I just finished reading Paul Bogard's The Ground Beneath Us, (I recommend it), which among other things warns us yet again about the serious issues--environmental, economic, public health, food security--associated with over-reliance on chemical and fossil-fuel intensive industrial agriculture. It's a good 40-years-later follow-up to Wendell Berry's classic Unsettling of America: Culture and Agriculture (Sierra Club Books, 1977).

It also reminded me of a much more technical and difficult book I read a few years back, Jozef Visser's Down to Earth, subtitled "A Historical and Sociological Analysis of the Rise of 'Industrial' Agriculture and the Prospects for the Re-rooting of Agriculture in the Local Farmer and Ecology. Visser, who has graduate degrees in chemistry and a long career in agricultural chemistry, returned to graduate school later in life to produce this book, which is his dissertation from the University of Waginengen (Netherlands). A pdf is available free at the link above, and I recommend it.

The journal Hydrological Processes has recently been publishing a series of articles and commentaries in tribute to the estimable Keith Beven, the recently retired hydrologist from the University of Lancaster. One of his many fundamental contributions has consisted of drawing attention to the importance of, and making fundamental insight into, the phenomenon of macropores and preferential flows. One of those commentaries, by Markus Weiler, addressed these contributions as well as unresolved issues in understanding and simulating preferential flow.

No hillslope hydrologist, geomorphologist or pedologist would dispute the existence or frequent occurrence of preferential water flux in soils, or its importance in many cases at the scales of soil physics to hillslopes. However, Weiler points out that the observed differences in flow pathways at the pedon or hillslope scale are not necessarily detectable at the watershed scale. Does macropore flow matter at the catchment scale? Weiler's answer is yes, though he points out that many scientists believe otherwise.

From my student days onward, the aspects of nature that interested me most were the apparent anomalies--the things that were uncertain and unpredicted; that weren't like they were supposed to be. Nature contains both regular, ordered, predictable aspects, and irregular, disordered and unpredictable facets. As scientists we are taught to focus on the former and eliminate, ignore, or circumvent the latter.  But anyone who spends much time in the field knows that our planet is a source of infinite variety and ever-increasing uncertainty (because the more you learn, the more you realize that you don't know). But what always fascinated me was not that (for instance) the soils or streams or eastern North Carolina or central Kentucky fit, and can be predicted by, some broad pattern. It was the fact that you can often auger the ground at two spots less than a meter apart and find completely different soils, or walk or canoe a stream channel and easily find features not explained or predicted by the conventional scientific wisdom.

In 2011, a massive flood swept through the Lockyer Creek valley in southeast Queensland, Australia. The environmental, economic, and geomorphic impacts were immense, and Australian geoscientists immediately set out to document, understand, and contextualize them. The “Big Flood” project, led by Jacky Croke, has already produced 19 scientific journal articles, and they just this week went live with their web site, with numerous resources for scientists, managers, and the general public.

Floodwaters in Grantham, QLD, 2011 (http://www.thebigflood.com.au/whathappened.html)

The project has already produced some novel results with respect to flood geomorphology and hydrology, and is unique as far as I know with respect to direct efforts to integrate geoscience research with public policy, public education, and practical land and water resource management.

I recommend you check it out.

When more rain falls than the soil can absorb or plants can use, it has to go somewhere, and that movement is driven by gravity. Because concentrated flows are more efficient than sheet flows, concentrated and channelized flow paths are more likely to occur than diffuse flows. These pathways are also more likely to be reused, and to be enhanced by erosion associated with those flows. Similarly, when two of these threads of flow meet, they typically combine (less total surface area for the same amount of water = greater efficiency). Thus these channelized flows tend to form branching channel networks.

The formation of stream and river channels and networks is thus an emergent property of efficiency selection--those most efficient flow paths are more likely to arise in the first place and to be preserved and enhanced. The fact that most of these systems eventually lead to the sea (though globally, a surprisingly large minority drain to interior continental basins) is due to the fact that the flows are gravity driven, and for water, the ocean is the low point.

Geomorphological Flickering

As environmental systems approach critical thresholds or tipping points, they may experience increased variability, which in the literature on critical environmental state transitions has been referred to as “flickering” (e.g., Lenton, 2011; Scheffer et al., 2012; Dakos et al., 2013). This is primarily the case for noisy, stochastic systems, which is not the case for many lab and mathematical models, but is emphatically so for most real-world environmental systems. As Dakos et al. (2013) put it:

Most work on generic early warning signals for critical transitions focuses on indicators of the phenomenon of critical slowing down that precedes a range of catastrophic bifurcation points. However, in highly stochastic environments, systems will tend to shift to alternative basins of attraction already far from such bifurcation points. In fact, strong perturbations (noise) may cause the system to “flicker” between the basins of attraction of the system’s alternative states. As a result, under such noisy conditions, critical slowing down is not relevant, and one would expect its related generic leading indicators to fail, signaling an impending transition.

To me, colluvium—at least conceptually—is pretty simple. When soil or sediment is eroded (or mobilized via mass wasting) from a hilltop or hillslope, moved downhill, and redeposited before reaching a stream valley, then those deposited materials are colluvium.

But not everyone shares my perspective. Many soil scientists and engineers, for example, restrict colluvium to deposits associated with mass movements. Some geomorphologists attach additional criteria beyond those of my simple definition. This issue is important beyond basic issues of scientific communication, because the identification and measurement of colluvial deposits is critical for studies of sediment budgets and mass balances of hillslopes and drainage basins, and for understanding regolith development and pedogenesis.

From 2010 through the first two-thirds of 2015, at least 211 scientific articles with the term “tipping point” and 109 with “regime shift” in the title were published (according to the Web of Science database, as of 23 November 2015). These span a broad range of science, technology, and engineering, but the geosciences are well represented. In recent years the concept of tipping points in the global environment related to climate change, regime shifts, ecosystem collapse and other phenomena has garnered a great deal of both scientific and public attention. “Tipping point” is often used in public (and sometimes scientific) discourse to refer to impending doom, or at least major environmental changes with uncertain and potentially negative impacts. However, tipping points are not necessarily associated with negative impacts on humans. Nor are they inevitably associated with direct or indirect human agency, as Earth history is marked by numerous tipping points and regime shifts.

Axioms of the Dominant Controls Concept

The “dominant processes concept” in hydrological modeling argues, in essence, that there are too many potentially relevant hydrological processes to feasibly or efficiently include them all in a single model. However, in any given watershed a handful of processes dominate the hydrological response, and an effective model may be developed based on those. This argues for adapting models to local conditions and needs, rather than attempting to construct “one size fits all” models designed to handle any watershed, anytime, anywhere. Grayson & Blöschl (2000a) are credited with initiating the DPC; I encountered it through Bellie Sivakumar (2004, 2008).