The 23 unsolved questions of water, & the evolution of hydrology

Questions drive the progress of science. They help provide a context, a map, a direction, helping us orient between the known and the unknown. Questions can open up new fields of knowledge, and the solving of them can connect seemingly disparate fields of knowledge. Sets of questions create research programmes, that influence the research efforts of many groups, organizations and universities.

What do we know about the water cycle? What are the puzzles, the unknowns, the hard-to-model parts of the water cycle? What causality chains, what complex system emergence behavior of water do we not yet grok? Which part of water’s local and global behavior can we not yet predict?

Gunther Bloschl is a professor of hydrology at the Univeristy of Vienna, whose research work had demonstrated that climate change was increasing flood risk. In 2017, as head of the International Association of Hydrological Sciences, he sent out a missive - “To all hydrologists of the world: A Call to Arms! What are the 23 Unsolved Problems in Hydrology that would revolutionise research in the 21st century?” The idea was that by coming to figure out the key questions about water, hydrologists could develop a collective sense of the water field, counteract the fragmentation that was happening in the field, and delineate directions to focus research efforts in the future. The hope was that a set of questions would better link water research around the world.

The effort was modelled after the famous 23 unsolved math problems that the great mathematician David Hilbert delineated at the beginning of the 20th century. Those problems had a big influence on the direction of research in mathematics. Research on many of them helped open up new branches of mathematics. Fields medals (the equivalent of Nobel prizes in mathematics) were given for the solving of a number of problems.

[The paper describing David Hilbert’s Mathematics Problems]

Before we dive into what the 23 unsolved problems of water, lets first take a look at the history of hydrology. Hydrology is a field that has evolved and shifted, its contours and boundaries morphing like a river carving a path across the landscape. Its a research discipline that has, over time, expanded its metaphors and models, its methodologies and apparatus. Hydrology has found itself dividing into multiple subfields, while at the same time, incorporating a multiplicity of disciplines, developing new integral themes, and undergoing its share of punctuated-equilibrium type paradigm shifts. Water is a multi-faceted topic, which many different groups and demographics have studied from many different angles. As hydrology has evolved, it has found itself combining and merging its knowledge with that from other groups.

Hydrology was developed as the Egyptians constructed basin irrigation, the Romans built aqueducts, the Iranians engineered qanats, the Chinese made waterwheels, and the Mesopotomians stacked flood walls. Hydrology studied the questions of where did the water for rivers come from? How does river speed vary? How fast do aquifers fill up? Where did the rain come from? Why did the Nile flood some years, and some years not?

In the nineteenth century, in the West, hydrologists developed their understanding of river engineering and urban drainage, and the equation for groundwater flow was figured out by Henry Darcy. In the twentieth century, hydrology became a science, distinguishing itself from water infrastructure engineering. Hydrology developed more sophisticated ways of measuring data in the rivers, snowpack, glaciers, and aquifers. Hydrologists were trained at technical universities and used their knowledge of river flow and watershed response to help the engineers who were being employed in the worldwide boom of dams, reservoir, and aqueduct building.

In the 1970s and 1980s, a paradigm shift happened in hydrology, as computer modelling of local watersheds and global atmospheres became possible. Syukuro Manabe, who won the Nobel prize for his carbon greenhouse model that launched the modern climate movement, was the first to connect local watershed models with global atmospheric models. He modelled the soil’s ability to absorb rain in the local watersheds, which then through evapotranpsiration and heat flux, affected rain and climate on larger global scales.

Peter Eagleson, a professor of engineering at MIT, wrote a famous, prescient paper in 1986 [Eagleson 1986] that proclaimed the emergence of a global hydrology, which connected local watersheds to the global water cycle and climate. Water was a vast connective network. He wrote ‘Changes in atmosphere and/or landscape characteristics modify the earth's metabolism through changes in its biogeochemical cycles. The most basic of these is the water cycle which directly affects the global circulation of both atmosphere and ocean and hence is instrumental in shaping weather and climate.” He added “vegetation cover has a profound influence on the heat and moisture budges of the land surface… The microclimates of forested and cleared areas differ markedly.” He pointed out that precipitation recycling (aka small water cycle) meant that evapotranspiration from one place, can turn into preciptitation in another place. Large macroengineering efforts would impact the climate e.g. the effort to divert Soviet rivers southward would affect climate as “by depriving the Kara Sea of a large fresh water inflow, this latter diversion may alter ice cover and thus change the regional albedo.

Eagleson predicted the rise in importance of teleconnections in atmospheric science. Teleconnection is the process by how weather in one location in the world, can affect weather in another location in the world. This can, for instance, happen if a jet stream or large scale atmospheric circulation, being affected at one locale, then propagates that change to another part of the world. Teleconnections could thus explain the “negative correlation between the winter snow cover over Eurasia and the intensity of the following summer monsoon in India.” Deforestation or macro water engineering projects could impact weather elsewhere around the globe.

As hydrology developed it began to recognize the importance of the vegetation and soil to the water cycle more and more. Ecohydrology came into its own as a discipline.

Eagleson wrote [2000] “We need to get away from a view of hydrology as a purely physical science. Life on earth has to be a self-evident part of the discipline. In particular, I am thinking of vegetation, and its powerful interactive relationship with the atmosphere, at both a local and global scale. In attempting to get the full picture we must not be afraid to express the role of plants in our mathematical equations.” Ignacio Rodriguez-Iturbe, a renowned Venezuelan hydrologist at Texas A&M University, wrote “I believe that the spatiotemporal linkage between the hydrologic and ecologic dynamics will be one of the most exciting frontiers of the future. It is full of challenging and unexplored questions which go to the heart of hydrology and which are of fundamental importance for understanding the environment in which we live and the state in which it will be inherited by future generations.” [Rodriguez-Iturbe 2000]

Ecohydrology studies how rain gets redistributed by the vegetation canopy and soil, how rivers and basins support life, how local ecology can affect global weather through teleconnections, it studies the mechanisms underlying vegetation-soil-climate dynamics, and the “basic processes that control the stability and sustainability of natural environmental systems” [Andrea 2022].

[The earth’s critical zone - source Wikipedia]

Gordon Grant, a hydrologist, and William Dietrich, a earth science professor wrote “One of the newest and most exciting frontiers.. is the critical zone. The critical zone is where water moves, vegetation grows, roots spread, organic matter decomposes, soil develops, and rock weathers. It's also where we, and most life, lives, and is therefore “critical” to our survival. Studying the critical zone has rapidly become an international and interdisciplinary science effort utilizing field studies, long term observatories, and new geophysical measurement techniques. These studies are revealing insights into a broad range of previously unexplored topics: where do trees get their water, how does rock weather, and where does water go when it rains. Understanding the critical zone is vital to addressing key environmental and social problems: maintaining soil productivity in intensively managed landscapes, ensuring that forests don't die during droughts, and improving landscape resilience to wildfires, floods, and hurricanes. Today, the term “critical zone” provides an essential organizing principle for the earth and biological sciences just as “ecosystem” did for ecology half a century ago.” [Gordon, Dietrich 2017]

If you are unfamiliar with the field of hydrology, you might be surprised by how much details can go into hydrological simulations. Models take into account rainfall, air temperature, topography, and hydrogeology. More detailed models can take into account how different soil types absorb rainfall, how the carbon cycle and phosphorous cycle affect the soil, how earthworms affect soil permeability, how leaf density affects photosynthesis and rain falling through, how different crops use up water, how different tree roots bring up water from the aquifers at different rates, how waters seep through aquifers, how snow melts, how wetlands evolve, and how geomorphology changes. Putting more details into your model though does not guarantee you get better predictions. “Each model has got its own unique characteristics.” [Gayathri 2015]. In order to improve a models predictive power, one has to adjust certain parameters so that output numbers fit historical data. In this sense aspects of the models are more engineering than physics. Different models are built for different catchments. “A common joke among large-scale modelers is that every catchment hydrology group has its own model that works excellently in its own experimental catchment. Deriving more generic relations between mechanisms of runoff generation on the one hand and climate and geological setting on the other hand could be greatly helped by tying these individual catchments studies together with high-resolution global hydrological models” [Bierkens 2015]

The growth of the field of ecohydrology has created a paradigm shift amongst a certain segment of water resource engineers. There is now more likely to be proposals of nature based solutions - of using forests, wetlands, bio-swales, floodplains, and healthier soil to manage water flow. The sponge city concept is getting traction. Dam removal may even be proposed. Some water resource engineers have found themselves coming to the same water management conclusions as permaculturists and indigenous people. The World Bank which had heavily funded traditional infrastructures like dams and dykes in the 20th century, now also espouses, at times, a different worldview. On its blog, one article writes “Green infrastructure and nature-based solutions can play a critical role in improving water security, including mitigation and adaption to climate change. It is often more resilient, flexible, and reversible than traditional infrastructure and it can allow clients to gradually adapt to changing circumstances.” Christine Moreau (2022) wrote that the design of water infrastructure “switching from civil engineering to soil bioengineering (*the practice of using soil and plants for water management) is not only a technical change, but also requires a shift from a ‘predict and control’ paradigm to an ‘adaptive management’ paradigm”

………

In 2017, the call was sent out to hydrologists to figure out the unsolved questions of water. The initial discussion was held on LinkedIn, and then scientists were invited to gather in Vienna for in person deliberation. Out of this, over 230 hydrologists ended up coauthoring the paper “Twenty-Three Unsolved Problems in Hydrology (UPH): A Community Perspective” that appeared in the Hydrological Sciences Journal. [Blosch et al 2019].

Below are the questions the collective of hydrologists came up with. (Some of them are a little technical, so you may have to chew on them a bit, and look up some of the terms if you are unfamiliar with them. The extra effort can be worth it.)

The questions are broken up into different sections i) time variability and change ii) space variability and scaling iii) variability of extremes iv) interfaces in hydrology v) measurements and data vi) modelling methods vii) interfaces with society. In the unsolved problems paper they refer to many other papers for more in depth discussion of these discussions. After each section of questions, I will quote and reference from some of these papers, and explain some of the questions.

23 UNSOLVED PROBLEMS OF WATER

Time variability and change

1. Is the hydrological cycle regionally accelerating/decelerating under climate and environmental change, and are there tipping points (irreversible changes)?

2. How will cold region runoff and groundwater change in a warmer climate (e.g. with glacier melt and permafrost thaw)?

3. What are the mechanisms by which climate change and water use alter ephemeral rivers and groundwater in (semi-) arid regions?

4. What are the impacts of land cover change and soil disturbances on water and energy fluxes at the land surface, and on the resulting groundwater recharge?

This initial set of four questions are about how climate change and environmental change will affect the water cycle, how it will affect evapotranspiration, rain, runoff, groundwater recharge, permafrost thaw and snow melt. Its about how to figure out the tipping points in the system, noting that “regime changes in complex systems are notoriously difficult to identify”

An example of a not properly understood tipping point is permafrost melting in the Artic. The paper notes that “the combination of gradual climate change and multiple ecological feedback processes, many of which include and are propagated by water, can cause Arctic ecosystems to shift, from one set of mutually reinforcing feedbacks to another”.

A second example of a hard to predict tipping point is when does ocean water start surging into aquifers. This is a troublesome phenomena happening all over the world. In Orange County, in Los Angeles, as groundwater has been depleted, ocean water has surged in, turning the drinking water saline. Similar phenomena have been happening in the Nile Delta, Israel Coastal and Cyprus Akrotiri aquifers.

A third example of something not understood is the rapid climate changes that have occurred 25 times over the last glacial period. One of them happened 11,500 years ago when there was a fluctuation of 8 degrees over 40 years. These fluctuations are called Dansgaard–Oeschger events. “Present day general circulation climate models simulating glacial conditions are not capable of reproducing these rapid shifts. It is thus not known if they are due to bifurcations in the structural stability of the climate or if they are induced by stochastic fluctuations.” [Ditleveson 2010]

Question 4 is about how land cover change affects water flow. To study how harvesting from forests affects stream flow, researchers will take two very similar forests, one being the control, and the other one being the one they harvest from. With over 500 cases of these paired experiments, there are not consistent results - “Paired watershed studies have revealed everything from increases to decreases in annual streamflow in response to forest harvesting…. this illustrates the difficulty in predicting, a priori, the outcome of forest cover change on streamflow.” Its still to be figured what are the key variables that determine how streamflow will change.

Space variability and scaling

5. What causes spatial heterogeneity and homogeneity in runoff, evaporation, subsurface water and material fluxes (carbon and other nutrients, sediments), and in their sensitivity to their controls (e.g. snow fall regime, aridity, reaction coefficients)?

6. What are the hydrologic laws at the catchment scale and how do they change with scale?

7. Why is most flow preferential across multiple scales and how does such behaviour co-evolve with the critical zone?

8. Why do streams respond so quickly to precipitation inputs when storm flow is so old, and what is the transit time distribution of water in the terrestrial water cycle?

These questions have to do with how the behavior of water varies across space. Its about how behavior of water at one given scale, will affect the behavior of water at another scale, where the scales are i) local (order 1 m); e.g., macropores, ii) hillslope (order 100 m); e.g., preferential flow paths of water, iii) catchment (order 10 km); e.g., soils, and iv) regional (order 1000 km); e.g., geology. The question arises of how you statistically model the correlation of water behavior between different scales. As an analogy lets say you were trying to model the movement of a large crowd of people on a piece of land. If you can measure how the jostling changes at the size scale of a few people, how does that jostling shift translate into the movement of the crowd as a whole, and vice versa. How would different variables like the vegetation and geomorphology of the land affect how the movement of the people scale?

In hydrology sometimes there is a lack of data to input into models. In those cases it is useful to have some hydrological laws, which do not require calibration, that can be used to predict where and how the water will flow.

One of the known laws in hydrology is Darcy’s law which explains how the flow of liquid in the land is affected by the spaces in the soil and rock a pressure drop, by the viscosity of the liquid, and how a pressure drop can suck the liquid through. The question is, are there other laws? Is there a law for how water behaviour scales?

James Dooge, an engineer who helped transform hydrology from an empirical technology into a science, wrote, in a special issue of Trends and Driections of Hydrology, about looking for hydrological laws that scale across regions “If results are to be obtained at the catchment scale that contribute toward developing hydrologic laws rather than the fitting of empirical expressions to data with an unknown signal to noise ratio, then the scientific method must be followed either explicitly or implicitly. The first step in such a venture must be the generation of plausible hypotheses that can be tested” [Dooge 1986]. A collective of eminent hydrologists wrote in 2013 that the finding of new hydrological laws “could only be reached by an improved understanding of the underlying hydrological processes, demanding a shift of the research focus away from parameter fitting towards process understanding and model structural diagnostics.” [Hrachowitz 2013]

Dooge suggests figuring out which parameters in the microscale processes of the model that can be simplified, or which factors in the model can be left out, while not significantly reducing the predictive power of larger (meso)scale processes. Different soils, vegetation, drainage networks, rainfall patterns, and energy budgets may emerge the same patterns. He offers that the study of scaling relations in geomorphology and ecological adaptation may shed light on scaling laws in hydrology. He points to metabolic scaling law in biology where metabolism scales with their size to the 3/4 power, as an example of a scaling law. Dimensional analysis, Dooge suggests, may help derive hydrological scaling laws, in the same way dimensional analysis led to Kolmogorov’s law of turbulence.

Questions 7 and 8 are related to the critical zone and where the rain goes after it falls. The critical zone is the region from the top of the vegetation canopy to the soil and down to the groundwater and bedrock. When rain falls and moves through these levels it affects the growth of the vegetation and the soil, which in turn affects how future rain moves, leading to complex co-evolutionary dynamics. Grant and Dietrich [2017] ask “What controls the spatial pattern of the critical zone across different landscapes? Does the vegetation know what kind of bedrock lies beneath the root zone, and does the bedrock know what is growing on top of it? How will a climatically changed atmosphere and a land surface modified by human activities affect the deeper critical zone and vice versa? These fundamental inquiries lead directly to more applied questions. Can landscape degradation be reversed? Can intensively managed soils retain carbon, nutrients, and the capacity to grow food? Can the critical zone be managed to improve water quality…… Following the simple question as to where water goes when it rains leads to one of the most exciting frontiers in earth science: the critical zone—Earth's dynamic skin. Only recently recognized as a distinct zone, it is challenging to study because it is hard to observe directly, and varies widely across biogeoclimatic regions. Yet new ideas, instruments, and observations are revealing surprising and sometimes paradoxical insights, underscoring the value of field campaigns and long-term observatories. These insights bear directly on some of the most pressing societal problems today: maintaining healthy forests, sustaining streamflow during droughts, and restoring productive terrestrial and aquatic ecosystems. The critical zone is critical because it supports all terrestrial life; it is the nexus where water and carbon is cycled, vegetation (hence food) grows, soil develops, landscapes evolve, and we live. No other frontier is so close to home.”

Variability of extremes

9. How do flood-rich and drought-rich periods arise, are they changing, and if so why?

10. Why are runoff extremes in some catchments more sensitive to land-use/ cover and geomorphic change than in others?

11. Why, how and when do rain-on-snow events produce exceptional runoff?

In ancient Egyptian days people wanted to know how likely each year the Nile would flood. Today hydrology knows a lot more about how to answer this question, but there are still quite a lot of unknowns. The probability that it floods it each year is not independent of what had happened in previous years, as the system has a memory.

Hall and colleagues wrote about understanding the flood regime changes in Europe “Hydrologists…are now exploring the question of whether floods are increasing and, if so, why. The difficulty lies in the erratic nature of floods, as one big flood event does not indicate an increasing trend in flooding…. In order to understand flood regime changes it is important to be clear about their physical causes. Merz et al. (2012) defined three groups of potential drivers of change (river channel engineering and hydraulic structures, land use change and climatic change) …….. land use and management changes have been shown to affect evapotranspiration, water infiltration into the soil and surface and subsurface water storage and therefore flood-generating processes…… While at the catchment scale the hydrological effects of hydraulic structures are well understood, at least for individual case studies, less is known about the effect of land use/management and climate variability on the flood regime.” [Hall 2014]

At the climate level, El-Nino, La-Nina, and the Indian Ocean dipole, influence the irregular oscillation between flood and drought years. There are nonlinear couplings between the ocean and atmosphere that affect preciptation over land. The models to predict these events are not as reliable as other types of weather forecasting models, with scientists currently able to can get a sense of the likelihood of these events a couple of months to a year ahead of time. Recent research is also finding that deforestation and land-use changes affect El-Nino behavior [Ting-Hui Lee 2023]

Rogger wrote about land-use changes effects on floods - “The time scales involved in the process interactions may range from event to seasonal to centennial scales…. When agricultural practices change, the topsoil characteristics may respond very quickly, while the subsoil may respond more slowly, and both are modulated by seasonal fluctuations of biotic activities associated with the energy and water balances. The coupling across different time scales adds complexity to the catchment system making cause-effect relationships less obvious. Critical transitions or tipping points may occur, leading to sudden changes in the system behavior…..Land use change impacts on floods therefore involve a plethora of closely intertwined process dynamics that make their analysis and the prediction of any impacts at the catchment scale extremely challenging. For example, clear-cutting in forest plantations decreases interception and evapotranspiration which increases antecedent soil moisture and consequently decreases soil storage capacity. The use of heavy machinery on agricultural land tends to cause soil compaction and a decrease in soil infiltration, resulting in increased surface runoff. The process interactions involve a number of positive feedbacks enhancing small disturbances and negative feedbacks, where the effects of disturbances are dampened due to counteracting processes. An example of a positive feedback is erosion caused by agricultural intensification resulting in a reduction of soil depth, a reduction of soil storage capacity, and an enhancement of surface runoff which in turn increases erosion. An example of a negative feedback and how it might change with time is related to the interaction of deforestation with soils: initially there may be an increase in soil moisture but, for internally erodible soils, this may result in the development of subsurface pipe systems, which in turn may reduce soil moisture and therefore reduce flood generation.” [Rogger 2017]

Interfaces in hydrology

12. What are the processes that control hillslope–riparian–stream–groundwater interactions and when do the compartments connect?

13. What are the processes controlling the fluxes of groundwater across boundaries (e.g. groundwater recharge, inter-catchment fluxes and discharge to oceans)?

14. What factors contribute to the long-term persistence of sources responsible for the degradation of water quality?

15. What are the extent, fate and impact of contaminants of emerging concern and how are microbial pathogens removed or inactivated in the subsurface?

The unsolved problems paper discusses the need to understand more about how water moves from one type of area to another type of area, i.e. understand the interface between atmosphere–vegetation–soil–bedrock–streamflow–hydraulic structures. It discusses the need for more research into how sub-processes that are dealt in one discipline interact with sub-processes of another discipline e.g. water chemistry, ecology, soil science, biogeochemistry

One interface that was singled out as being less understood was groundwater recharge at the regional and continental level, with the local level being more understood. The amount and process of discharge into the ocean is not so well modelled. How pollution moves through the aquifer system before leaving is hard to calculate.

A question arises of how to connect the movement of water in different areas into one coherent framework. Vit Klemes, a previous president of the International Association of Hydrological Sciences, suggested that ‘it is highly likely that instead of mastering partial correlations, fractional noises, finite elements, or infinitely divisible sets, the hydrologist would more profitably spend his time by studying thermodynamics, geochemistry, soil physics, and plant physiology’ (Klemeš 1986, p. 187S)”

Measurements and data

16. How can we use innovative technologies to measure surface and subsurface properties, states and fluxes at a range of spatial and temporal scales?

17. What is the relative value of traditional hydrological observations vs soft data (qualitative observations from lay persons, data mining etc.), and under what conditions can we substitute space for time?

18. How can we extract information from available data on human and water systems in order to inform the building process of socio-hydrological models and conceptualisations?

Modelling methods

19. How can hydrological models be adapted to be able to extrapolate to changing conditions, including changing vegetation dynamics?

20. How can we disentangle and reduce model structural/parameter/input uncertainty in hydrological prediction?

In order to have models that can extrapolate to changing climate and environmental conditions, the hydrologists felt a need to move from calibration based hydrological models, which are based on parameters adjusted to meet experimental data, to process-based hydrological models which work with more the underlying physics, and more explicity track flow paths.

Interfaces with society

21. How can the (un)certainty in hydrological predictions be communicated to decision makers and the general public?

22. What are the synergies and tradeoffs between societal goals related to water management (e.g. water–environment–energy–food–health)?

23. What is the role of water in migration, urbanisation and the dynamics of human civilisations, and what are the implications for contemporary water management?

…….

In the conclusion of the unsolved hydrology problems paper they write “Most of the unsolved problems identified here are questions that perhaps cannot be solved conclusively, but can likely be realistically advanced in the next couple of decades. This is in line with Hilbert’s recommendation on choosing unsolved problems ‘A mathematical problem should be difficult so as to pose a challenge for us, and yet not completely inaccessible, so that it does not mock our effort.’ On the other hand, there were no really unexpected questions that came up in the process. Burt and McDonnell (2015) noted that hydrology has perhaps reached a stage, similar to geology in the early 1920s, where more daring activities (and outrageous hypotheses) were needed to inject a renewed sense of purpose” e.g. Wegener’s continental drift theory was an outrageous hypotheses once. “These crazy theories can create a paradigm shift in the field if they turn out to be right.”

The paper adds “While the notion of hypotheses in hydrology has received renewed interests in recent years (e.g. Baker 2017, Blöschl 2017, Pfister and Kirchner 2017), most of them are not outrageous. One of the few examples is the idea of an “active biotic pump transporting atmospheric moisture inland from the ocean” (Makarieva and Gorshkov 2007) that has attracted numerous comments in HESS (Hydrology and Earth System Sciences journal). Another example is the idea of a “planetary boundary as a safe operating space for humanity” (Rockström et al. 2009).”

During the gathering to figure out the unsolved problems, the hydrologists expressed a sense that hydrology had broken into many subdisciplines that were not communicating enough with each other. They felt that hydrology needed to integrate more with issues of the larger water cycle on an earth-wide scale, as, quoting Lall [2014], “the planetary focus would entail the integration of capability to understand and predict local hydrologic processes into a context that brings climate, meteorology, agriculture, and social dynamics together into an exploration of what may be, and what is possible in a water networked world”. Water is a network, to understand one part, you also have to understand the other parts.

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This is a reader supported publication.

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