014 - 01:29 pm

 

Resistance of environmental systems is their capacity to withstand or absorb force or disturbance with minimal change. In many cases we can measure it based on, e.g., strength or absorptive capacity. Resilience is the ability of a system to recover after a disturbance or applied force to (or toward) its pre-disturbance condition—in many cases a function of dynamical stability. In my classes I illustrate the difference by comparing a steel bar and a rubber band. The steel bar has high resistance and low resilience—you have to apply a great deal of force to bend it, but once bent it stays bent. A rubber band has low resistance and high resilience—it is easily broken, but after any application of force short of the breaking point, it snaps back to its original state.

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