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1 The Hydrologic Sciences The abundance of liquid water sets Earth apart from almost every planetary body yet discovered in the galaxy. The hydrologic cycle, or the movement of water through evaporation, atmospheric transport, precipita- tion, and river and groundwater flows, shapes the terrestrial surface of the planet and transports the resulting solutes and sediments from mountain- tops to the ocean depths. Water supply and temperature together determine the form, life history strategies, and productivity of vegetation, ultimately controlling rates of photosynthesis in the biosphere. Liquid water serves as habitat to an immense variety of aquatic species and as a necessary resource for all terrestrial species. Understanding the storage and movement of water through the biosphere is essential for understanding the physical structure, chemistry, biodiversity, and productivity of the biosphere. Although water is renewable, it is not inexhaustible. Throughout his- tory, civilizations and ecosystems have flourished with the presence of water, executed engineering feats to secure its presence, and collapsed due to the lack thereof. Today, human influences are even greater, dominating the natural cycle of freshwater and causing environmental changes that are argued to have moved the planet into a new geologic period termed the "Anthropocene" (Crutzen, 2002; Vince, 2011). Global population growth has led to increased demand for water to support agricultural, industrial, and drinking water needs, with water withdrawals outstripping water sup- ply in many parts of the world. Climate variability and change, land use change, and demographic change place varying stress on the planet's water resources. Access to safe water supplies remains a challenge in many parts of the world. The rates of species extinction are highest for freshwater 15

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16 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES organisms because of habitat destruction, changes in water quantity, and water quality degradation and the introduction of foreign species looms large (Dudgeon et al., 2006). The risk of future extinction of freshwater biota is projected to be five times higher than that of terrestrial biota and two times higher than that of coastal mammals (Ricciardi and Rasmussen, 1999). At the core of these challenges is hydrologic science. WHAT IS HYDROLOGIC SCIENCE? Hydrologic science or hydrology is, at its most basic level, the "science of water" that embraces topics from research on fundamental processes through operations associated with flood protection, drinking water sup- ply, irrigation, and water contamination. The National Research Council (NRC) report Opportunities in the Hydrologic Sciences, known as the "Blue Book," defined hydrologic science as a distinct geoscience--"a g eoscience interactive on a wide range of space and time scales with the ocean, atmo- spheric, and solid earth sciences as well as with plant and animal sciences." However, hydrologic science is also firmly anchored by phenomena that have direct and important relationships with the well-being of humans and natural systems. In fact, as noted by Thomas Dunne, hydrologic science: will remain vital only if (1) it discovers new phenomena, processes, or relationships governing the behavior of water and its constituents and (2) it focuses on real hydrologic phenomena, such as floods, droughts, drain- age basins, material storages and fluxes, and even large-scale engineering effects such as streamflow modification, soil conservation, or channel modifications (NRC, 1998). Hydrologic scientists are driven in their research by curiosity about how the natural environment functions. How water shapes landscapes is intimately interrelated with life on land as well as in water bodies and is a primary ingredient in the planet's climate engine. Some of the curiosity, however, arises directly from a desire to solve problems associated with a variety of hydrologic phenomena, as suggested by Dunne. This blend of "curiosity-driven" and "problem-driven" research in hydrologic science is one of the aspects that makes the field so vibrant and exciting and repre- sents a defining feature of the future challenges and opportunities for the field. "[Water is the] elixir of life, the climatic thermostat and the global heat exchanger." Opportunities in the Hydrologic Sciences, NRC, 1991

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THE HYDROLOGIC SCIENCES 17 Hydrologic science's origins lie deep within the engineering applica- tions community. Greeks and Romans, who were pioneering hydraulic engineers, built aqueducts. Herodotus, Plato, Aristotle, and Hippocrates theorized about the hydrologic cycle, and this understanding was advanced in the Renaissance and the years following. Yet it was because of water's role in human affairs, primarily its availability and avoiding hazards, that the evolution of the science of hydrology followed the leadership of civil and agricultural engineers. During the 17th, 18th, and a large part of the 19th centuries the scale of hydraulic engineering efforts was modest yet, in the United States, dominated by engineers concerned with water supply and sanitation. The evolution of hydrologic sciences in the United States throughout the late 19th century and the 20th century was one of ever expanding scope largely driven by societal needs. In 1879, the United States established itself as the primary supporter of water research by forming the U.S. Geological Survey, which has contributed major advances in areas such as sediment transport in streams and rivers, groundwater, and water chemistry since that time. At the turn of the 20th century, hydrologic science was intro- duced into U.S. universities. This introduction was largely in departments of civil engineering with a focus on floods, surface runoff, water sup- ply, soil-plant-water relationships, agriculture, and groundwater but also within geography departments focusing on river and streamflow and other surface-water processes related to geomorphology. Subsequent teaching and research on hydrologic processes was introduced into agriculture and forestry departments, largely focusing on soil-plant-water relationships, and geosciences departments, largely focusing on groundwater. In 1930 the Hy- drology Section of the American Geophysical Union (AGU) was created. By the mid-1900s research focused on various aspects of the hydrologic cycle and was being conducted in government and university labs throughout the United States. In 1991--some 20 years ago--the NRC released the aforementioned report Opportunities in the Hydrologic Sciences, which was a thoughtful reflection upon the field of hydrologic science. The "Blue Book" envisioned hydrologic science as a distinct geoscience and set forth a corresponding re- search agenda for the field. In the years following its publication, the docu- ment stimulated discussion and various actions, which culminated in the widespread recognition of hydrologic science as a separate field within the Earth Sciences. These actions included strengthening of existing university programs and establishing new ones, as well as establishing the Hydrologic Sciences Program within the National Science Foundation's (NSF's) Direc- torate of Geosciences. The "science of water" was recognized as a critical component of geosciences linking the atmosphere, land, and oceans and contributing to the understanding of life on Earth.

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18 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES The hydrologic community took on a separate identity as scientific hydrologists1 and those in the community were and are proud to call them- selves such. Hydrologic science developed into a discipline with a number of interrelated subdisciplines, some of which were created and active before the Blue Book's publication. These subdisciplines can be roughly catego- rized as pertaining to either the subsurface or the surface, despite the great deal of overlap in processes and interactions between the two and related research. An example of the former is groundwater hydrology, which deals with the movement of water in the upper layers of Earth's surface, com- pared to catchment hydrology, which is the study of surface water fluxes, particularly runoff, and transport of substances within a catchment or hydrologic basin. The NRC formed the Committee on Hydrologic Sciences (COHS) in 1998 to provide a mechanism for continued promotion, integration, and advancement of hydrologic science at the interface with other related sci- ences. The COHS has produced and organized several reports, such as In- tegrating Multiscale Observations of U.S. Waters (NRC, 2008) and Global Change and Extreme Hydrology: Testing Conventional Wisdom (NRC, 2011). Other entities have issued visionary manuscripts to promote the field. For example, the Royal Netherlands Academy of Arts and Science published a forward-looking agenda in hydrologic sciences for its country and the globe (Royal Netherlands Academy of Arts and Sciences, 2005). The "Chinese Blue Book" took a holistic view of groundwater science in China and recommended priority research areas and strategies for advanc- ing groundwater research and education (Zheng et al., 2009). Membership in AGU's Hydrology Section has more than doubled since 1998 (A. Orr, personal communication, May 19, 2010). Today it is acknowledged that a successful and distinct science has evolved, and much fundamental progress has been made (Box 1-1). TECHNOLOGICAL AND SCIENTIFIC ADVANCES Over the past few decades and accelerating in time, leaps in technol- ogy have enabled unprecedented measurement, observation, and funda- mental advances in the conceptual understanding of hydrologic processes. Just comparing the technological capabilities available in 1991 to those available today attests to near-term opportunities available to realize the scientific vision and test the scientific hypotheses set forth in the original Blue Book and advanced in the following chapters. The Blue Book stated: 1 Here and throughout the report the term "scientific hydrologists" includes hydrologists, engineers, and those in related disciplines and subdisciplines participating in hydrologic research.

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THE HYDROLOGIC SCIENCES 19 BOX 1-1 Water in Geosciences: Celebrating the Past and Looking Ahead, a Special Session at the 2009 AGU Fall Meeting A special session was held at the 2009 AGU Fall Meeting titled "Progress in Hydrologic Sciences since the Blue Book" with the aim of taking a look at the scientific developments in hydrologic sciences since the Blue Book was published and to discuss perspectives for the open problems that lie ahead. The session was organized in honor of the founding Director of NSF's Hydrologic Science program (L. Douglas James) on his retirement after 17 years of leadership and dedicated service to the hydrologic science community. The presentations in this special session provided a perspective of the past accomplishments in hydrologic science with an eye toward the future. The session opened with remarks by Peter S. Eagle- son, Professor Emeritus, Massachusetts Institute of Technology, who chaired the 1991 NRC Blue Book committee, on the origins of the Blue Book and his perspec- tives on where future opportunities lie. The talks that followed emphasized the vitality of hydrologic science at its disciplinary core as well as at the boundaries with atmospheric sciences, biogeochemistry, geomorphology, social sciences, and engineering. The talks also presented major breakthroughs in land-atmosphere interactions such as the effect of deforestation on hydrometeorological predictions, advances in generalized scaling theories of floods, the value of remote sensing data of precipitation, vegetation, and soil moisture in improving hydrologic model- ing and prediction, new methodologies for characterizing hydrologic uncertainty, advances in data assimilation, advances in coupling geochemical and surface- groundwater systems, the role of social sciences in hydrologic prediction, and the need to translate increased scientific understanding into better management of water resources systems. The talks also presented a worldview perspective of water and international efforts in hydrologic science and practice, a renewed educational agenda for hydrologic science at the interfaces of geosciences using community collaboration, and the use of high-performance computing capable of resolving processes at scales of the order of meters in advancing hydrologic predictions in an Earth systems perspective. The need for changing perspectives to engage interdisciplinary synthesis and data-driven exploration to develop pre- dictive models that can function in a changing environment was noted. "In the history of the hydrologic sciences as in other sciences, most of the significant advances have resulted from new measurements." This remains as true today as when it was originally stated in 1991 although since then there have been game-changing innovations in measurement technologies. Many instruments are now deployed with remarkable pervasiveness and exchange data wirelessly with little to no latency. Devices are available to detect chemical and biological constituents with remarkable sensitivity. Taken together, hydrologic science is poised to advance in leaps and bounds enabled by the new measurements and the insights they afford.

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20 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES Paired with or perhaps as a result of these game-changing innovations is a change in the context of how hydrologic science is done. Human im- pacts and inputs now represent a major wellspring of research questions and directions in the hydrologic sciences. The recognition of human-driven climate change has challenged hydrologic scientists and engineers with new questions and has resulted in development of new paradigms. For example, hydrologic science is now challenged to understand, quantify, and delineate the contribution of human land use change to flooding in comparison to those changes driven solely by anthropogenic changes in greenhouse gases. Hydrologic science has seen a transition in philosophy from strict river control to river control achieved through maintenance of river ecosystems and their natural geomorphology--a fundamental and controversial move away from flood conveyance to floodplain maintenance. A subdiscipline studying the exchange between surface and groundwater, termed "hypo- rheic exchange," has emerged. Conceptual advances in the hydrologic sciences extend beyond these Earth-centric environments as well. Evidence of water cycles on other planets, in particular Mars, has led to the entirely new science of exohydrology and the development of water-cycle models for planets. Hydrologic science has evolved into a science that both derives strength from other sciences and provides strength to other sciences and societal issues. Although the advances mentioned above are not all-inclusive, these and other advances correspond to and describe an evolution of the context in which hydrology is done--an evolution that points the field in certain directions (for example, the development of models linking hillslopes to river connectivity across the landscape). Certain advancements in particular have allowed the community to understand, respond to, and address issues within this changing context. The committee anticipates that the field of hydrologic sciences will use these advances to surge ahead, in part because capabilities in such areas as imaging the Earth, measuring minute quantities of molecules in water and in organisms, performing calculations on amaz- ingly fast computers, and employing techniques developed in microelec- tronics will enable the community to formulate and answer new questions and to approach recalcitrant old questions effectively. To underscore the advanced state of water science in 2012, the committee highlights four areas where progress has revolutionized hydrologic science--chemical analytical instrumentation, remote sensing and geophysics, embedded sensor systems, and computation. These also exemplify areas in which expectations of further progress indicate significant opportunities for future advances in hydrologic science.

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THE HYDROLOGIC SCIENCES 21 Chemical Analytical Instrumentation In Chapter 3 on critical and emerging scientific areas, the Blue Book acknowledged that tools and knowledge from the fields of environmental chemistry and aqueous geochemistry are key to understanding water move- ment, establishing erosional and climatic histories, and describing impor- tant biogeochemical processes such as solute transformation, ecological function, and contaminant fate. Since the publication of the Blue Book, there have been significant gains in making fast, accurate, and low-level chemical measurements of compounds in aquatic environments. These ad- vances have greatly benefitted the hydrologic science community by helping to answer a wide range of hydrologic questions and providing the informa- tion for posing new ones. Mass spectrometry (MS) is a technique used to determine the mass of particles or the chemical structure of molecules and was first applied in the mid-19th century. Since then, MS has been continually refined and advanced. Today it is a ubiquitous scientific tool that has evolved, for ex- ample, with the use of novel ionization techniques, through coupling with chromatography, and with the use of mass analyzers in a series. As a result, a variety of instrument configurations can produce more sensitive, cost- efficient, and specialized measurements. Geochemical applications of MS have dramatically changed since the Blue Book was published. Back then inductively coupled plasma mass spectrometry (ICP-MS), commercially introduced in the 1980s, was the state of the art for detecting most trace elements, especially transition and heavy metals, and stable isotope MS was (and still is today) used in a number of hydrologic applications (e.g., sepa- ration of baseflow and overland flow, dating of groundwater, etc.). Today, MS techniques in many labs utilize "nontraditional" stable isotope analysis, which included elements such as lithium and boron and now includes rock- forming and biologically important elements such as silicon and calcium, as well as many trace metals such as copper, molybdenum, and even mercury. These new techniques provide insights into geochemical and biogeochemi- cal processes as well as describe water pathways. In addition, development of the measurement technology to detect cosmogenically produced isotopes such as 10Be and 26Al in minerals has transformed the field of geomorphology by providing dates of topographic surfaces and rates and patterns of erosion processes. These measurements help set the stage for advancing the understanding of hydrologic processes (and the coupling to biotic and chemical processes) as well as providing ways to age-date hydrologic events such as glaciations and extreme floods. In another application, noble gas spectrometry is being used to estimate the age of groundwater, which is important for understanding impacts of groundwater pumping and rates of recharge.

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22 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES The technology used to analyze more "traditional" stable isotopes (H, C, and O) has evolved to include portable, high-sensitivity cavity ring down spectroscopy approaches that are very low cost and involve far less mainte- nance and training compared to benchtop isotope-ratio mass spectrometers. These new endeavors will lead to more robust databases. For example, the Global Network of Isotopes in Rivers2 globally monitors 18O and D in river water, which allows for better assessment and evaluation of anthro- pogenic activities such as water storage and irrigation practices as well as climate change on river runoff. Chromatography, a method used to separate and analyze complex chemical mixtures, is used extensively in scientific research. Like mass spectrometry, some instruments developed based on this fundamental tech- nique have evolved over the past 20 years and have spread to a variety of scientific disciplines and subdisciplines, while others have remained mostly unchanged. Ion chromatography, used to separate ions and polar molecules based on their charge, allowing rapid and precise analysis of dissolved major ions such as fluoride, chloride, nitrate, nitrite, calcium, magnesium, and many others at low concentrations, has not changed significantly over the past 20 years. Yet newer chromatography techniques can now separate and detect of organic compounds (naturally and anthropogenically de- rived). These include the development of highly selective stationary phases and fundamental changes to the liquid chromatography (LC) hardware. For example the relatively recent development of ultra performance liquid chro- matography (UPLC) pushes the bounds of traditional high-performance liquid chromatography (HPLC) by providing excellent analyte resolution coupled with rapid sample analysis. Perhaps the most significant advance over the past two decades is the coupling of HPLC with MS (single and in tandem), which was accom- plished through the development of electrospray ionization, a groundbreak- ing method used to ionize high-mass compounds with subsequent analysis by MS.3 This method significantly expanded the suite of compounds iden- tifiable by modern analytical techniques, and is critical in the identification of new contaminants in water around the globe. Used with HPLC or UPLC and tandem MS such as triple-quads or quadrupole-time-of-flight MS, this highly selective technique can elucidate complex chemical mixtures such as contaminants of emerging concern in complex environmental matrices. The sophistication of many of these newer techniques and instruments has allowed for more detailed temporal or spatial sampling and analysis, enabling hydrologic scientists, engineers, aquatic geochemists, and envi- 2 Seehttp://www-naweb.iaea.org/napc/ih/IHS_resources_gnir.html. 3 John B. Fenn and Koichi Tanaka were awarded the Nobel Prize in Chemistry in 2002 for working on this ionization method.

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THE HYDROLOGIC SCIENCES 23 ronmental chemists to unravel the impact of such processes as photosyn- thesis and photo-oxidation on transition-metal dynamics and speciation. An astounding number of new contaminants ("contaminants of emerging concern") have been detected in the aquatic environment. New and more sensitive geochemical and isotopic techniques have added to the ability to age-date and trace groundwater movement over various temporal and spatial scales. The presence of sophisticated analytical equipment extends into the field, as well. Hydrologic scientist are now deploying new in situ ("in position") analyzers or field-deployable boxes, especially for nutrient chemicals such as nitrate, that were originally developed by the ocean- ography community. These allow for higher-frequency temporal analysis and potentially almost real-time data return. Finally, fourier transform ion cyclotron resonance (FTICR) MS is one of the latest developments of MS to be applied to the hydrologic sciences. Developed in the 1970s, this high- resolution technique is one of the advanced methods--if not the most ad- vanced method--of mass analysis with unprecedented resolution especially for larger and more complex organic molecules such as dissolved organic matter, proteins, and so on. Remote Sensing and Geophysical Techniques The inherent and pervasive irregularity of Earth's surface and sub- surface properties presents an impasse in the ability to characterize and predict hydrologic processes. However, recent breakthroughs in satellite and remote imaging and sensing and ground-based geophysical techniques have provided unprecedented opportunities to break this impasse by col- lecting and analyzing a massive amount of field data. A few examples are provided, below, to highlight the currently available techniques that have contributed greatly to new ideas but have yet to be fully exploited in ad- vancing hydrologic science. Weather radar has facilitated spatially extensive estimates of rain- fall that are unavailable using sparse rain gauge networks. For example, the completion of the Next Generation Radar (NEXRAD or WSR-88D4) surface-based radar network over the United States led to a synoptic view of evolving and migrating rainstorms that transformed both the researcher and the public view of this fundamental flux in the hydrologic system. The dynamic national composite precipitation maps, albeit with coarse mea- surement accuracy, inspired a new line of hydrometeorological research, including work done as part of the Advanced Hydrologic Prediction Service at the National Oceanic and Atmospheric Administration.5 For example, 4 See http://www.roc.noaa.gov/WSR88D/. 5 See http://water.weather.gov/ahps/.

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24 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES there is now an almost real-time assessment capability for estimating the average recurrence interval of extreme rainfall events. National precipita- tion maps that track storms across the continent have brought about a new public appreciation of weather-related natural hazards. The upgrade of radars to allow for polarimetric measurements promises to significantly enhance the accuracy of continental precipitation mapping and to result in research quality high-resolution mapping of this important forcing of the hydrologic cycle. The accuracy of current estimates of global precipitation monitoring, which are on a larger scale and based on new spaceborne sensors, is to within about 40 percent for precipitation rates in the range of 1 mm/h to 10 mm/h (Hou et al., 2008). Drizzle, heavy precipitation, and solid-phase precipitation are even more difficult to capture. The current data sets are formed by combining data from passive and active satellite sensors with data from precipitation gauges. The spatial resolution and data refresh rates are limited by today's technological capabilities. In the coming decades with the launch of the National Aeronautics and Space Administration's (NASA's) Global Precipitation Measurement (GPM) project,6 which in- cludes follow-on missions building on the success of the Tropical Rainfall Measuring Mission (TRMM) (Figure 1-1), a constellation of spaceborne sensors on board multiple satellite platforms will allow for mapping of global precipitation fields with unprecedented relative accuracy across a larger range of rain rates and with higher spatial and temporal resolutions. Remotely sensed observations of many other land-surface conditions from current and forthcoming sensors on board spaceborne satellites and suborbital aircraft will provide ever greater streams of unprecedented high- resolution data on surface soil moisture, soil surface temperature, topog- raphy, vegetation structure and health, snow cover, and other variables. (For example, the satellite microgravity measurements described in Box 2-3 have proven extremely valuable for estimating groundwater depletion.) These data will allow hydrologists to examine patterns that could not oth- erwise be easily discerned and will potentially lead to new theories about hydrologic processes in relation to land-surface properties. The forthcom- ing Earth-observing NASA satellites now include instruments and missions that are principally justified by applications in the water-cycle sciences. For example, NASA's Soil Moisture Active Passive mission7 that is in develop- ment and due to launch in 2014 is designed to produce high-resolution estimates of the near-surface (0-5 cm) soil moisture field and its frozen or thawed status. Similarly the GPM mission,8 also in development and due 6 See http://pmm.nasa.gov/GPM. 7 See http://smap.jpl.nasa.gov/. 8 See http://pmm.nasa.gov/GPM.

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THE HYDROLOGIC SCIENCES 25 FIGURE 1-1 The microwave radar and radiometer instruments on board the Earth- orbiting Tropical Rainfall Measuring Mission (TRMM), launched in 1997, al- low unprecedented insights into the three-dimensional structure of precipitating clouds. This image is of Typhoon Neoguri in the Southern Pacific, April 17, 2008. SOURCE: NASA's Earth Observatory, available online at http://trmm.gsfc.nasa.gov/ trmm_rain/Events/neoguri_17apr08_0729_utc_15dbz.jpg and through the TRMM extreme event image archives, http://trmm.gsfc.nasa.gov. to launch in 2014, will map global precipitation as part of a constellation R02116 of Earth-observing satellites. One of the main goals of the Surface Water and Ocean Topography mission, Figure 1-1 is under study for launch later 9 which bitmapped, uneditable in the decade, is to provide estimates of surface water extent, elevation, and slopes and therefore storage and storage change in lakes, reservoirs, wetlands, and r ivers (especially flood plains) based on spaceborne measure- ments. Discharge will be modeled from those measurements. In February 2000, during an 11-day mission using the Space Shuttle Endeavour, the Shuttle Radar Topography Mission produced a near global- scale, high-resolution (as fine as 30 m) digital topographic database of Earth. These digital elevation data quickly became the gateway data set for watershed studies and enabled studies of landscapes that had not been 9 See http://swot.jpl.nasa.gov/.

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34 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES begun. For example, the study of vegetation patterns has, until relatively recently, been segregated by discipline. Ecologists explored connections based on specific competition, biogeochemists sought explanations based on heterogeneous distributions of soil and rock chemical properties, and hy- drologists and geomorphologists considered controls on fluxes of water and sediment. In reality, of course, complex interactions exist among a host of factors, and hydrologic science is important in almost all critical processes. As a result of this and other similar examples, new disciplines have emerged that define these new areas of research (e.g., hydro climatology, hydro meteorology, geobiology, hydroecology, hydrogeomorphology, ecogeo morphology, and Earth-surface dynamics). Hydrologic science is central to all of these fields and, in being so, is becoming redefined by these fields. Interdisciplinary research is a mode of research by teams or individuals that integrates information, data, techniques, tools, perspectives, concepts, and/or theories from two or more disciplines or bodies of specialized knowledge to advance fundamental understanding or to solve problems whose solutions are beyond the scope of a single discipline or field of research practice. Facilitating Interdisciplinary Research, NRC, 2004 In addition to the scientific desire to delve into fascinating questions at disciplinary interfaces, emergence of high-profile, multifaceted problems such as water management and ecological restoration in California, resto- ration of the Everglades, Mississippi River nutrient loading and Gulf Hy- poxia, sediment management in the Missouri River, and water management in the Colorado River basin highlights how collaboratively driven work is a critical component of research that focuses on real hydrologic phenomena. The committee mentions a few examples of areas where past collaborative research yielded major new insights and where future concerted effort is needed. Paleohydrology is the science concerned with the study of hydrologic systems as they existed before direct observation and modern hydrologic records. This interdisciplinary field uses methods of analysis and informa- tion from hydrologic science, climate science, botany, and geology. Concern about the impacts of climate variability and change and the correspond- ing desire for forecasting changes have heightened interest and resulted in increased activity in this area, beginning in the early 1990s (NRC, 1991) extending to the present. An increasing number of high-resolution records have been developed over the past several decades, especially from tree

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THE HYDROLOGIC SCIENCES 35 rings. These records are important because they provide a more complete picture of past hydrology, documenting both spatial and temporal patterns of hydroclimatic variability over the past several millennia, and allowing recent hydrologic trends and events to be placed in a long-term context. Significant contributions include documentation of the spatial extent of droughts in many regions over North America over the past 2,000 years that indicate persistent droughts of much greater length and severity than any in the past century over broad areas of the western and southwestern parts of the United States (Figure 1-6). These droughts are also manifested in a multitude of watershed-scale reconstructions of streamflow, which are now being used by a number of water resource management agencies to plan for drought. The linkages between ecology and hydrologic science at the land sur- face are complex and, like paleohydrology, scientific progress requires interdisciplinary collaboration. Water from the atmosphere as rain, snow, or dew obviously is essential for plants to thrive. But plants also affect the soil, which determines how much water is held for use by vegetation and how much is "lost" from the near-surface soils. Similarly, the atmosphere drives evaporation and transpiration (the movement of water from soils through plants to the atmosphere). Plant canopies create an environment FIGURE 1-6Droughts lasting for several decades have been identified by pa- leoclimatologists. Megadroughts during the 14th, 15th, and 16th centuries were determined using tree-ring reconstructed summer Palmer Drought Severity Indices (PDSI) averaged and mapped over the western United States. SOURCE: Reprinted, with permission, from Stahle et al. (2007). © 2007 by Springer Science and Busi- ness Media. R02116 Figure 1-6 bitmapped, uneditable

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36 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 1-7 In agroecosystems crop yield is affected by groundwater depth, which in turn is affected by the use of water by plants. To identify groundwater depths that optimized crop yield, the authors combined data on corn, soybean, and wheat yields with topographic maps and groundwater-depth sampling. In this figure, piecewise regressions with two breakpoints were R02116 used. SOURCE: Reprinted, with permission, from Nosetto et al. (2009). © 2009 Figure 1-7 by Elsevier. bitmapped, uneditable with different temperature and humidity relative to, say, a concrete parking lot and so affect the behavior of the atmosphere. Furthermore, soil proper- ties affect how readily water and nutrients can be taken up from the soil by plant roots but again the growth of the roots themselves changes the soil structure, which is relevant to vadose zone hydrology, the subdiscipline concerned with water in soils.

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THE HYDROLOGIC SCIENCES 37 Hydroecologic interrelationships are of considerable practical impor- tance, for example in agriculture (Figure 1-7). Beginning in the late 1970s and 1980s, leading researchers from traditionally "agricultural" and "hy- drogeology" backgrounds began to recognize that hydrologic processes from the soil surface to the water table were continuous and that col- laboration among soil scientists and geologists was a fruitful way to make progress. Cross-fertilization between the agricultural and geologic com- munities led to advances in solute transport models, commercialization of field measurement systems, inclusion of vadose zone monitoring in waste disposal design and remediation, and recognition of the role of land use change and paleoclimates on fluxes to the water table. Mechanisms exist to promote these and other interdisciplinary collabo- rations and are discussed in more detail in Chapter 5. For example, in recent years a group of scientists has worked collaboratively within the Critical Zone Network to investigate "processes within the Critical Zone, defined as the Earth's outer layer from vegetation canopy to the soil and groundwater that sustains human life."11 Critical Zone Observatories (CZOs) study the operation and evolution of the Critical Zone. (For more information, see Box 3-1.) These collaborations are in the spirit of discovering fundamental relationships among physical and biological processes where expertise from the fields of geosciences, hydrologic science, microbiology, ecology, and soil science participates. Problems being addressed range from acid mine drain- age to release of arsenic to groundwater. HYDROLOGIC SCIENCE: LOOKING AHEAD Important challenges lie ahead in understanding the complexity of the Earth system, and water will never cease to play an important role in that system (NSF, 2009). Almost 20 years after its publication, the Blue Book remains fresh and compelling, and its statements take on even greater ur- gency in view of a stressed planet: [Water] ... is a hazard, a resource to be managed, and an enabler and sus- tainer of civilization. It is important to and affected by physical, chemical and biologic processes within all compartments of the earth system: the atmosphere, glaciers and ice sheets, solid earth, rivers, lakes and oceans. Water vapor is the working fluid of the atmospheric heat engine; water, as the primary greenhouse gas, is instrumental in setting planetary tempera- ture; water, through fluvial erosion and sedimentation, together with tec- tonics, shapes the land surface; water, is the universal solvent and the agent of element cycling. Finally, water is necessary for life.... (NRC, 1991) 11 See http://www.czen.org/.

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38 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES The hydrologic community should be ready to face the complex water- related challenges of today and tomorrow by continuing disciplinary and interdisciplinary research toward a predictive understanding of the atmo- sphere-hydrosphere-biosphere-lithosphere system from the microscopic to the global scale, by continuing the transformation of hydrologic education to ensure the workforce needed for the years ahead, and by translating new scientific understanding to decision-making tools for solutions that achieve sustainable outcomes. The NSF recognized the significance of emerging water issues and the need to strengthen and adjust its hydrologic science research efforts to ad- dress these issues. NSF acknowledges that understanding the complexity of the Earth system, which is driven by water, is critically important. It also recognizes the importance of interdisciplinary efforts among its various programs, divisions, directorates, and other agencies. The present study was undertaken by the NRC's Water Science and Technology Board at the request of NSF Earth Sciences officials.12 The committee's charge is to review the current status of hydrology and its subfields and the coupling with related geosciences and biosciences, and to identify promising new opportunities to advance hydrologic sciences for better understanding of the water cycle that can be used to improve human welfare and the health of the environment (Box 1-2). With respect to the restrictions of the study to not make budgetary rec- ommendations or to critique existing NSF programs, embedded within the statement of task was certain language that the committee interpreted as a request from the NSF for specific advice pertaining to the foundation. This includes reference in the task to current research modalities,13 educational opportunities, and strengthening observational systems, data management, modeling capacity, and collaborations including interfaces with mission agencies. These capabilities are integral to the NSF Hydrologic Science program and other NSF programs within the foundation; they represent the mechanism(s) used to promote the foundation's mission.14 In addition, 12 Original language from the study proposal that was included in the grant from the National Science Foundation to the National Research Council authorizing and scoping the study is as follows: "The primary focus of this study will be the NSF program in hydrologic science but given the importance of water issues to the nation, the report should also serve the academic/educational community, other agencies with programs in hydrology and water resources, Congressional staff, the Office of Science and Technology Policy, professional societ- ies, and other entities with missions related to Earth sciences and water resources." 13 The committee interprets the term "modalities" in the statement of task as referring to capabilities within the NSF and other federal agencies used to advance hydrologic research including contracts and research grants, instrumentation and facilities, and so forth. 14 To "promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."; see http://www.nsf.gov/about/.

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THE HYDROLOGIC SCIENCES 39 BOX 1-2 Statement of Task This study will identify the challenges and opportunities in the hydrologic sci- ences, including (1) a review of the current status of the hydrology and its subfields and of their coupling with related geosciences and biosciences, and (2) the identi- fication of promising new opportunities to advance hydrologic sciences for better understanding of the water cycle that can be used to improve human welfare and the health of the environment. The goal is to target new research directions that utilize the capabilities of new technologies and not to critique existing programs at NSF or elsewhere. The resulting report will not make budgetary recommendations. Specifically, the study will: · Identify important and emerging issues in hydrology and related sciences, · Assess how current research modalities impact the ability of hydrologic sciences to address important and emerging issues, · Identify needs and research and education opportunities for making sig- nificant advances in hydrologic sciences, and · Assess current capabilities in and identify opportunities to strengthen observational systems, data management, modeling capacity, and collaborations needed to support continued advancement of hydrologic sciences, and also their relationships to and value for mission-related agencies and, reciprocally, how observational systems of mission-related agencies relate to and contribute to hydrologic sciences. these capabilities are integral to other agencies and organizations that sup- port research in the hydrologic sciences. The Committee on Challenges and Opportunities in the Hydrologic Sciences met six times, heard presentations from scientists and engineers who work in several areas of hydrologic science and related disciplines, and solicited input from the community at an open town hall meeting at the 2009 Fall Meeting of the American Geophysical Union. Despite funda- mental progress in the field, the challenges and opportunities that lie ahead have intensified rather than diminished in view of the increasing pressure on Earth's water resources. Many open questions demand renewed and strategic research in the field. Water shapes landscapes and life, which, in turn, affect water flows and stored volumes of water. How do hydrologic systems, landscapes, and their biological communities co-evolve? Scientists are learning more and more from records encoded in rocks, trees, and ice. How does knowledge about the hydrologic past prepare us for society's water future? Does the planet face shrinking ice and growing deserts? How are bioclimatic zones evolving? Humans continue to intervene in the hydrologic cycle of Earth via diversions, dams, and pumps. What are the

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40 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES consequences of this planetary "replumbing?" Can sufficient clean water be supplied where and when humans and natural ecosystems need it? Through a host of natural and human-induced changes, ecosystems have been altered extensively. How can scientists tell if freshwater ecosystems are broken and, when they are found to be broken, how can they be fixed? How much water does an ecosystem need? Arguably the most important advance in public health came toward the end of the 19th century when water contamination was recognized as the cause of widespread disease outbreaks such as cholera, and sanitation was introduced. Yet today soci- ety still grapples with issues related to water quality. Can water quality be assessed and managed to protect human and ecosystem health? How can new detection, treatment, and modeling tools be used to protect the public against contaminants of emerging concern? The questions posed above are examples of intriguing puzzles that en- gage scientists and engineers in hydrologic science and related disciplines. A discussion of such questions could be organized along many different path- ways. This committee chose to write three separate chapters, which stand on their own but are intimately linked, that cover fundamental questions in hydrologic science and related biogeosciences. All three chapters present examples of "curiosity-driven" and "problem-driven" research; both are important. The sum of these chapters is not an exhaustive list spanning the entire range of hydrologic and related research. Rather, it is intended to enumerate some of the most challenging concepts and to identify some of the research areas most important to promoting progress in the field. The research opportunities in each chapter are arranged into several sections paired with a succinct boldface statement intended to extend the meaning of the section title as well as to interest and inspire the reader. Some of these areas will obviously overlap, a reflection of the intertwined, high-level challenges facing the community. Each section drills down into more specific, italicized questions and ends with specific exemplary ques- tions for readers seeking more detail. The tiered structure of the central chapters is intended to cater to an audience ranging from the aspiring hydrologist or engineer seeking an introduction to hydrologic research to an established scientist seeking detailed information. The chapters contain numerous boxes and figures to draw attention to interesting contributions and examples, and to support concepts articulated in the surrounding text. The committee cited publications only when critical: to properly attribute a figure or image, quotation, point of fact, or explicit concept, thus avoiding a literature review. The reference section at the end of each chapter contains references not only called out in the text but also from key citations ("Sug- gested Reading") that provide additional source material as an educational tool and a resource for aspiring scientists. The Suggested Reading lists are composed of review papers, synthesis documents, or landmark papers to

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THE HYDROLOGIC SCIENCES 41 provide the reader with material containing both depth and breadth on a given issue. The report is designed to be of use to members of the hydrologic com- munity, mainly the research community, which includes not only academics but scientists and engineers from the private sector, federal agencies (most notably the NSF Hydrologic Science program and other Earth Science pro- grams within NSF, when appropriate), decision makers interested in water research and policy, and those with Earth sciences and water resource- related missions interested in where hydrologic science fits into the surface- earth sciences. The report is also written for graduate and undergraduate students seeking inspiration, general knowledge, or guidance when selecting a focus within the field. Although the primary audience is the hydrologic community, the challenges and opportunities are intentionally broad, illus- trating the necessity of interdisciplinary work needed to face the complex water related challenges of today and tomorrow.15 The signature of a scientific challenge is that it is compelling--both in the domain of intellectual curiosity as well as in the domain of conse- quences for human and ecosystem welfare. The following chapters, titled "The Water Cycle: An Agent of Change," "Water and Life," and "Clean Water for People and Ecosystems," outline major areas of opportunity and challenge for hydrologic science. The content of each is intended to be a vision statement, identifying areas that deserve emphasis because they pres- ent challenges and opportunities which are both intellectually compelling and socially relevant to human and ecosystem welfare. The fifth and final chapter also discusses the challenges and opportunities, but in the context of accomplishing these goals for the hydrologic community and, more specifically, NSF. This chapter points out that "translational hydrology"-- highly collaborative work that includes social scientists and a wide variety of stakeholders--will be required to establish a healthy, resilient, and sus- tainable planet. Opportunities in the Hydrologic Sciences cemented the foundation of the field. Hydrologic science in the 21st century is a broad field that en- compasses all of traditional hydrologic science as defined in Opportunities in the Hydrologic Sciences and extends into areas that are traditionally of interest to other fields and related subdisciplines. This report builds on that foundation by stressing not only further building of hydrologic science, but also the interdisciplinary potential of a science with an established foundation. 15 The committee was asked to report on challenges and opportunities in the hydrologic sciences. The charge was not to discuss the distinction between "scientists" and "engineers."

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