Debates—The future of hydrological sciences: A (common) path forward? One water. One world. Many climes. Many souls

All the world's a stage, and all the hydrologists merely actors. Each plays many roles—an accountant or statistician dealing with limited data; a physicist; an engineer; a chemist; a policy maker; a risk manager; and a diviner. Seven roles or perhaps more. Today, I am asked to play a speculator. It was a dark and stormy night, when our hero, Columbus, set sail for the storied land of India. He found the bountiful Americas. The natives were henceforth dubbed Indians. An interesting story of prediction and validation—if your prediction goes awry, then a correction by declaration is in order—a bias correction in modern hydrologic lingo. I had learned in kindergarten that Columbus's voyage disproved the hypothesis of the day that the earth was flat. It turns out that this is a myth perpetuated in the 19th century. The notion of a spherical earth was quite well accepted by 1490. The unknown at the time of Columbus's voyage was the size of the Earth, and the precise position of Asia. The controversy was as to whether the distance to Asia was as low as Columbus estimated or much larger as the scholars of the day thought. The concern was whether the ships would run out of food and water before they reached Asia. It turns out that Columbus grossly miscalculated the distance to Japan as 5000 km, when it was really 20,000 km. The ships arrived at the shores of the eastern Caribbean, with a mutinous crew, at the edge of starvation. Miraculously, Columbus "discovered" the unknown Americas almost precisely at the distance he had conjectured would be Japan. Today, India struggles to deal with the resource constraints of feeding and productively employing over a billion souls. The Americas continue to be a haven for resources. Perhaps, bravado can overcome poor prediction based on bad science, or sometimes you just have to be very lucky and set the course of history. With this example in mind, I offer an idiosyncratic reflection on the hydrologic sciences to establish context and then return to the task at hand to suggest a common path forward on matters related to water that transcend some of the traditional boundaries of hydrology. We live on the blue planet, unique in our solar system. It has abundant water in all its phases, and hence supports a myriad forms of life. Hydrology is often described as the study of the occurrence, movement, and quality of water. It seems then, that hydrology has to be the most fundamental and exciting of the earth sciences. One can start with a basic question. What led to the formation and abundance of water on the Earth? Or another—why and how the renewable supplies of water are distributed as they are on the planet? Or what are the geochemical determinants of the chemical and biological constitution of water at any place of interest? Or why is water essential for the origin and maintenance of life? Or why in aqueous media, like-charged substances actually attract one another, when it is expected that they will repel each other? Or how, landscapes are shaped by water, and in turn shape the course of water? Or how variations in water fluxes constrain the development of patterns of drainage networks and vegetation, or patterns emerge at different scales in transport in porous media? Many more such questions can be posed. All three phases of water play a fundamental role in the organization of the thermodynamic equilibrium, and hence of the climate dynamics of the planet. Most major questions or uncertainties as to future climate projections for the 21st century are water related. They include: (a) the likely behavior of clouds, (b) the change in the transport of moisture and latent heat associated with this moisture through the atmosphere, (c) the form, intensity, and frequency of precipitation, (d) the extent and nature of sea ice, (e) the strength of the currents such as the Gulf Stream that transport heat from the tropics to the poles, (f) the degree of acidification and stratification of the major oceans, (g) the changes in regional vegetation, the associated evapotranspiration, and water balance, and (h) the changes in snow and ice cover and associated changes in planetary albedo. In turn, these uncertainties propagate to concerns as to the variable precipitation, runoff, recharge, and the chemical and biological quality of water that is available to humans and other forms of life at different places around the world. Today, as many nations report a high level of water scarcity and/or pollution, there is broad concern about our water future given projected climate changes and population increase. An understanding and prediction of the state of water and its evolution across the planet is increasing in importance for science and for society. Hydrology is a many faceted science. Hydrologists often identify themselves as surface hydrologists, groundwater hydrologists, contaminant hydrologists, catchment hydrologists, urban hydrologists, hydrometeorologists, hydroclimatologists, hydropedologists, vadose zone hydrologists, social hydrologists, physical hydrologists, statistical hydrologists, engineering hydrologists, etc. While this speaks to the tremendous breadth and depth of the field, it also reflects a certain lack of synthesis toward hydrology as a planetary science. As one reflects on the questions raised in the previous two paragraphs, it appears that they are peripheral to most of what is taught in a hydrology curriculum or is in the forefront of research published in the established journals in the field. Indeed, what we perceive as the hydrological sciences is largely focused on terrestrial hydrology, and within that on topics that are of primary interest from a functional perspective as to the organization and temporal movement of water and its constituents as they interact with the landscape (natural or as modified by humans). It may be fair to say that typically hydrologists attempt to model the spatio-temporal response of an open terrestrial system with external forcing that varies in space and time. Prescribed time varying boundary and initial conditions are translated into a response of certain state variables, and the thrust is to identify how the attributes of a particular domain will determine such a response. This is the prediction problem, focused on the identification of a function that relates inputs and outputs, and there is usually a complementary forensic or inverse problem where the parameters of this function are to be estimated given some data. Given a limited observation of the state of a system, the hydrologist seeks to either reconstruct attributes (e.g., past history) of the initial and boundary conditions or the key parameters of the model. In either case, at a conceptual level, the domain for the parameterization is implicitly fixed to a river basin or hillslope or urban scale, even if the analysis is performed at a global scale. This process is not so different from other physical and life sciences or engineering disciplines. The difference is that often, much of the temporal information content is determined directly from the exogenously specified climatic or human modification of the boundary conditions of the problem—as one expects in an open dynamical system. Consequently, the fundamental questions of water on the planet, examples of which were listed earlier, do not emerge as the centerpiece of hydrological inquiry. Rather, we seem to focus on the response of a system to external input, and consider a limited set of feedbacks (e.g., land-atmosphere interactions research) to the driving climate and human systems. I realize that this statement may offend many hydrologists who work within an earth systems perspective and are interested in the nonlinear dynamics of the planetary system of which water is a key component. Indeed, this culture exists, and I speak instead to the dominant threads in our field. Hydrologic science considers the dynamics of the conservation of mass, momentum, and energy at any scale at which a model is constructed. There is much research on how one may try to close the balance in these equations given limited information as to the parameter that vary in space, and on the exogenous forcing of the system. Clever schemes for parameter estimation and model calibration are devised, as are empirical models to predict time series, interpolate data, and estimate transfer functions. However, unlike some of the other earth sciences, there is less emphasis on phenomenological research aimed at discovering patterns or generating hypotheses related to the evolution of the system. This is not altogether surprising, given that in the usual representation much of the long term hydrologic dynamics is generated by the exogenous forcing. When we seek causal answers as to why there are patterns, trends, or oscillations in hydrologic time series, clustering of events, or extreme floods and droughts, inevitably one looks to forcings external to the typical hydrologic model. These can be a manifestation of the global climate or reflect changes in the watershed due to human activity, and the hydrologist tries to see how they are manifest in some local system. Consider the typical efforts today to project hydrological conditions in the future. General Circulation models of the ocean-atmosphere dynamics provide scenarios that are "downscaled" for use with hydrologic models to generate passive responses to these climate forcings. Exploration of the possible feedbacks of the land surface into planetary fluxes and future states become the domain of the climate scientist. With this example, I do not intend to deride hydrologists, or to suggest that they need to become climate scientists to be useful. Rather, I suggest that the hydrologist who tries to understand the long-term dynamics of water will necessarily operate at the larger scales that synthesize across climate and other factors, and may hence be better able to understand and predict the dynamics of the system of interest. The current era is being called the age of the anthropocene—a time when planetary water, land, and air are being changed substantially by human activity. Water has always shaped human habitation, and in turn, humans have played a key role in shaping the residence time, quality, and course of water. Engineering habitats and water courses have been the fundamental ingredients of human settlement, navigation, migration, and trade for centuries. It is the rate and scale of these changes and the associated feedbacks on human and hydrologic systems that is of recent interest. In this spirit, in the beginning of the 21st century, a few of us (Jose Salas, Jery Stedinger, Richard Vogel, and I) coined the term hydromorphology to describe the "Structure and Evolution of Hydrologic Systems at the Scale of Years to Centuries." By analogy to geomorphology, hydromorphology would focus on the long-term evolution of hydrologic systems from river basin to planetary scales, and encompass the full range of hydrologic processes across the planet. To give a flavor of the thinking, some broad questions that were articulated by the group included: how has water influenced the history of man and life on Earth? How has man determined the history, distribution, and pathways of water? How have climate variations and change determined water and life at different scales, places, and times? How has water constrained and determined climate? How will the physical and chemical attributes of water, climate, engineered systems, landscapes, and human use of water coevolve in the future? At an abstract level, in addition to looking at the long-term evolution of the boundaries of the usual hydrologic system description, the idea was to start looking at a global water system in such a way that the human and physical system interactions are endogenous to the conceptual model of hydrologic evolution. This would lead to a more closed system with the forcing being primarily the external radiation input to the planet, and the interaction between the earth system processes and human dynamics focused on water as the central medium of interest. This would place existing hydrologic modeling paradigms in the role of defining structural relationships or transfer functions between the traditional forcings of hydrologic models and their outputs, and hydromorphology as the paradigm that allows systems level thinking about the potentially interacting earth systems that prescribed the boundary conditions of the traditional hydrologic models. Speculation as to feedbacks across these earth systems that could lead to interesting structure in space and time could then follow. Empirical investigations in this area are limited by data, but one could still generate hypotheses and explore their implications. Hydrologic models may then be described through dynamic transfer functions with parameters vary across time and space in response to interactions that are local and global across natural and human systems. For instance, one could hypothesize that extreme floods or droughts in a region due to a recurrent climate regime that persists as wet or dry for years or decades, may depending on its recent history and oscillatory phase, stimulate investment in flood control and/or water storage structures in a region, which would then lead to subsequent modifications in regional hydrologic fluxes, and land use (e.g., diversions for irrigation of crops) which could in turn modify the regional land-atmosphere interactions and change the frequency and/or intensity of precipitation in the same region or at larger scales. Such a process would reflect interactions between local, regional, and planetary scales of activity, and diagnosing, understanding, and predicting the impacts and consequences of such interactions at all these scales would be of interest. A hydromorphological perspective on water would thus bring together a scientific framework that establishes water as the important vehicle for understanding critical planetary science questions that link climate (ocean and atmosphere dynamics), ecology, geomorphology, and human dynamics. In summary, I laud the many accomplishments of hydrology as a terrestrial science dedicated to the understanding and prediction of the functioning of water systems at many scales up to the river basin, and welcome an era where we may embrace the functioning of our blue planet as a system defined by water through all its media and scales, with an engaging opportunity to understand the very march of past and future human civilizations. Is there likely to be a unified theory across such an inquiry? Does there need to be? In addition to individual curiosity and intrinsic search for knowledge, science appears to move forward in response to themes that are deemed important given some scientific, institutional, or social mandate. The themes of hydrologic change, global hydrology, and social hydrology are being articulated as timely and important. Climate change and its potential impacts continue to be an overarching concern, with interest in prediction, mitigation, and adaptation. The projected global population growth, and its resource needs is another clear concern. The rapidly degrading state of the surface and groundwater stores in many places provides a focal point. Collectively, these themes and concerns point to a time when water issues appear to be poised to define a critical and urgent research agenda that is not yet as visible on the global stage as it needs to be, in part due to the local nature of the science and the institutional concerns, and in part due to the diversity of seemingly independent issues related to water, each of which has importance. Water is a primary planetary sustainability concern and it behooves us to shape the discussion from that perspective. A question that is often voiced in academic and governance forums is: "will we be able to feed the 9 billion people we expect on the planet by 2050?" This clear question and the agenda that surrounds it provides a focal point for investigations that range from the routine to deep scientific inquiry and technology development. Even though "food" means many things, there is a global community that is organized around understanding and addressing this question from the perspective of population demographics, physical, and economic productivity, nutrition, population migration, distribution inequities, climate change and its implications for the changes in geographic suitability of crops, genetic and biotechnologies, pests, and of course resource endowments—land, water, and nutrients. Future productivity in a traditional agriculture model is highly contingent on climate and the availability, distribution and effective, efficient of water. the of climate can be with efficient irrigation that not the regional then food for 9 billion people seems In the context of Climate mitigation, there is a question as to the future energy for the planet, and there is an exciting diversity of from for renewable to for and as to the use of economic to stimulate changes in human behavior for resource and I can on and these and other The point is that through a yet the interest and of a group of scientists are and focused on what could be a question. What can be a question that would be for planetary I can we a of water of quality for all life around the by 2050?" and the use of this is to for the functioning of the The question may across as a water question just to cover the for or hydrology. The and may also questions as to are some that this as a planetary hydrology question with the themes articulated so in this a global such as droughts and often reflect a organization of the global of the and atmosphere and hence of the hydrological a regional perspective these appear to be A understanding of these would us understand when and which of the may floods and This may the of one water use across and could be to for the possible climate To a climate water is to what a hydrologist considers a or a contaminant in a surface or groundwater transport It is but in a few it change the of the as hydrologists at models of contaminant they need to at models of water transport in the atmosphere to understand and on global water The of as an area of study is a in this We have a of water use around the as the world's population its billion This has been manifest through intensity of agriculture and the associated often at low and low of It is routine to of highly water being in is that 9 or even billion with food from the planet is but if we are water as to our of what is where and and we see a possible of agriculture in and by and/or other with potential feedbacks to the atmosphere and that could changes in and global Even as in and the Americas continue to be there appears to be about in and in It is time we the of such exciting regional at the planetary scale, and of by which nature and humans could from a to a global food potentially planetary current to understanding issues at a river basin scale is based on a context that is defined largely by the of the century, and the current needs of existing human and the water, a human is well its scarcity and/or development and there is across and nations as they perceive that is being by the other As conditions change and local due to population these may be An understanding of the global context of the changes and its regional may in the fair of potential and predicting water is an open research Most of future urban water in the that were over the or even the were to be In our work in we found that of water use based on the usual models to water or even water as through Even as water balance at a watershed scale is it is that we can on in the of an understanding of water use patterns across the many of human activity, the economic and policy that determine this use in an are better We are at a time of change in the way the climate, land, and water function As the and temporal of hydrology, vegetation, and human change, it is not possible to the question a in our understanding of the regional, and local interactions in planetary hydrology and the that It is projected that much of the world's population will live in urban by continue to emerge across the world. They an increasing of the work and also of the global of and It is that one can and even in in and This that a global and distribution system that is with water and climate is and may be in place by economic hydrologic a hydrologic these a in the hydrologic field. They can be for groundwater and for the of and of and may in turn of water of a changed quality to the They also heat which the regional and precipitation thus their interaction with the larger climate may the interaction between the of the atmosphere and the to for the many that these the surface and groundwater hydrology modifications by such their paradigm for understanding at a level that a particular Today, a of the water trade place through from What would be an from a water sustainability and from an economic perspective given the and variable human or or vegetation, has been a to to such as extreme has been and will change across the planet as water and climate change, and as humans such as into typically with on the hydrologic is limited today by boundaries and but are that many of the such as may open up for agriculture and At a planetary scale, one needs to understand what this to climate may do to water through potential climate and through in water with the in I in the that as a of science we have tremendous to understand and predict local hydrologic surface and water groundwater and surface water contaminant The planetary focus would the of these into a context that climate, and social dynamics together into an of what may be, and what is possible in a water world. It also the for looking at how landscapes across the may to climatic and human for that conceptual and hydrologic models to work better in at in of system evolution. In many we are at a time of history to that of Climate change is of human planetary is hydrologic As with the spherical but not all seem to that we are on the of a very different planet. we not precisely dynamics are yet as a system it is When man nature as we do some of the states of nature we will be just more or less may be and we at a given in the they may a we the system at the scales that nature and man we may in the function that across place and time. This may provide a path forward for hydrology, the and the the local and the One could out a research or of prediction, technology and science or time to the question. I from this at this and However, I do that the answers will looking into the hydrologic past at the planetary scale, a years or and the use of and information systems that our use of water, an area we understand as The past will yet be a to the as we on a voyage of and that prediction has been the of hydrologic it is understanding that we be This is an point. from their is that instead of just parameter one needs to how to identify when the model dynamics a from by for that are determinants of the of of given in initial conditions or of the organization of in space or time, or the in which external or are through space and time in the system. These are not attributes that they or usually focus and consider the dynamics of and a from and It is interesting that years this largely a of hydrologic has of key processes to an or of the that lead to the response. their we need to where we are as a in being able to and of and residence time, or how an of systems as by and can be across these scales through to the models at each of the scales. What can the of statistical or models that are for such and how do we use I these two a focused to to the scientific and human of hydrology. The distance between the other two in this is to their distance to this They research needs that better models and process understanding but are at to in a of and a of about hydrology. They issues that are important as to the we can I focused on the role of water in the planet and where hydrology can in an I am that much of hydrologic research has into and or and we be on fundamental questions of water on the planet and the role of people in its In I have to the water, one that I are to the of hydrologists to and about the interactions between water, climate, and people and how we and this To two who have in a a on the cover of in to the of hydrology. At in I was to that the had given an and he was perhaps the most visible water of around the world. taught that and and and critical thinking science to the I am for from many but Salas, and Richard