ӻӿӺ ஼ၟആJ. Jpn. Soc. Soil Phys.
土壌の物理性No. 155, p.3 ∼ 7 (2023)

The “long and winding road” from soil physics
to catchment hydrology and hydrogeomorphology

Roy C. SIDLE

When the Editor-in-Chief of the Journal of the Japanese Society of Soil Physics, Professor Hirotaka Saito, invited
me to submit a paper to inspire graduate students and early career soil physicists to explore diverse avenues in theirrespective careers, I was more than happy to oblige. My ongoing afﬁliation with Tokyo University of Agriculture andTechnology during the past ﬁve years has given me many opportunities to further expand my interests and contribute toresearch in hydrological and sediment connectivity in catchments (e.g., Kharismalatri et al., 2019; Miyata et al., 2019),
hydrogeomorphic processes in earthquake affected volcanic soils (e.g., Sidle et al., 2018; Koyanagi et al., 2020; Arataet al., 2023), and, most recently, assessing the beneﬁts of living mulch on soil hydrology (Nishiwaki et al., 2022) andexamining hydrogeomorphic factors affecting rice paddy ﬁelds. These latter studies with Professor Saito, Dr. Nishiwaki,and Denis Bwire, have forced me to apply my background to completely new areas of inquiry, challenging the old adage“you can’t teach an old dog new tricks”.
Unlike the many graduates that entered the ﬁeld of hydrology by in past years via their backgrounds in civil engineering,
geography, and geology, as noted by the well-respected hydrologist Vit Klemeš (1986), I decided to study hydrology to‘escape’ from an engineering major during my undergraduate program at University of Arizona. In the late 1960’s,hydrology was a newly developing interdisciplinary major at University of Arizona fostering a unique approach to thescience (the Hydrology Department was still being organized). Professor Art Warrick was an important inspiration forme at that time as a struggling engineering student who was looking to make a better connection to the environment.
I look back on this as a very positive decision for my career, and I continue to tell students that the world needs bothengineers that focus on the optimal answers to problems and scientists who strive to unravel the underlying processes. Iwas much better at the latter than the former. Soil physics seems to ﬁt somewhere between these two disciplinary arenas
with typically more emphasis on quantitative scientiﬁc inquiry.
While I would not characterize any stage of my career as being entrenched in soil physics, I experienced a good
introduction to this discipline during my undergraduate and Master’s studies at University of Arizona under the inﬂuenceof Art Warrick and in classes with Dan Evans. Then during my PhD studies at Pennsylvania State University, I took
another soil physics class from Dan Fritton and proceeded to apply miscible displacement theory to the transport ofnitrate and heavy metals in soils and soil water. This latter research involved a very helpful connection with Rien vanGenuchten who was a post-doctoral researcher at Princeton University at that time. Rien collaborated with me on the ﬁnal
paper from my PhD research, which is some of the ﬁrst work to demonstrate the importance of preferential ﬂow in soils(Sidle et al., 1977). I vividly remember walking back and forth from my ofﬁce to Penn State’s computer center late atnight (when run time was less expensive) with two boxes of punch cards that contained my solute transport model hoping
I didn’t trip and drop the boxes.
While I moved away from relatively small-scale soil water and solute transport research over the years, one common
denominator that has stayed with me is the importance of preferential ﬂow in soils and weathered substrate. This initiatedfrom the ‘unintended discovery’ of this important phenomenon during my PhD research. As such, although my researchhas become generally broader with time, I have not forgotten the importance of understanding some of the small-scale proMountain Societies Research Institute, University of Central Asia, Bishkek, Kyrgyzstan.

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土壌の物理性第 155 号(2023)

cesses imbedded in soil physics (e.g., Sidle, 2006). But unfortunately, the ﬁelds of soil physics and catchment hydrology
still seem to be evolving separately.
It is interesting to me that some of the ‘new discoveries’ in catchment processes are based on decades old ideas thathave been repackaged in attractive ways. A prime example of this was a ﬂurry of recent articles published in high level
journals elucidating the concept of “two water worlds”. These studies used detailed and expensive tracer investigations
to show how plants use soil water held at different matric potentials compared to water that discharges to streams andgroundwater (e.g., Brooks et al., 2010; Evaristo et al., 2015; Berry et al., 2018). While these studies have likely produced
greater precision in estimates of, for example, transpiration versus evapotranspiration ratios, this in my opinion does notseem like a quantum leap beyond what soil physicists knew for nearly a Century. That is, gravitational water ﬂows rapidly
through soils to recharge groundwater and support streamﬂow, and unsaturated water held in micropores at high tensions
does not appreciably contribute to groundwater or streamﬂow, but instead is available for uptake by plants as long istensions do not exceed the so-called ‘wilting point’. While I am obviously over-simplifying this comparison and probably
doing these studies some level of injustice, it appears to me that that more emphasis on soil physics and less focus on
expensive isotope analyses would have better addressed the problem and put this issue to bed.
As my career evolved, I became interested in landslide processes and related sediment transport in catchments. Inearly ﬁeld investigations in coastal Alaska where it was almost impossible to do any detailed laboratory or instrumental
analyses, I was able to observe the importance of preferential ﬂow in relation to rapid piezometric response in hillslope
hollows. We published a few papers on this phenomenon, including likely the ﬁrst paper showing pore water pressure
conditions during the occurrence of a small landslide (Sidle and Swanston, 1982). At that time, I was not aware of some ofthe studies in Japan, particularly those by Professor Tsukamoto. Just after leaving Alaska, I had the pleasure of meeting
Professor Tsukamoto at an IAHS meeting in Corvallis, Oregon, which provided my initial insights into the excellent
research being conducted in Japan.
An unexpected opportunity arose in 1991 that literally changed my career and life. I received a nine-month Science& Technology Agency Fellowship from Japan based on an invitation from Professor Makoto Tani (at that time he was at
Forestry and Forest Products Research Institute, FFPRI). This initial visit opened my horizons to some of the excellent
hydrology research that was being conducted in Japan, much of which was published in Japanese with little recognitionin the western world. This and numerous follow-up visits provided the basis for developing the relatively new scienceof hydrogeomorphology, including numerous publications with Japanese scientists. Working with FFPRI researchers wedeveloped the hydrogeomorphic concept of stormﬂow generation (Sidle et al., 2000) supported by extensive ﬁeld studies
and long-term monitoring at various spatial scales. Through this research I recognized the beneﬁts of nested catchment
studies in elucidating spatial and temporal stormﬂow runoff response in headwater catchments. Within this context, wealso employed soil physics theory to assess experimental tracer test data collected from soil pits (e.g., Tsuboyama et al.,1994). Combining these data and inferences at different spatial and temporal scales shed important light on the dominant processes that contribute to storm runoff during changing antecedent moisture conditions. Importantly, wetness
thresholds were identiﬁed that dictate conditions that control the release of subsurface runoff to streams from geomorphic
hollows (zero-order basins) and those that facilitate the self-organization of preferential ﬂow across larger hillslope domains (Tsuboyama et al., 2000; Sidle et al., 2001). Later work with John Nieber using soil physics theory modelled how
discrete macropore segments can connect under conditions of increasing soil wetness (Nieber and Sidle, 2010).
Ultimately, a parsimonious conceptual model was developed based on the hydrogeomorphic paradigm that simulated
discharge from hillslopes and zero-order basins using multi-tank models and a kinematic wave model to route runoff
from riparian areas and zero-order basins through channels (Sidle et al., 2011). The objective of this model was thisdemonstrate that better prediction of runoff from various ﬂow paths and catchment elements (zero-order basins withvariable soil depth, riparian zones, linear hillslope segments, soil matrix, and preferential ﬂow) can be achieved by asemi-distributed model that effectively captures runoff processes rather than a highly parameterized physics-based modelthat incorporates only small-scale hydrologic information (e.g., saturated hydraulic conductivity, Ks , values). While
this hydrogeomorphic model for headwater catchments did not directly utilize soil physics theory, the subsurface ﬂow
analogues were derived from miscible displacement ﬁeld experiments that elucidated dynamic ﬂow contributions andthreshold behavior in macropores and the soil matrix.
This scale conundrum related to parameterization of hydrologic models exposes an important issue concerning howhydrologists misuse soil physics data as well as the need for soil physicists to quantify important parameters (e.g., Ks ) at
scales useful in catchment hydrology. This ‘wicked’ problem is typically glossed over by simply calibrating catchmentdischarge by Ks as part of an inverse modelling exercise. While such modelling may be useful to generate simulatedメッセージ5discharge that matches measured catchment runoff, such practices embrace equiﬁnality ‒ i.e., getting the right answer
for the wrong reasons (Klemeš 1986). As such, my knowledge of distributed catchment processes together with myrather meager soil physics expertise has pushed me to better understand ﬂow pathways and dynamics in catchments andhow these can be parsimoniously incorporated into models (e.g., Sidle, 2021). During a presentation of some the earlier
work related to how preferential ﬂow self-organizes across hillslopes based on antecedent moisture thresholds and thencontributes to runoff generation, a rather well-known hydrologist (unnamed here) accused me of being a ‘reductionist’.
While that may be somewhat true, I would argue that by ignoring ﬂow paths at multiple scales (including detailed scales),one may conclude erroneous mechanisms and pathways that contribute to runoff. And if the objective of the research isto assess impacts of land use practices on runoff and material transport, then we are setting ourselves up for erroneous
inferences. Unfortunately, I do not see much attention focused on this important issue, either in catchment hydrology orsoil physics. Thus, this appears to be an important intersection where these disciplines could collaborate better to ‘move
the needle forward’.
After holding professor positions at University of British Columbia and National University of Singapore where I
conducted research on catchment hydrology, erosion processes, and landslides, I joined Kyoto University’s Disaster Prevention Research Institute in 2002 as the ﬁrst foreign professor succeeding Professor Okunishi who advanced early ideas
in hydrogeomorphology. This opportunity came with many challenges and rewards and was the most productive periodof my career due to an excellent group of post-doctoral researchers and graduate students that worked in my laboratory.
This research produced important ﬁndings related to modelling preferential ﬂow with implications for slope stability (e.g.,Tsutsumi et al., 2005), hydrogeomorphic linkages between landslides/debris ﬂows and sediment transport in catchments(e.g., Imaizumi and Sidle, 2007), and insights into scale effects on runoff connectivity (e.g., Gomi et al., 2008). The
three lead authors on these papers now hold full professor positions at Mie University, Shizuoka University, and Nagoya
University, respectively, and are some of the leading voices in their ﬁelds in Japan. During the six years I spent at Kyoto University, I also beneﬁted from numerous interactions with Japanese scholars in ﬁelds of engineering, soil science,water resources management, geography, and geomorphology, as well as numerous international colleagues, particularly
related to continuing catchment hydrology studies in Southeast Asia. Another accomplishment during my time at Kyoto
University was writing the American Geophysical Union monograph “Landslides: Processes, Prediction and Land Use”
with my colleague and good friend Hirotaka Ochiai (Sidle and Ochiai, 2006). This book examined landslide processes
from multiple perspectives, including the important role of soil physical properties and subsurface hydrology. Overall, I
found that the freedom I had to explore hydrological processes across various scales and within the context of land use
scenarios fostered scientiﬁc discovery and was appreciated by most of my Japanese colleagues.
During the past decade or so, my research horizons have expanded to deal with issues such as fate and transport ofcontaminates in managed catchments while I was Director of the Ecosystems Research Division of US Environmental
Protection Agency, sustainability science issues during my time in Australia at University of the Sunshine Coast, andeffects of climate trends, water supplies in the cryosphere, and natural hazards on mountain communities in my current
role as Director of the Mountain Societies Research Institute at University of Central Asia. The latter has been by far
the most challenging experience in my career dealing with not only the rugged and remote terrain in the Pamir, but also
working with impoverished mountain communities in Tajikistan along the precarious Afghanistan border. In approaching
many of these recently encountered scientiﬁc venues, I keep refocusing on hydrological processes and how these shapelandforms, deliver water and pollutants to streams and rivers, affect mountain food security, and manifest with changing
climates. So, while focus may change, it should be supported by robust process understanding.
Thus, as I reach the twilight of my ‘winding road’ that has allowed me to work in six continents, I feel grateful that
I have been intimately exposed to all three parts of this water-based journey — soil physics, catchment hydrology, andhydrogeomorphology. Early in my career, I learned the value of careful ﬁeld investigations, an aspect that has stuck withme through today. I sometimes see this important attribute being lost on some younger colleagues and students whofocus mostly on modelling without properly understanding how hydrological processes manifest in the ﬁeld, particularly
at different scales. The opportunity I had to work in coastal Alaska as a young hydrologist/soil scientist with US Forest
Service Research instilled a deep interest in landslide initiation and other hydrogeomorphic processes, as well as a respect
for the importance of ﬁeld investigations. I am indebted to the many colleagues in Japan who have supported my workover the years and contributed to what I feel are the most important research products of my career. I learned so muchfrom these scientists in places like Kyoto Univrsity, FFPRI, Tokyo University of Agriculture and Technology, Niigata
University, and University of Tsukuba. The varied environments I have worked in have shaped my understanding ofhydrological and hydrogeomorphic processes. These range from the mountains and landscapes of Appalachia, Paciﬁc

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Northwest USA, coastal Alaska, Japan, the Andes, China, India, Denmark, Holland, Australia, New Zealand, SoutheastAsia, Italy, France, Saudi Arabia, Central Asia, and Afghanistan. Few people have been afforded such opportunities andI feel very grateful for these experiences, including working on publications with scientists in most of these regions.
In closing, I would urge soil physicists, catchment hydrologists, and hydrogeomorphologists to collaborate more on
issues of common interest to improve quantiﬁcation and insights related to complex hydrological processes that spanvarious scales and media (e.g., overland ﬂow, ﬂow in the vadose zone, contributions to stream and river runoff, interactions
at the soil-atmosphere interface), especially related to addressing environmental management issues. Working together,
these combined disciplines can help solve some of the ‘wicked problems’ facing our ever-changing environment. As I
noted in an earlier paper, one of my favorite ballads (“The Long and Windy Road” by Paul McCartney) contains a linethat embraces these challenges and some frustrations that I have tried to address during my career ̶ “The wild and windynight, that the rain washed away, has left a pool of tears crying for the day”.

References

Arata, Y., Gomi, T., Sidle, R.C., Saito, H. and Wang, G. (2023): Soil–water response in a volcanic ash hillslope affected by ﬁssuresand microtopographic changes caused by the Kumamoto earthquake, in Japan. Hydrol. Process., 37(8), e14947.
Berry, Z.C., et. al (2018): The two water worlds hypothesis: Addressing multiple working hypotheses and proposing a way forward.
Ecohydrology, 11(3): e1843.
Brooks, J.R., Barnard, H.R., Coulombe, R. and McDonnell, J.J. (2010): Ecohydrologic separation of water between trees andstreams in a Mediterranean climate. Nature Geosci., 3(2): 100–104.
Evaristo, J., Jasechko, S. and McDonnell, J.J. (2015): Global separation of plant transpiration from groundwater and streamﬂow.
Nature, 525(7567): 91–94.
Gomi, T., Sidle, R.C., Miyata, S., Kosugi, K. and Onda, Y. (2008): Dynamic runoff connectivity of overland ﬂow on steep forested
hillslopes: scale effects and runoff transfer. Water Resour. Res., 44(8): W08411.
Imaizumi, F. and Sidle, R.C. (2007): Linkage of sediment supply and transport processes in Miyagawa Dam catchment, Japan. J.
Geophys. Res., Earth Surf., 112: F03012.
Kharismalatri, H.S., Ishikawa, Y., Gomi, T., Sidle, R.C. and Shiraki, K. (2018): Evaluating factors for controlling sediment connectivity of landslide materials: a ﬂume experiment. Water, 11(1), 17.
Klemeš, V. (1986): Dilettantism in hydrology: transition or destiny? Water Resour. Res., 22(9S): 177S–188S.
Koyanagi, K., Gomi, T. and Sidle, R.C. (2020): Characteristics of landslides in forests and grasslands triggered by the 2016
Kumamoto earthquake. Earth Surf. Process. Landforms, 45(4), 893-904.
Miyata, S., Gomi, T., Sidle, R.C., Hiraoka, M., Onda, Y., Yamamoto, K. and Nonoda, T. (2019): Assessing spatially distributed inﬁltration capacity to evaluate storm runoff in forested catchments: Implications for hydrological connectivity. Sci. Total Environ.,669, 148–159.
Nieber, J.L. and Sidle, R.C. (2010): How do disconnected macropores in sloping soils facilitate preferential ﬂow? Hydrol. Process.,
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Nishiwaki, J., Koseki, T., Asagi, N., Saito, H. and Sidle, R.C. (2023): Changes in soil hydraulic conductivity in sweet potato ﬁeld
with living mulch. Agric. & Environ. Letters, 8(1), e20106.
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Sidle, R.C. and Ochiai, H. (2006): Landslides: Processes, Prediction, and Land Use, American Geophysical Union, Water Resources
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Sidle, R.C. and Swanston, D.N. (1982): Analysis of a small debris slide in coastal Alaska. Can. Geotech. J., 19(2): 167–174.
Sidle, R.C., Gomi, T., Akasaka, M. and Koyanagi, K. (2018): Ecosystem changes following the 2016 Kumamoto earthquakes inJapan: Future perspectives. Ambio, 47, 721–734.
Sidle, R.C., Kardos, L.T. and van Genuchten M.Th. (1977): Heavy metals transport model in a sludge–treated soil. J. Environ. Qual.,
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Sidle, R.C., Kim, K., Tsuboyama, Y. and Hosoda, I. (2011): Development and application of a simple hydrogeomorphic model forheadwater catchments. Water Resour. Res., 47(3): W00H13.

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Sidle, R.C., Noguchi, S., Tsuboyama, Y. and Laursen, K. (2001): A conceptual model of preferential ﬂow systems in forested
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Sidle, R.C., Tsuboyama, Y., Noguchi, S., Hosoda, I., Fujieda, M. and Shimizu, T. (2000): Stormﬂow generation in steep forested
headwaters: a linked hydrogeomorphic paradigm. Hydrol. Process., 14(3): 369–385.
Tsuboyama, Y., Sidle, R.C., Noguchi, S. and Hosoda, I. (1994): Flow and solute transport through the soil matrix and macroporesof a hillslope segment. Water Resour. Res., 30(4): 879-890.
Tsuboyama, Y., Sidle, R.C., Noguchi, S., Murakami, S. and Shimizu, T. (2000): A zero-order basin — its contribution to catchmenthydrology and internal hydrological processes. Hydrol. Process., 14(3): 387–401.
Tsutsumi, D., Sidle, R.C. and Kosugi, K. (2005): Development of a simple lateral preferential ﬂow model with steady state application in hillslope soils. Water Resour. Res., 41(12): W12420.