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.

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

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.

Just published in Geomorphology, with Pavel Šamonil: Biogeomorphological domination of forest landscapes: An example from the Šumava Mountains, Czech Republic.

This paper originated from a more general study of forest biogeomorphology in unmanaged forests of the Czech Republic. In higher elevations and upper slopes mainly in the Šumava National Park, we noticed an almost complete lack of stream channels and surface runoff, except on or near roads and associated drainage features (and in the valley bottoms).

Pavel Šamonil (right) and yours truly.

Elsevier has announced a May 1 predicted shipping date for my forthcoming book, Landscape Evolution, cheerfully assuming that someone will want it. Below is the mockup of the back and front covers, complete with descriptive blurb and cover art by Petr Mores. You can pre-order at https://www.elsevier.com/books/landscape-evolution/phillips/978-0-12-821725-2.

In my own work on pedology, soil geography, and soil geomorphology, there are at least three overlapping concepts of soil memory (or pedological memory) that at least generally, if clumsily, parallel the pedological literature as a whole.  

One is based on the fundamental idea of soils as products of the environment, reflecting the combined, interacting influences of geological (or other) parent material, hydrological and geomorphological processes, climate, biota, all changing over time, and affecting each other and affected by the soil itself.  This is the factorial or state factor model of soils going back to Dokuchaev and Jenny, and the conceptual basis of soil geography, surveying and mapping. Inverting the logic—soils as evidence of the environment rather than the environment as an explainer of soils—is the basis of paleopedology and paleoenvironmental interpretations of paleosols. This concept of soil memory is that soils “remember” the environmental factors influencing their genesis and development.

Carsten Lorz has used the memory of soils such as this one in Saxony, Germany, to reconstruct Quaternary pedogenesis. 

Off-road vehicle trails, especially those used extensively by ATVs (all terrain vehicles, four-wheelers, quads), are notorious for producing a lot of soil erosion. Over a period of several years, I was involved in a project funded by the U.S. Forest Service, to determine where the soil eroded from trails in the Wolf Pen Gap ATV trail system in the Ouachita National Forest, Arkansas was going. That the trails are eroding was never in question--the questions were how much and how fast, what the off-trail impacts on streams are, and where the eroded sediment goes.

Sediment-laden runoff from an ATV trail in the Wolf Pen Gap complex near Mena, Arkansas.

The last of a series of articles based on this work has finally been published, as shown below!

This was actually the second-to-last that we produced, but it took longer to get published. The last one was in print last year:

Every geomorphologist, soil scientist, farmer, backhoe operator or other person who digs holes in certain areas is familiar with them—floaters. I am not sure how widespread the terminology is, but the phenomenon is ubiquitous in areas where the regolith is derived mainly from underlying bedrock. Floaters are large rock fragments, unattached to underlying bedrock, within a soil or weathering profile.

Floaters in a limestone weathering profile, central Kentucky.

For farmers and excavators, floaters are mainly an annoyance. For pedologists and geomorphologists they can also be an annoyance. This is not only due to the difficulties they pose for digging and sampling, but because—particularly with augers, probes, and core samplers—they can easily be mistaken for underlying bedrock, resulting in underestimates of the depth and thickness of soils, regoliths, and weathering profiles.

But can these floaters tell us anything about weathering profile, critical zone, and regolith evolution?

This post continues the story of a collaboration with artist Petr Mores to combine scientific and artistic perspectives to tell stories of landscape evolution.

One of the first issues we encountered was that Petr, while wonderfully experienced in depicting nature from the ground up, was not accustomed to representing underlying soils and geology. Pavel and I, on the other hand (like many other pedologists and geomorphologists) are quite familiar with showing soils, regoliths, weathering profiles, and parent rock in the form of two-dimensional profiles and cross-sections, either in highly simplified forms or in annotated photographs. 

Mountain stream (Petr Mores)

Typical two-dimensional representations of soil profiles (by Carsten Lorz, from Phillips & Lorz, 2008).