jdp's blog

THEY TOLD US SO

When I was an undergraduate student at Virginia Tech from 1976-79, in several environmental science, geography, and ecology classes we were taught about global warming and climate change due to human activities such as burning fossil fuels and deforestation. The physics behind warming due to increasing atmospheric concentrations of carbon dioxide, methane, and other greenhouse gases had been well known for nearly a century by that time, and the fact that carbon dioxide concentrations in the atmosphere were increasing was also firmly established. Empirical evidence suggesting that human-caused climate change was already occurring was beginning to accumulate. 

THE GEOMORPHOLOGICAL VOICE OF THE BIOSPHERE

Vernadsky (1926) developed the concept of the biosphere as a planetary membrane that captures, stores, and transforms solar energy. The proportion of solar energy captured by the biosphere is small compared to that represented by climate processes, but large compared to other energy sources for landscape processes A tiny fraction of net primary productivity doing pedologic and geomorphic work (e.g., bioturbation, bioweathering, bioerosion, organic matter formation) is (as a global average) a greater energy input for landscape evolution than geophysical processes (Phillips, 2009a).

The soil and the biosphere have been characterized as an “excited membrane” or skin at the planetary surface stimulated by solar energy (Vernadsky, 1926; Nikiforoff, 1959). Can other aspects of landscapes—particularly landforms and topography—be characterized as an “excited membrane?”

AVULSIONS, STABILITY, AND SCALE

Alluvial rivers are dynamic, and often characterized by lateral migration, formation and eventual abandonment of meander loops, formation of anabranches or distributaries, and relatively abrupt shifts in channel courses. An understanding of river behavior, and effective management of resources in alluvial valleys, requires some understanding of the conditions under which these phenomena occur, and of the relationship between local and broader, reach-scale changes. This can be approached via the concept of dynamical stability.

DISTURBANCE IN THE BOTTOMLANDS

I’ve been spending a lot of time lately kayaking through tupelo/cypress (Nyssa aquatica and Taxodium distichum) swamps in eastern North Carolina, partly in connection with my continued dabbling in research, but largely for fun. I’ve also become interested in what I call “ravine swamps,” which exist along the valley side slopes of the Neuse River estuary and are dominated by tupelo gum (aka water tupelo, swamp tupelo) and bald cypress. Heck, I even own a little bit of one next to my house. 

Ravine swamp, Craven County, N.C., in winter. The scar on the foreground tree is from a floating log repeatedly bashing against it during a flood event (probably Hurricane Florence).

In my efforts to learn more about these bottomland hardwood swamps I read Status and Trends of Bottomland Hardwood Forests in the Mid-Atlantic Region (Rose and Meadows, 2016), and came across this: 

A FISTFUL OF DOLLARS

Landscape Evolution: Landforms, Ecosystems, and Soils has been published in the electronic version, and the hardcopy will be available soon. I have mentioned previously that I wish it was less expensive--$150 list, though you can still get it for 15% less if you preorder from the publisher: https://www.elsevier.com/books/landscape-evolution/phillips/978-0-12-821725-2

THE RESILIENCE TRIANGLE

The state of an Earth surface system (ESS) is determined by three sets of factors: laws, place, and history. Laws (L=L1, L2, . . . , Ln) are the n general principles applicable to any such system at any time. Place factors (P=P1, P2, . . . , Pm) are the m relevant characteristics of the local or regional environment. History factors (H = H1, H2, . . . , Hq) include the previous evolutionary pathway of the ESS, its stage of development, past disturbance, and initial conditions. Geoscience investigation may focus on laws, place, or history, but ultimately all three are necessary to understand and explain ESS. 

I first started using the law-place-history (LPH) framework as a pedagogic device in my teaching, and as a sort of aide memoire framework in my research. Eventually I began invoking it more explicitly in my research and writing, and expressing it formally, as in the passage above lifted from Phillips, 2018. 

THE SONGS OF LANDSCAPE EVOLUTION

My approach to the study of the structure, function, and evolution of Earth surface systems (landscapes) is based on the premise that they are controlled and influenced by three sets of factors, summarized as laws, place, and history. Law represents laws per se, such as the conservation of mass, energy, and momentum, and also for other generalizations and representations that are independent of location and time. Place represents the environmental context in which laws operate – ‘other things’ that are rarely equal. Path-dependent, historically contingent aspects of landscapes comprise the history. These may include sensitivity to initial conditions, previous evolutionary or developmental pathways, stages of development, disturbance histories, and time available for system development or evolution. 

My work and my approach also emphasize multiple possible evolutionary pathways and outcomes, the sensitivity of many Earth surface systems to seemingly minor variations and disturbances, and the amazing variety of landscapes resulting therefrom. These are discussed in fascinating/excruciating detail in my to-be-published-any-day-now book, Landscape Evolution.

FLOW PARTITIONING & SOIL GENESIS

Preferential flow occurs at all scales and in all phenomena in hydrological systems. By this we mean that rather than more-or-less uniform sheets of runoff moving across the ground surface, or more-or-less uniform bodies of water soaking into and seeping through soils and groundwater aquifers, most flow is concentrated in preferential flow paths.

Preferential flow of surface water—the universal tendency for flow to become channelized and form channel networks—is so obvious that it has been more or less taken for granted, though the mechanisms, and resulting structures and fluxes remain a highly active area of research. In soil and regolith, preferential flow has long been recognized, but its ubiquitous importance has only been widely recognized in recent decades. There are two main reasons for this, I think. Some forms of preferential flow, in soil pipes, large root channels, conduits, etc. are obvious, but other forms, such as macropore flow, are not readily recognized in the field. Second, a Darcian approximation of flow through porous media as a uniform mass often works fairly well, even where this type of flow is not necessarily dominant—presumably because the local preferential flow variations tend to average out in the aggregate.

THE SUBCRITICAL EASE OF BREAKING ROCKS

Several years back when I was working on the energetics of landscape evolution, focusing on the biological subsidy to geophysical energy sources, I tried to get my head around the energy required to turn fresh rock into transportable debris by weathering. I looked at factors such as the chemical activation energy of rocks, and various strength/stress measures (tensile & compressive strength, etc.). My geochemistry and rock mechanics background is/was too feeble for me to get very far with this beyond drawing two very broad conclusions:

1. It takes a lot of energy to break down rock.

2. There has to be more to it than the basic force/resistance principles (for example shear stress vs. shear strength) that I am familiar with, and that work perfectly well in some contexts. 

 

Breaking rocks the hard way

INSTABILITY & COMPLEXITY IN BANK EROSION

River and stream bank erosion is normally assessed—including studies I was involved in--by reaches. That is, the erosional (or stable or accreting) status is assessed along sample reaches of a certain length (e.g., 100 m long segments), or lengths of shoreline are classified—e.g., a 40 m erosional stretch, a 120 m stable stretch, 30 m accreting length, etc. Or, where rates are measured, they are presented (not illogically) as mean or characteristic values for a reach.  They are also generally averaged over time—so, for example, if the bank retreated 1 m in a decade, a rate of 0.1 m yr-1 is reported. Again, in many cases that is entirely reasonable, as (depending on the study design) there is often no way of knowing for sure if the erosion occurred at a more-or-less steady rate, all in one flow event, or somewhere in between. For some alluvial streambanks, there also exists uncertainty as to whether the observed retreat represents nothing but erosion, as opposed to net erosion—for instance, 1.5 m of retreat over 10 years, offset by 0.5 m of accretion somewhere during that time.

Eroding banks, Old River, Louisiana.

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