WaterUnderground

# groundwater flow

## A do-it-yourself Jupyter notebook to constrain sediment permeability

Post by Elco Luijendijk, Junior lecturer in the Department of Structural Geology and Geodynamics at Georg-August-Universität Göttingen and WaterUnderground founder Tom Gleeson (@water_undergrnd), Associate Professor in the Department of Civil Engineering at the University of Victoria.

Most of the groundwater on our planet is located in sedimentary rocks. This is why it is important to know how easy or hard it is for water to flow through pores in sediments, which is governed by permeability. Unfortunately, permeability is extremely variable. Wouldn’t it be great if we could estimate permeability based on sediment types (for which a decent amount of data exist)?

Enter the 150+ year challenge to estimate the permeability of sediments with universal equations. Most of the equations work well for one sediment type, such as pure sands or clay. For instance, the Kozeny-Carman equation from the 1920s tends to work well for most granular materials such as sand or silt. However, pure sands or clays are rare, and most of what’s out there are mixtures.

Evaluating how well existing and new equations work for mixed sediments is tricky business. Searching high and wide only three datasets with 78 samples were found that contained all the required information (grain size distribution, clay mineralogy). Needless to say, more data are needed to improve the predictive equations. In a paper published a few years ago we found that in most cases, the permeability of the sediments could be estimated in a two-step process:

• calculate the permeability of clay and granular (sand/silt) components, and
• calculate the permeability of the mixed sediment by taking the geometric mean of the two components weighed by the clay content of the sediment.

The resulting workflow was published as a series of equations that are not particularly easy to work with. That is why we recently decided to take advantage of the general awesomeness of Jupyter notebooks to publish a do-it-yourself notebook to calculate permeability on GitHub (https://github.com/ElcoLuijendijk/permeability_notebooks). For those of you new to Jupyter notebooks: these are documents that contain a readable mix of text, code, data and figures and can be used to publish studies in such a way that you can reproduce the analysis and make the figures yourself (much like R Markdown).

The Jupyter notebooks to calculate permeability consist of a main notebook and additional notebooks to calculate the specific surface area of sediments. Also included are all the calibration datasets Jthat were compiled for the publication. You can use the data to evaluate how well the permeability equations match these datasets, or you can set up a new spreadsheet with data from your own study area which can then be used by the notebook to calculate permeability. The notebook automatically generates several figures like the one below (Figure 1).

There is also an additional notebook that calculates first-order estimates of permeability from well log data collected by geophysical tools that map the density or water content of sediments. Such well log data can be more widely available than detailed sediment records and may help estimate permeability for the deeper subsurface (>100s of m), where permeability data are generally scarcer than at the surface.

Comparing these datasets and equations with the Jupyter notebooks highlight the gaps in quantifying permeability. These notebooks and datasets are out there for the world, so join the effort to make more accurate predictions of permeability (and groundwater flow) in sediments!

Figure 1: Figure produced by the Jupyter notebook showing measured vs calculated permeability using an example dataset of mixed natural sediments.

## Of Karst! – short episodes about karst

Post by Andreas Hartmann Assistant Professor in Hydrological Modeling and Water Resources at the University of Freiburg.

#### Episode 4 – Karst Groundwater: quick and slow at the same time?

We often associate groundwater with large water storage and very slow water movement for instance compared to rivers. But is it possible that groundwater flow can be as quick as stream flow and, at the same aquifer, flow for several months or years before it is reaching the surface again? Of karst, it is possible! When chemical weathering is able dissolve carbonate rock, cracks and fissures may grow to a subsurface channel system that can take vast amounts of water flow (see Of Karst! – episode 2).

The schematic figure below shows how this affects water flow in a karst system. At the surface, water may flow for some distance (external runoff towards the recharge area or internal runoff within the recharge area), before it reaches a dissolution widened vertical crack or fissure. On its way, part of it may slowly infiltrate into the soil but the stronger the rainfall event, the more water will infiltrate quickly into cracks and fissures after being redistributed laterally. Consequently, slow and quick infiltration will be followed by slow and quick vertical flow through the vadose zone. The former through the carbonate rock matrix, the latter through the interconnected system of dissolution caves. Finally, recharge and groundwater flow take place, again quickly through the caves and slowly through the matrix.  When passing the system through the cave network, water can enter and leave the system within several hours. When taking the slow and diffuse path, the transit through the system may take months to years.

Because of this behavior, hydrogeologists often speak about the Duality of Karstic Groundwater Flow and storage, although it is known that there is a wide range of dynamics between quick flow through the caves and slow flow through the matrix and that lateral redistribution between the interconnected caves and the matrix takes place at almost every part of the system.

Figure 1: Schematic description of karstic groundwater flow and storage (Hartmann et al., 2014; modified)

A rather uncomfortable lesson on quick flow processes in karst was learned by a group of school students on a trip through a karstic cave in Thailand. Due to the quick recharge processes explained above, the groundwater tables could quickly rise blocking the return path of the group and resulting in a dramatic rescue mission:

In order to predict the impact of interplay of quick and slow karstic groundwater processes on cave water levels or water resources in general, karst-specific simulation models are necessary. If you are interested in those, follow the Water Underground blog’s postings and wait for Of Karst! Episode 5, which will introduce karstic groundwater modelling.

Andreas Hartmann is an Assistant Professor in Hydrological Modeling and Water Resources at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com

`Further reading: Hartmann, A., Goldscheider, N., Wagener, T., Lange, J., Weiler, M., 2014. Karst water resources in a changing world: Review of hydrological modeling approaches. Rev. Geophys. 52, 218–242. doi:10.1002/2013rg000443`

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## Community advice to young hydrologists, Part 1

We at Water Underground loved reading Young Hydrologic Society’s post titled “Community advice to young hydrologists” – an advice column written by a network of established scientists in the field. We appreciated the column so much, in fact, that we have decided to re-blog the post to you (with YHS’s consent, of course). We’ve split up their post by question, and have added in hyperlinks to all contributors and related material (as has always been our inclination). Happy reading!

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## Question: What book or paper has been most influential to your career and why?

Groundwater by Freeze and Cherry – this textbook, now out of print, was a critical reference as I began my graduate training in hydrogeology and I still refer to it today.

Jean Bahr (University of Wisconsin)

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I can think of no single one.  However, papers that were a combination of field observations and clever analyses leading to new insights always are intriguing.  Papers which I find of little value are those that propose a new modeling approach with little to no field verification, or which use existing models to reach some conclusion.  For example, we seem to be seeing a proliferation of papers using complex models to highlight some “new” effect of climate change on the hydrologic cycle, with no grounding in hindcasts. (See this, also) The musings of Keith Beven always have been insightful, including his Advice to a Young Hydrologist.

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I can’t identify single “most influential” books or papers – I learned early to read as widely as possible, and not just within narrow/specific research problems of direct interest. I have been inspired by a range of articles – including books on philosophy, history of physics, etc. – which broadened my approach and ways of looking at a given problem. Indeed, some of my most influential work developed from studying methods and approaches in statistical physics and physical chemistry.

Brian Berkowitz (Weizmann Institute of Science)

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The most important influence was a person – Mike Kirkby and particularly the undergraduate course on quantitative hydrology he taught at the University of Bristol when I was taking my degree there (later, I would do a post-doc with him at Leeds that resulted in the development of Topmodel). That gave me a lot of reading to do – but it was probably not the hydrological reading that had most influence, but rather the papers on theoretical geomorphology starting with Horton BGSA 1945, then picked up by Kirkby, Frank Ahnert and others in the late 1960s. I struggled to understand them (at the time I wanted to be a geomorphologist but I have never quite finished getting the water part right) but they left me the idea that it was possible to theorize about environmental processes and systems in approximate but useful ways.

During my PhD the most influential paper was undoubtedly Freeze and Harlan JH 1968, and the papers about the field site I was applying my model to by Darrell Weyman (HSB 1970, IAHS 1973). If I had talked to him a little more (he was doing his PhD at Bristol while I was an undergraduate) or read those papers more carefully, then I might have been more realistic in my PhD modelling.

The most important book at that time was Zienkowicz, Finite Element Modelling (that was the technique I was trying to master). Hillslope Hydrology edited by Kirkby was also important but came later.

Keith Beven (Lancaster University)

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Paper: Scale of Fluctuation of rainfall models by I. Rodriquez-Iturbe. It formed the basis for my MSc research that I did during 11 months in Davis California (As a Dutch Student from Wageningen). It was extremely difficult stuff, but I kept on it and it understanding gave me the stamina to really dig into a subject. It was the basis for my first paper entitled “Analytically derived runoff models based on rainfall point processes” in WRR. To obtain better background I also read in depth the influential.

Marc Bierkens (Utrecht University)

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Dooge’ 1986 Looking for hydrologic laws in WRR. This paper gives a broad perspective on science, including scales.

Günter Blöschl (TU Vienna)

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Konrad and Booth (2005), Hydrologic changes in urban streams and their ecological significance, American Fisheries Society Symposium, 47:157-177.  This paper is a bit outside my area of expertise, but I think the linkage they make between physically measurable streamflow changes and stream ecology represents a fundamental shift in thinking from engineering hydrology to more of an eco-hydrology perspective.  They illustrated that we need to go beyond analyzing just changes in peak flow or low flows (or fixed percentiles), to look at more derived metrics that better capture hydrologic regime change.

Laura Bowling (Purdue University)

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That is a very hard question. As a Geography undergraduate student, I had to write a particular essay on the “all models are wrong” theme and this involved critiquing two papers which completely changed my worldview about models and modelling: Konikow and Bredehoeft’s 1992 ‘Ground-water models cannot be validated’ Advances in Water Resources 15(1):75-83.  and Beven’s 1989 ‘Changing ideas in hydrology – the case of physically-based models’ Journal of Hydrology.

But in the last year, I would say it has been Lab Girl by Hope Jahren (2016) who is a gifted and talented scientist and writer and has the knack of intertwining the natural world with tales of remaining brave in your career. I wish I’d had the opportunity to read it earlier in my career.

Hannah Cloke (University of Reading)

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Ecological and General Systems – H.T. Odum. This book explores general systems theory in the context of ecosystem behaviors. It is holistic, comprehensive, and full of important insights about the structure and dynamics of systems.

Matthew Cohen (University of Florida)

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It is a novel by Milan Kundera: “Slowness”. My natural tendency is to rush up, be as fast as possible, quickly fix things… Yet, speed often leads to miserable outcomes. Many lines of Kundera’s book are still in my mind, and they work as a continuous reminder for me that only slowness allows thoughtful consideration, serious reflection, and appreciation of reality. Realizing this has strongly influenced my academic career as it made me focus on the quality (and not the quantity) of my work.

Giuliano Di Baldassarre (Uppsala University)

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Several hydrogeology-related texts were very helpful for me.  These include some of Mary Hill’s papers, John Doherty’s PEST manual (as much for the philosophy as the instruction), some of Jasper Vrugt’s early papers, and work by both Wolfgang Novak and Steve Gorelick on measurement design. The real recommendation would be to find authors that you enjoy and read as much of their work as possible – in this category, I would add Shlomo Neuman, Randy Hunt, Hoshin Gupta, Dani Or, Keith Beven and Graham Fogg. I am sure that I am forgetting more than I have listed. I think it is equally important to read broadly. Rather than provide a list, I’ll encourage you to look at my recent paper in Ground Water (Sept 2016) for some suggestions!

Ty Ferré (The University of Arizona)

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Book:  Groundwater Hydrology by David Keith Todd, 1st edition, 1959. As a 3rd-year undergraduate in hydrology at the University of New Hampshire in 1973, this book (and course by Francis Hall) kindled my interest in groundwater and completely changed my career path, which previously was essentially an aimless sleepwalk through my major in mathematics.

Paper/report:  Kaiser, W. R., Johnston, J. E., and Bach, W. N.. 1978, Sand-body geometry and the occurrence of lignite in the Eocene of Texas: The University of Texas at Austin, Bureau of Economic Geology Geological Circular 78-4, 19 p.  This paper demonstrated in stunning detail how modern borehole geophysical data together with understanding of the geologic genesis of sedimentary deposits could be used to create unprecedented subsurface maps of aquifer/aquitard system heterogeneity and structure. This led me down the long path of better integrating groundwater hydrology and geologic depositional systems.

Graham Fogg (UC Davis)

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My interests have been in predictive hydrometeorology. The following were influential books at the start of my carrier in the late 70s and early 80s: Dynamic Hydrology by Eagleson; by Wallace and Hobbs; Applied Optimal Estimation by Gelb (ed).  These represented the fields of hydrology, meteorology, and estimation theory with applications to prediction, and were the necessary pillars to build predictive hydrometeorology.

Konstantine Georgakakos (Hydrologic Research Center in San Diego)

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Haitjema and Mitchell-Bruker (2005) which taught me to think of groundwater as a process that interacts with topography, climate and geology in complex but predictable ways.

Tom Gleeson (University of Victoria)

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The paper that has been most influential to my career is most certainly  “Johnston, P. R., and D. H. Pilgrim (1976), Parameter optimization for  watershed models, Water Resources Research, 12(3), 477–486. I read this paper during my graduate work in the early 1980’s and was intrigued by their report that “A true optimum set of (parameter) values was not found in over 2 years of full-time work concentrated on one watershed, although many apparent optimum sets were readily obtained.”

On the one hand this paper clearly identified an important problem that needed to be addressed. On the other (as I often remark during talks on the subject), I think it was remarkable as an example of a paper reporting the apparent “failure” of the researchers to achieve their goals … how often do we see people reporting their failures in the literature these days :-). More of this kind of work – reporting a scientific study and accurately reporting both successes and failures … but especially failures … is critically important to the progress of science, so that people can both contribute to solutions and also avoid unsuccessful forays down paths already tried.

In any case, the paper clearly pointed me towards an important problem that led to me adopting a path of research over the past decades, which led to the development of the SCE and SCEM  optimization algorithms (and indeed a whole field of optimization developments), studies into impacts of model structural deficiencies, multi-criteria methods for parameter estimation, the diagnostic model identification approach, and more recently the Information Theoretic approach.

The 1990 paper by Michael Celia et al on the numerical solution of Richards equation, recommended to me by Philip Binning at the beginning of my Honours Project at Newcastle Uni. This paper made a big impression on me because it provided a very clear exposition of how to solve a challenging modelling problem – and played a bigly role in getting me interested in research.

Dmitri Kavetski (University of Adelaide)

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The Ecological Studies Series, published by Springer, was the most influential in my career because several books published in the Series (e.g., Forest Hydrology and Ecology at Coweeta edited by Swank and Crossley and Analysis of Biogeochemical Cycling Processes in Walker Branch Watershed edited by Johnson and Van Hook) sparked my interest in forest hydrology and biogeochemistry. In tandem with the superb mentorship of Prof. Stanley Herwitz (Clark University), I decided to embark upon a career as a forest hydrologist as a sophomore in college. I never looked back.

Delphis Levia (University of Delaware)

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The papers of the series “Plants in water-controlled ecosystems” (2001, Advances in Water Resources 24), by Laio, Porporato, Ridolfi, and Rodriguez-Iturbe have been among the first and most influential I have read. Their clean, analytical approach to the complex interactions among vegetation, soil, and climate remains deeply inspiring. As an example of inter-disciplinary work (actually outside hydrology), I would like to mention the book by Sterner and Elser (2002) “Ecological stoichiometry. The biology of elements from molecules to the biosphere” (Princeton University Press) – a great example of how integrating knowledge from various sources around a common theme can yield deeper understanding and perhaps even lay the foundation of a new discipline.

Stefano Manzoni (Stockholm University)

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The Hewlett and Hibbert 1967 conference paper “Factors affecting the response of small watersheds to precipitation…” is perhaps the best paper ever written in hydrology. For a full homage, please look here. The paper is field-based, theory focused and a blend of bottom-up and top-down research, before that was even ‘a thing’. It inspired me in my graduate research in the 1980s; I continued to read it and ponder it in my first years as a professor, as I strived to follow in Hewlett’s footsteps. He was my mentor even though he retired before I could ever meet him.

Jeff McDonnell (U Saskatchewan)

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In general, the books that have been most influential to me refer to sister disciplines. The reason is that I found illuminating to study methods and models used in statistics and economics for the purpose of applying them to hydrology for the first time. Thus, the most influential book to me has been “Statistics for long-memory processes”, by Jan Beran. The very reason is that I found there a detailed explanation of models that were useful to get to target with my Ph.D. thesis.

Alberto Montanari (University of Bologna)

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Chamberlin TC. 1890. The method of multiple working hypotheses. Science 15: 92-96 (reprinted in Science 148: 754–759 [1965]). I read this paper as part of a second-year course in Archaeology, which I took as an elective in my undergraduate program. Although the writing style is somewhat archaic, this article introduced me to the value of hypothesis-based thinking in science and the need to avoid favouring a pet hypothesis or model. It is instructive also to read the many follow-up essays to gain a broader perspective on hypothesis-based research and, more broadly, the “scientific method.”

Dan Moore (University of British Columbia)

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I think I was more influenced by my peers, colleagues, mentors, supervisors and friends as I learn better through discussions and challenges. One of the more memorable papers is one of Manning (Manning, R. (1891). “On the flow of water in open channels and pipes,” Transactions ofthe Institution of Civil engineers of Ireland.) and it’s associated history. In this paper he actually suggested a far more ‘complex’ formulation than the formula which is today widely known as the Manning equation – history has it that it was never adopted widely as well as many subsequent more more sophisticated formulations. Science doesn’t work linear and we are sometimes less rational or objective (if the latter is actually possible) than we believe.

Florian Pappenberger (ECMWF)

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“Show me a person who has read a thousand books and I’ll show you my best friend; show me a person who has read but one and I will show you my worst enemy.” I have been influenced by many and I can’t say one is *the* most influential or important alone.  At the moment, I am reflecting on (McCuen RH. 1989. Hydrologic Analysis and Design. Prentice Hall: Englewood Cliffs.) As far as being a hydrology textbook it is not particular special, but it is written extremely clearly with a lot of good step-by-step workflows.  Most importantly, the book integrates throughout its whole development the concept of analysis versus synthesis, and this has been central to how I approach my research.  We do both analysis and synthesis.

Gregory Pasternack (UC Davis)

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This is very difficult to say. I must admit that my academic work started from engineering practice and I only started reading the international literature very late in my career. But a book that has been very influential to me was the book by Fischer et al. (1979) “Mixing in inland and coastal waters”. Fischer soon died in an accident after this book was published. The book introduced me to the fundamentals of mixing processes in estuaries, on which I had done substantial field research and had developed my own practical engineering method, which I still use, but which lacked a fundamental theoretical basis. I am still working on finding this fundamental basis, and Fischer’s book put me on that track.

Hubert Savenije (TU Delft)

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It would be tough to answer what’s been the most influential to my career as a whole, but I could answer what was the most influential to my early career, and that was Menke’s Geophysical Data Analysis: Discrete Inverse Theory.  I labored through that book for years during my PhD. My copy has dog-eared pages and writing throughout as I tried to figure out inversion methods.  Finally getting my head around the mathematics of inversion really opened up some doors for me early on.  Davis’ Tools For Teaching also really helped me think about how to be as effective a teacher as I could be.

Kamini Singha (Colorado School of Mines)

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Books are hardly ever influential once you are actually ‘in’ research. Early on, look for the best review articles in your field. They will ‘set the scene’ for you.

Keith Smettem (The University of Western Australia)

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Opportunities in the hydrologic sciences”, National Academy Press. This landmark book which defined hydrology as a science appeared right at the start of my PhD. It provided a nice framework for my own research and that of my fellow PhD students in those days.

Remko Uijlenhoet (Wageningen University)

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It is difficult to select one single work from the literature that has been influential over my entire career in groundwater flow and transport modeling.  But, there is one book that I used as a grad student that I still refer to today.  It is “Conduction of Heat in Solids” by Carslaw and Jaeger.  The book is a treatise on analytical solutions to diffusion equations.  The lesson for me is that knowledge from other disciplines (in this case thermal engineering) can be applied to problems in hydrology.  Another lesson is that we can learn a lot and gain important insights through wise approximations that have analytical solutions.

Al Valocchi (University of Illinois at Urbana-Champaign)

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Abramowitz & Stegun: Math is something you look up, not something you try to memorize.

Nick van de Giesen (TU Delft)

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In hydrology, some of the most influential books for me have been Handbook of Hydrology (edited by David Maidment) and Principles of Environmental Physics (Monteith & Unsworth). These two books are so rich in physics, empirical equations, recipes, and references. Of course the times have changed and nowadays you can google almost anything, but some of the chapters in these books are so well written that I still regularly use them. They also have the benefit that they summarise areas of research where things haven’t actually changed too much since the 80ies – the physics we use haven’t become that much more sophisticated, and sometimes in fact less so; whereas the field measurements on which a lot of the empirical rules and equations are based generally also haven’t been added much to since.

Outside hydrology, some books that have made me think differently about the field and my research include

Emergence: The Connected Lives of Ants, Brains, Cities, and Software (Johnson) – one of the first popular science books I read that made me think different (about ecohydrology)

The Sceptical Environmentalist (Lomborg) – I didn’t accept his reasoning but it was seductive and it forces you to really pick apart the logical and rhetorical flaws he uses.

Thinking, fast and slow (Kahneman) – which really made me realise the questionable quality of my analytical rigour and decisions in general (also those of anyone else, though!).

Albert van Dijk (Australian National University)

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Physical Hydrology by Dingman and Elements of Physical Hydrology are both great textbooks. Why: just lots of “basics” well explained, emphasizing the need to understand PROCESSES.

Doerthe Tetzlaff (University of Aberdeen)

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House at Pooh Corner, specifically, Chapter VI. In which Pooh invents a new game and Eeyore joins in.  The first paragraph is an awesome description of a classic watershed and affirms my theory that hydrology is truly everywhere… even on Mars.  Indeed, the search for “life” has largely been a search for “water.”

Todd Walter (Cornell University)

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Comparative hydrology, edited by Malin Falkenmark and Tom Chapman (1989). This book is one of the first to examine global hydrology phenomena. It asserts that a comprehensive and systematic description of hydrological processes is (i) possible (ii) not too complicated. Until then I’d thought the task was impossible, so I found the approach inspirational for my research.

Ross Woods (University of Bristol)

## Of Karst! – short episodes about karst

Post by Andreas Hartmann Assistant Professor in Hydrological Modeling and Water Resources at the University of Freiburg.

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Episode 3 – Learning about karst by … KARST IN THE MOVIES!

Before writing about karst hydrology in “Of Karst! Episode 4”, I have been urged to present some more visual information on karst landforms. Of Karst! Episode 1 focused on the abundance of hilarious karst landforms in nature. This episode focusses more on the appearance of karst features in famous movies and TV programs that may be familiar to some of us, although we may not have watched them through the eyes of a karst fanatic at the time.

In the next episode, we follow the path of the water from the karstic surface with karstic towers and dolines, through caves and conduits, to spectacular karst springs where waters emerge to the surface.

Movie makers have their reasons to pick spectacular landscapes for their stories and, Of Karst!, those landscapes are crowded with karst features. Let’s begin with James Bond. Created in the 70s, “The Man with the Golden Gun” finds a spectacular showdown just in front of a lovely tower karst at the Khao Phing Kan island in Thailand. Tower karst is a karst landform that is, characterized by residual hills of limestone rising from a flat plain or the ocean.

Figure 1: Bonds‘ duel with villain Scaramanga in front of a tower karst rock (Khao Phing Kan, Thailand; http://www.criminalelement.com, http://www.marinaaonang.com)

Similar landforms were chosen as scenery for a recent remake of the King Kong saga. Fighting with intruders and evil monsters from the deep subsurface (karst caves?), Kong had the pleasure living on the beautiful Cat Ba Island in Northern Vietnam, whose characteristic landscape evolved due to the strong dissolution of limestone.

Figure 2: Silhouette of Kong between the Tower Karst mountains of Cat Ba Island located at Ha Long Bay, Vietnam (https://c1-zingpopculture.eb-cdn.com.au, http://www.baolau.com).

The opposite landform to tower karst landforms are karstic dolines, which occur commonly as funnel shaped depressions on the surface, also formed by carbonate rock dissolution. These depressions do not only funnel the water downwards to the subsurface, but also create favorable conditions for the installation of (very) large radio telescopes. The largest of those was built a couple of years ago in China but a similarly impressive one can be found in Puerto Rico, where James Bond had to deal with his evil competitor Trevelyan in “Goldeneye”.

Figure 3: Bond fighting with evil Trevelyan in Goldeneye high above the Arecibo Observatory in Puerto Rico that was built just in the middle of a karst doline (https://i.pinimg.com, http://www.si-puertorico.com).

Underneath the tower karst and dolines, karst dissolution creates wide networks of karstic caves and conduits. With increasing dissolution of the carbonate rock, these features may also emerge at the surface, which was probably the case for the Azure Window at Malta. This karst landform was chosen as the background of a conversation of the famous Khaleesi and her spouse Drogo in “Game of Thrones”. Unfortunately, this amazing land form is not available for further movies as it was recently destroyed by a storm.

Figure 4: Khaleesi speaking to her beloved Drogo in Game of Thrones in front of the Azure Window in Malta (http://nypost.com).

Deeper in the subsurface, the famous Devetàshka cave in Bulgaria set the stage for a dramatic showdown in “The Expendables 2”, when Stalone’s plane crashed through the cave entrance that used to be the exit of groundwater flows emanating from karst. Imagine the tremendous amounts of water filling the karst system over thousands of years that are capable of forming a cave that can (almost) host an entire airplane!

Figure 5: Stalone’s plane crashing into the Devetàshka karstic cave in Bulgaria in The Expendables 2 (www.huffpost.com, www.wikipedia.org).

Due to the formation of dolines, caves and channels, karst springs are usually quite large in terms of their discharge. They also provide amazing sets for fantasy movies. Even though the springs of the St. Beatus Caves in Switzerland only inspired Tolkien for the scenery of the Rivendell, the town of the elves, their similarity is obvious.

Figure 6: Elves’ town Rivendell in Lord of the Rings, whose scenery was inspired by the karst spring of the St Beatus caves in Switzerland (http://www-images.theonering.org, http://tilomitra.com).

This movie-based tour through karst systems may have given you an impression how rainfall becomes discharge in karst systems. Of Karst!, Episode 4, will combine this impression with the hydrological, and more scientific point of view. It will speak to the complexity of these specific surface and subsurface land forms, and elaborate on why exploring and understanding these processes is worthwhile.

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Andreas Hartmann is an Assistant Professor in Hydrological Modeling and Water Resources at the University of Freiburg. His primary field of interest is karst hydrology and hydrological modelling. Find out more at his personal webpage www.subsurface-heterogeneity.com

## Where does the water in streams come from when it rains?

Post by Anne Jefferson, associate professor in the Department of Geology at Kent State University, in the United States.

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The title of this blog post might seem like a question with an obvious answer, or even a silly question to pose on a blog devoted to groundwater, but if you don’t see the connection between streamflow and underground water, you need to keep reading.

Water in streams during storms does come from the sky, but often it’s not the water droplets falling in a particular rain storm that are going rushing down the stream during that storm. Very little precipitation falls directly on the stream channel, because streams occupy very little of the landscape. So most water in a stream has to travel over or under the ground to get there. Traveling from some random point in a catchment to a stream takes time, and there are usually other water molecules between the freshly fallen raindrop and the channel.

Water molecules that can’t infiltrate into the ground become surface runoff. Surface runoff tends to reach the stream channel fairly quickly, because the water molecules encounter much less flow resistance than water moving through soil or bedrock and because all of the other water molecules between them and the stream are also on the same downhill express lane to the stream. So a water molecule that becomes surface runoff can reach a stream minutes to hours after the raindrop first hit the land surface.

On the other hand, raindrops that infiltrate into the ground take a much more leisurely approach to reaching the stream. Underground, flow resistance is really high, relative to what surface runoff encounters, because the water molecules have to squeeze through tiny and tortuous open spaces between soil particles, roots, and rock. They may even be trapped by capillary forces or stick to mineral or organic surfaces. Plus, underground, there are lots of other water droplets also slowly making their way to the stream. Being a water molecule that goes underground to reach a stream is not so much taking an express lane, as joining the back of a barely moving traffic jam that stretches as far as the eye can see. In the minutes, hours, and days during which a stream runs high following a rainstorm, that poor underground water molecule may only have moved a few meters to a few hundred meters toward the channel. Indeed, its main function in joining the back of the traffic jam was to give a pressure wave nudge to the water molecules closer to the channel, urge them to get a move on, and increase the rate they discharge into the stream.

With those general concepts in mind, let’s turn our attention to humid, forested landscapes. Most rain will infiltrate into the soil, disappear underground, and join the back of the traffic jam. As a consequence, when we look at the water rushing along in a stream channel, during or shortly after a storm, mostly we are not seeing water that fell from the sky in the storm. We’re seeing the ghosts of storms past. Catchment hydrologists call this “old” water. To put some numbers on it, 60-80% of the volume of streamflow during and immediately following a storm is typically old water. Even at the peak of the streamflow, ~50% of the water in the stream is old water.

The Cuyahoga River in Kent, Ohio under low flow conditions, October 2017.

The Cuyahoga River in Kent, Ohio, at high flow, March 2014.

How did hydrologists figure out that the water in streams during storms is old water (at least in forested landscapes)? It would be virtually impossible to go out to a catchment and measure the water flowing across all possible surface runoff, soil water, and groundwater paths.

Wouldn’t it be convenient if there was some way to label the water molecules as new or old and then just count the ratio of new to old water molecules in the stream at a given time? Fortunately for hydrology, variations in the isotopes of hydrogen and oxygen in the water molecules provide just such a tracer. Beginning in the late 1960s, hydrologists realized that they could use the storm-to-storm differences in the isotopic ratios of rainfall to identify water that came from the most recent storm versus water that didn’t. Thus the technique of isotope hydrograph separation was born. The popularity of the method grew rapidly, hydrologists began to try it out in catchments all over the world. This led to more recent work carefully laying out the assumptions and limitations associated with the technique and developing ever more sophisticated methods of analysis. Even more important than the method itself was that hydrologists, confronted with the reality that most of the water in the stream during a storm was old, had to then come up with mechanistic explanations for how that was possible. I don’t think it’s hyperbolic to say that isotope hydrograph separation really helped revolutionize the field of catchment hydrology.

When I teach hydrology, I teach students that in our forested corners of Ohio the water in the stream during storms is old and I explain the mechanisms. But I also think it’s important for them to understand the technique that helped generate those insights, and so I make sure to teach them about isotope hydrograph separation. With the support of an NSF grant, I developed a teaching module that gives students a chance to work with real data and do their own hydrograph separation. My hope is that the exercise will give them a deeper appreciation of the technique, the assumptions and uncertainties it contains, and the insights it is able to provide. This teaching module is available on the SERC website for anyone to use in classes or just for fun. There are data to download, step-by-step directions, and links to related readings. If this post has spurred your interest in streamflow generation or isotope hydrograph separation, I encourage you to check it out. Other good places to learn more about applications of isotopes in hydrology at the SAHRA website or in this book chapter by Kevin McGuire and Jeff McDonnell.

Kent State students measuring discharge in the stream where we conducted the hydrograph separation.

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Anne Jefferson is an associate professor in the Department of Geography at Kent State University. Anne‘s research focuses on watershed hydrology, groundwater-surface water interactions, and landscape evolution in human-altered and volcanic landscapes. Current projects of her’s focus on green infrastructure, stormwater management, and stream restoration. Keep up to date with Anne by clicking on any of the links below:

## Good groundwater management makes for good neighbors

Post by Samuel Zipper, postdoctoral fellow at both McGill University and the University of Victoria, in Canada. You can follow Sam on Twitter at @ZipperSam.

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Dedicated Water Underground readers know that this blog is not just about water science, but also some of the more cultural impacts of groundwater. Keeping in that tradition, today’s post begins with a joke*:

Knock, knock!

Who’s there?

Your neighbor’s groundwater, here to provide water for your plants!

Figure 1. Typical reaction to joke written by the author.

Ahem.

Perhaps this joke needs a little explanation. As we’ve covered before, groundwater is important not just as a supply of water for humans, rivers, and lakes, but also because it can increase the water available to plants, making ecosystems more drought resistant and productive. However, we also know that groundwater moves from place to place beneath the surface. This means that human actions which affect groundwater in one location, like increasing the amount of paved surface, might have an unexpected impact on ecosystems in nearby areas which depend on that groundwater.

Imagine, for example, two neighboring farmers. Farmer A decides retire and sells his land to a developer to put in a new, concrete-rich shopping center. Farmer B continues farming her land next door. How will the changes next door affect the groundwater beneath Farmer B’s land, and will this help or hurt crop production on her farm?

In a new study, my colleagues and I explored these questions using a series of computer simulations. We converted different percentages of a watershed from corn to concrete to see what would happen. Our results showed that the response of crops to urbanization depended on where the land use change occurred.

Figure 2. Conceptual diagram showing how urbanization might impact crop yield elsewhere in a watershed. From Zipper et al. (2017).

In upland areas where the water table was deep, replacing crops with concrete caused a reduction in groundwater recharge, lowering the water table everywhere in the watershed – not just beneath the places where urbanization occurred. This meant that places where the ecosystems used to be reliant on groundwater could no longer tap into this resources, making them more vulnerable to drought. However, places where the water table used to be too shallow saw boosts in productivity, as the lower water table was closer to the optimum water table depth.

In contrast, urbanization happening in lowland areas had a much more localized effect, with changes to the water table and yield occurring primarily only in the location where land use changed, because the changes in groundwater recharge were accounted for by increased inflows from the stream into the groundwater system.

So, what does this mean for the neighboring farmers we met earlier?

For Farmer A, it means the neighborly thing to do is work with the developers to minimize the effects of the land use change on groundwater recharge. This can include green infrastructure practices such as rain gardens or permeable pavement to try and mimic predevelopment groundwater recharge.

For Farmer B, the impacts depend on the groundwater depth beneath her farm. If the groundwater beneath her farm is shallow enough that her crops tap into that water supply, she should expect changes in the productivity of her crops, especially during dry periods, and plan accordingly.

*Joke written by scientist, rather than actual comedian.

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Zipper SC, ME Soylu, CJ Kucharik, SP Loheide II. Indirect groundwater-mediated effects of urbanization on agroecosystem productivity: Introducing MODFLOW-AgroIBIS (MAGI), a complete critical zone model. Ecological Modelling, 359: 201-219. DOI: 10.1016/j.ecolmodel.2017.06.002

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Sam Zipper is an ecohydrologist. His main research focuses broadly on interactions between vegetation and the water cycle, with a particular interest in unintended or indirect impacts of land use change on ecosystems resulting from altered surface and subsurface hydrological flowpaths. You can find out more about Sam by going to his webpage at: samzipper.weebly.com.

## Is highway de-icing ‘a-salting’ our aquifers?

Post by Mark Cuthbert, Cardiff University, and Michael Rivett, GroundH20 plus Ltd; University of Strathclyde.

If you live in a cold climate, have you ever wondered where all the de-icing salt (or ‘grit’ as we call it in the UK) that gets spread on the roads in winter time ends up, aside from that accumulating salty grime that coats your car? As you might expect, most of the salt gets washed off the highways as the salt has the desired effect of melting the ice, or carried away by rain. This salty ‘runoff’ ends up in streams nearby via pipes which drain the highway. However, that is not the end of the story…

We studied a major highway intersection on the edge of UK’s second largest city, Birmingham (more than a million people), to see how much of the salt spread on the highway ends up in groundwater. Our interest stems from concerns of the national regulator to understand not only how much salt ends up in streams, but also the potential long-term build-up of salt in underlying groundwater resources that are pumped for public and private water supply. Although the origin of salt in streams and lakes is relatively well studied, the pathways by which salt moves from highways to groundwater are poorly understood and quantified.

The various ways salt could be getting in to the groundwater are shown in Figure 1 and a bit of detective work was required to find out what was going on.

Figure 1*

This involved dressing in silly (and warm) clothes, and pulling our equipment around on a sledge, which was a lot of fun (see photos below).

The authors, Mark (left) and Mike (right), enjoying some urban winter fieldwork.

Our main field activities were fortuitously timed to include the severe winters of 2009-10 and 2012-13. We collected information about the amount of salt spread on the roads and measured the salt concentrations in the local streams, as well as in the shallow and deep groundwater wells in the area around the highway network.

7 km of motorway drained to the studied stream. Over the winter of 2012-13 we estimated that around 510 tonnes of de-icing salt was applied to the highway and major road network across the catchment. Most of that washed off the road via drains when it next rained or as the snow and ice melted, and ended up in the stream which flows under the highways. Some of it ended up on the roadside verges – this was not quantified, but would likewise eventually leach into the underlying groundwater over time.

We estimated about 12% of the de-icing salt, that’s around 63 tonnes over the 2012-13 winter, leaked through the bottom of the stream channel into the groundwater in the sandstone aquifer beneath, and may pose a risk to groundwater supplies in the area (see “9” in Figure 1, and results in Figure 2). While increases in groundwater chloride concentrations observed to date have been modest, and fortunately remain far below the drinking water standards, the steady year-upon-year build-up of salt in groundwater remains a concern. This is especially the case as the UK’s de-icing salt applications reached record levels in the recent severe winters and add to a potential de-icing salt legacy in our aquifers that has accumulated now over some 50 plus years.

Figure 2*

So how concerned should we be that so much salt is flowing down these streams and into the groundwater around our highways? There isn’t yet a simple answer to that but, with a global trend towards increasing urbanisation, this is an important area of ongoing and future research.

Further details of our published research to date on the Birmingham case study may be found at:

Rivett, M.O.,  Cuthbert, M.O., Gamble R., Connon, L.E., Pearson, A., Shepley, M.G., Davis, J., 2016. Highway deicing salt dynamic runoff to surface water and subsequent infiltration to groundwater during severe UK winters.  Science of the Total Environment 565, 324-338. http://dx.doi.org/10.1016/j.scitotenv.2016.04.095 (*figures reproduced with permission) [Read More]

## The Groundwater Wetlands and Bogs Study Group

The Groundwater Wetlands and Blogs Study Group is an unfunded, voluntary collaboration of professionals, formed in December 2012, focused on groundwater wetlands, bogs, and related systems.  The Study Group has about 250 members in 39 countries.

Study Group members communicate primarily through a disciplined Yahoo Group listserve.  We are not a social network nor are we an environmental advocacy group. The listserve is used primarily to exchange scientific and technical information.  Exchanges of information are usually brief and technically focused.

For such leadership as is necessary, the Study Group has a Leadership Project composed of a Facilitator and four additional members. Decision-making in the Study Group is simple… an idea will be suggested to the Facilitator or other member of the Leadership Project…will be discussed among the Leadership…and may then be posted to the entire group for discussion. The Post contributed by Tom Baugh who blogs at Hidden Springs.

One such idea was participation in the Joint Aquatic Sciences Meeting held in Portland, Oregon in May 2015. The Study Group offered a number of papers in Portland in a session that attracted about 150 participants. Another project is currently underway with the journal Freshwater Science where the Study Group will sponsor a special session of papers tentatively scheduled for the June 2016 issue of the journal.  Sometimes, project groups will form to address specific issues…several of these include a Journal Literature Project and a Science App Project.

Study Group membership is by invitation and there is no fee. Those interested in joining should contact Study Group Facilitator Tom Baugh at springmountain1@att.net. Post contributed by Tom Baugh who blogs at Hidden Springs.

Longstreet Springs, Ash Meadows National Wildlife Refuge. Photo by Tom Baugh

## Are we in the age of postmodern groundwater?

My humanities colleagues and friends are always talking about postmodernism or pomo for short (see this funny satire). I’ve been thinking a lot lately about ‘modern groundwater’ (stay tuned for a cool paper), so I started wondering if there is ‘postmodern groundwater’.

Modern groundwater is groundwater recharged since the huge spike it tritium in the early 1960’s due to above ground thermonuclear testing. But tritium has a short half-life, so atmospheric tritium concentrations have largely decayed back to pre-bomb spike concentrations (see graph below). So does this mean that we are in the age of postmodern groundwater or pomo gw?

Tritium concentrations in precipitation through time (from USGS)

Googling ‘postmodern groundwater’ comes up with nothing, so maybe I’m on to something new. The only thing online that is close seems to be Michael Campana’s more political and very interesting ‘Postmodern water cycle” shown here:

The Postmodern Water Cycle by Kate Ely, Umatilla Basin hydrologist extraordinaire for the Confederated Tribes of the Umatilla Indian Reservation, as posted on waterwired.

## Best groundwater song ever? “Once in a Lifetime” by the Talking Heads?

Contributed by Kevin Befus, University of Austin – Texas
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If there has ever been a song for hydrogeologists, “Once in a Lifetime” by the Talking Heads is the best. Here’s why I have taken this song on as my hydrogeologic theme song.

But first, here is a link to the music video, in all of its early 1980’s glory:

Music is great because the listener can interpret the music and lyrics with their biases. My bias in this song is not about the drudgery of life (1) . It is about water which is everywhere in this song, and maybe that means we humans see water as mundane, everywhere and maybe a bit menacing. I fully encourage you to evaluate your life through this song, but let’s get on to the hydrogeology!

Where does this song get the hydrogeology right? I was surprised.
“Water flowing underground” – groundwater moves and is not still (but can be super slow), except maybe in stagnation points that may also be dynamic (2).

“same as it ever was” – groundwater responds over long time scales, but may not always be in the same place (3) . Even still, water is eventually renewed and continues on its many paths through the water cycle, same as it ever was.

“Into the blue again” – back to the ocean, with an average retreat of 4000 yrs (4) ; shout out to the submarine groundwater discharge community (5)!

“After the money’s gone” – water can be something we retreat to as a source of comfort or leisure, but here’s an idea: what do we do with water problems when the money is gone? How do economics affect water resources? Do we turn off the pumps and let water flow to the blue again, L.A. (6)?

“Water dissolving and water removing” – shout out to hydrogeochemists and transport modelers (7) ; yes, you, Chebotarev (8)!

“There is water at the bottom of the ocean…remove the water from the bottom of the ocean” – there sure is, and more than we thought (9)! Maybe a water resource that will be tapped more and more.

“Under the rocks and stones” – well, there is water under rocks and stones, but also inside, brushing but sadly missing porosity and saturation. This doesn’t mean I don’t like this lyric.

“Silent water” – if there is any water on Earth that is silent (and that is unlikely, depending on the definition of what sound is), groundwater would be a good place to imagine a silent water droplet.

The underlying theme of passing time is what really gets me. Once in a lifetime, this water is flowing underground. What a great way to introduce the timescales of groundwater flow! Or, even begin a lesson on groundwater, ranging from basics to interactions at the coast or human impacts? How precious is this water if it can only be replenished once in a lifetime?

May we someday not have to say to ourselves, “my god, what have we done?”