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Home , Hydrology, Hydrosphere

of a Dynamic A Decadal Research Plan for Hydrologic Science

November 2007

Consortium of Universities for the Advancement of Hydrologic Science, Inc. of Contents

CUAHSI Scientific Advisory Team...... ii Foreword...... iii Scientific Understanding of a Vital ...... 1 Two Driving Forces in the Dynamic Earth...... 2 Improving Understanding of Hydrologic Science...... 3 1. Continental Dynamics...... 6 1.1 Definition...... 6 1.2 Advancing Continental Water Dynamics...... 7 1.2.1 Variability, Change and Adaptation...... 7 1.2.2. Spatial Heterogeneity, Scaling and Emergent Behavior ...... 7 1.2.3 Modeling and in Continental Water Dynamics...... 8 1.3 Opportunities...... 9 2. Community Science Goals...... 12 2.1 Linking the and the ...... 13 2.2 Upscaling hydrologic, biogeochemical, and geomorphic processes...... 13 2.3 Predicting the Effects of Human Development and Climate Change on Water ...... 15 3. Implementation Plan ...... 17 4. Education and Outreach...... 22 5. Linkages and Partnerships...... 24 6. Why Now?...... 26 7. References...... 27

- i - CUAHSI Scientific Advisory Team

John Wilson, Chair New Mexico Institute of Technology Roger Bales University of California, Merced Ana Barros Duke University Rafael Bras MIT Stephen Burges University of Washington Kevin Dressler, Manager Penn State University Chris Duffy Penn State University Dave Gochis NCAR Venkat Lakshmi University of South Carolina Upmanu Lall Columbia University David Maidment University of Texas Bridget Scanlon University of Texas John Selker Oregon State University Murugesu Sivapalan University of Illinois University of California, Irvine

- ii - Foreword

This science plan is a continuation of the work of many over the past few decades to advance hydrologic science. The 1991 NRC report Opportunities in the Hy- drologic Sciences led to the of the Hydrologic Science Program within the National Science Foundation’s Geosciences Directorate, a first step in supporting the advancement of hydrologic science as a distinct discipline within Earth Sciences. A fol- low-on NSF-sponsored workshop resulted in a call for an interdisciplinary initiative to integrate Water, , and Biota, published as the WEB report (Gupta, 2000, 2001; for the full report see http://cires.colorado.edu/science/groups/gupta/projects/web/). Key recommendations of the WEB report include (i) organizing hydrologic science ac- cording to scales, (ii) NSF support for mesoscale natural laboratories for conducting comprehensive interdisciplinary experiments and taking core observations, and (iii) de- veloping community infrastructure for experimentation and observations in item (ii).

The Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI) was founded in 2001 to develop the community infrastructure described in the WEB report. CUAHSI published an initial science vision for hydrologic community science (CUAHSI Technical Report #1), and focused on designing large-scale facilities (CUAHSI Technical Re- ports #2, 3, 4, 5, 7). These reports contained the concepts behind such facilities and identified needs and opportunities, yet lacked sufficient detail to define a multi-million dollar facility.

Experience from the CUAHSI Hydrologic Information Systems project has shown that a multi-phase approach is needed to move from concept to a community facility. The con- ceptual phase must be followed up by a pilot phase to scope a project more precisely and to determine the best approach to delivering a service. The pilot phase is then followed by a developmental phase when the specific tools and services are “hardened” by testing them with a limited clientele to ensure reliability and operational readiness to serve the community. Only then have the necessary attributes of an operational community service been fully de- fined. This science plan summarizes the results of these interim community planning efforts.

- iii - Hydrology of a Dynamic Earth A Decadal Research Plan for Hydrologic Science

Scientific Understanding of a Vital Resource The Earth is dynamic and changing, but so too is seeks an improved ability to observe and predict human civilization. Conflicts between and dynamic interactions and feedbacks between vari- civilization are increasing. Nowhere is this more ous natural and human components of the critical evident than in conflicts involving water. The de- zone, and to assess the consequences of change. mand for water by agriculture and urban devel- Because of the integrative role of water, hydrolog- opment competes with the water requirements ic science has evolved as an interdisciplinary sci- of terrestrial and aquatic ecosystems. A warming ence that finds its frontiers at internal boundaries climate affects the amount, timing, and form of within hydrologic science (for example between , disrupting the efficiency of human and ) and at bound- engineered systems designed to harness and con- aries with other disciplines (including biogeo- trol that water. Human modifications for farming , , , , and living combine with climate change to alter the , and social/behavioral sciences). form and shape of water movement through the environment, especially the propensity for . However, successes at these frontiers, while im- pressive, are still limited in scope, setting and co- Water is also the tie that binds many dynamic as- herency. Why? Interdisciplinary hydrologic science pects of nature. Changing weather patterns and is largely beyond the reach of individual investiga- water availability interact with vegetation change tors. Integrated observations and modeling must and influence on hillslopes. moisture stretch over many processes and settings, and feeds back to cloud formation and precipitation. across a range of time and space scales. They re- Groundwater affects the biogeochem- quire a variety of technologies and talents not avail- istry of and the aquatic ecology of es- able to the individual or even to the small group. tuaries. And discharge supports hyporheic Modern developments in sensors and sensors net- aquatic and interacts with riparian vegetation. works, and information systems including compu- tational science, are not tuned to the needs of hy- These human and natural systems interact in drologic scientists and so are not readily accessible. the critical zone, the continent’s outermost sur- face defined from the vegetation canopy to To address these needs, and to take advan- deep groundwater, and from the atmospheric tage of new technological opportunities, hy- boundary layer, above, to the underlying bed- drologic science, through Consortium of rock, below. It extends across continents, from Universities for the Advancement of Hy- mountain headwaters to the coastal . drologic Science, Inc. (CUAHSI), has adopt- ed a community goal for the coming decade: Environmental sciences grapple with a breadth of To develop an integrated and quantitative rep- processes in the critical zone, with hydrologic sci- resentation of continental water dynamics that ence taking the leading role in understanding the spans boundaries within hydrology and with other integrative role of water as it moves though con- disciplines. tinents. Understanding that is challenged by the Success of achieving this goal will be tested by non- disparate dynamics of water in the atmosphere, in trivial, successful that cross boundaries, surface , and underground. Like other en- explaining how perturbations to one process inter- vironmental sciences, hydrologic science advances act with the coupled behavior of other processes. through integrated observations and modeling. It

- 1 - Two Driving Forces in the Dynamic Earth Although the Earth and human civilization are ex- We are not prepared to deal with the coming water periencing many changes, two stand out as vital is- crises caused by the convergence of climate change sues that society must address jointly. Climate change and increasing population. Consider the following: and increasing population are converging to create • We do not know how much water there is. Although there a water crisis affecting people and the environment. are thousands of and river gages, we routinely Climate change. Global circulation models predict monitor only these two components in the hydro- significant temperature changes over the next- cen logic cycle. We have no dynamic estimates of the tury, and, with less certainty, predict changes in the stores of water available within a river basin. In amount, timing, and form of precipitation (i.e., rain some areas where groundwater is an important or ). Knowing that temperature and precipita- water source, withdrawals are measured. However, tion will change is only the first step. How will these estimates of are not widely changes affect and interact with people and the en- available and imprecise where we do have them. vironment? For example, how will expected tem- Hence we don’t know the “hydrologic carrying perature increases change the rate and the pattern of capacity” of a river basin. How many people can evaporation and transpiration? If more rain falls dur- live within the basin and what are the ecological ing extreme events, how will partitioning of precipita- impacts of their water use? tion at the surface, among recharge, runoff, and • Water in the river today may have fallen as rain decades transpiration, change? How will these evaporation and to millennia ago. Only within the last 20 years have precipitation changes influence the water available for techniques been developed to estimate the age of vegetation and for human use? Will natural com- “young” ground water. Results indicate that, even munities shift? Can agricultural crops still be grown in in quite small river basins, a rain drop takes consid- the same places? Will human-engineered water-relat- erable time to travel to the river. Hence contami- ed infrastructure still be adequate to meet water de- nants that the water carries, such as nutrients, can mand reliably and to control floods safely? Predicting take several decades before they appear in a river climate effects is far more difficult for the terrestrial and, similarly, clean-up efforts can take decades environment than for the atmosphere because, unlike or longer to show any effect. Our knowledge of the continuous fluid of the atmosphere, the terrestrial travel time for , the transport vector for environment is highly heterogeneous, adaptive biolog- hydrophobic contaminants in , is even more ical processes control much of the terrestrial system, limited than for water. and the terrestrial subsurface is difficult to observe. • We cannot reliably predict the effect of a change in precipita- tion on water availability or , or on ecosystems. Population pressures and development. At The response of a river basin to a change in annual the same time as climate change alters evaporation average precipitation depends on a number of fac- and precipitation, human development and control tors—its distribution and form, for example—yet, of water resources has reached unprecedented lev- even if those are specified, we can’t say that a given els. Population density in coastal areas is stressing increase in precipitation will result in a propor- water supplies. Agricultural is overdraft- tional increase in runoff or recharge. Predicting ing groundwater. Citizens are demanding water for the indirect effects of precipitation change is still maintenance and restoration of aquatic ecosystems. more uncertain. For example, we can’t predict reli- Traditional solutions, whether structural controls for ably if river nutrients or would protection or end-of-pipe treatment of waste- increase or decrease in response to a 10% increase water, are inadequate to address the problems faced in annual precipitation because of the interactions by society. In all these cases, the knowledge base is among physicochemical and biological processes. lacking to make scientifically informed judgments We also can’t predict how vegetation or aquatic that allow us to make rational investments in hazard ecosystems will respond to precipitation change, protection or to make trade-offs among competing especially when the response is a truly indirect ef- economic, social, and ecological demands on water. fect, as through a change in groundwater level or discharge. - 2 - Improving Understanding of Hydrologic Science In light of the crises created by the convergence of not predictable from small-scale processes. In other climate change and increasing population, there is a words, are macroscale processes the result of emer- pressing societal need for improved understanding gent behavior, and if so what are their properties? of the Earth’s critical zone and how it will respond 3. Predicting the effects of climate change and to these predicted changes over the next century. human population on water resources. With the We proposed to address this need through a more understanding developed under the first two chal- comprehensive, systematic understanding of con- lenges, we can more reliably make these predictions. tinental water dynamics—that part of the global A fundamental goal of hydrologic science is to be hydrologic cycle acting over and on the continental able to predict the response of water and the critical land masses and involving the dynamic interaction zone to climate and human perturbations to a level of water and related processes within the critical of confidence approaching those now possible for zone. At the most general level, continental wa- atmospheric and ocean systems, despite the hetero- ter dynamics requires analysis of fluxes and stores geneous nature of the continental environment. of water throughout the continent, of the interac- tions water experiences along pathways through and These challenges share the same science requirement: among these stores, and of the velocity and travel understanding the controls on stores, fluxes, flowpaths, time along these pathways. These four fundamental and residence time of water. These four properties properties—stores, fluxes, flowpaths and residence provide a sufficient hydrologic description to examine time of water—link continental water dynamics with water’s interaction with the , chemosphere, , ecology, climatology, geomorphol- and biosphere. To be most useful to society, the con- ogy, oceanography, and social/behavioral sciences. trols on these properties must be understood across Understanding these properties of continental wa- all spatial scales, but most importantly at a sufficiently ter dynamics will enable better prediction of direct large scale to be meaningful for resource management. and indirect effects of climate change and human activities on water resources and the environment. To comprehensively address these challenges requires coordinated scientific investigation and large scale This report proposes a community research infrastructure that extends from a plot or hillslope, strategy that will develop a more comprehen- across the scale of a continent. The Consortium sive, systematic understanding of continen- of Universities for the Advancement of Hydrolog- tal water dynamics. Over the coming decade ic Science, Inc. (CUAHSI) has been formed by the if will focus on the following three challenges: academic hydrologic science community in the U.S. 1. Linking the hydrosphere and the biosphere. to develop plans for and to promote the establish- Recent research in , the interface be- ment of the large-scale infrastructure needed to tween hydrology and terrestrial ecology, has shown address interdisciplinary hydrologic science in a co- that interactions among biological, soil, and hydro- ordinated and coherent fashion. Infrastructure is a logic processes are strong and adaptive. Traditional necessary complement to existing single-investigator views of a one-way hydrologic forcing of the bio- research and is needed to promote interdisciplinary logical system do not capture these interactions and research. However, it can’t be established by individ- adaptations. , a poorly measured store ual investigators, or even by individual universities, of water, has emerged as the “gatekeeper” linking acting alone. This is CUAHSI’s role. This CUAHSI atmospheric, biological, and hydrologic processes report helps to set out the rationale for these plans. 2. Scaling hydrologic, biogeochemical, and geo- morphic processes. Our current understanding of There are four key approaches to systematic under- these linked processes comes from laboratory simu- standing of continental water dynamics and, in par- lations under controlled conditions and field studies ticular, the three science challenges outlined above: at relatively small scale. We need to determine wheth- er these processes combine in a predictable way to 1. New observations. Gaining new understanding of control macroscale processes or if these process are continental water dynamics requires integrated ob- - 3 - servations of the hydrologic cycle, particularly at Although increased understanding of hydrology of a the interfaces in that cycle, such as the land surface dynamic earth requires new observations, determin- and atmosphere, or groundwater and surface water. ing what new observations to collect, where to col- Existing data are spatially incoherent—rainfall is lect them, when to collect them, and how frequently measured at one place, groundwater at another and to collect them, is complicated by the spatial hetero- soil moisture at a third, as as too sparse in time geneity and dynamism of the continental environ- and space to test hypotheses. Furthermore, multiple ment. Three stages are proposed to explore these is- lines of evidence, such as the use of chemical trac- sues: (1) Benchmarking current understanding through the ers, are needed to add power to such tests. The de- use of simulation models on “digital observatories” velopment of coherent multidisciplinary data sets is constructed from existing data records (including a central goal for the hydrologic science community. remotely sensed data and paleodata), while prototyp- 2. Interdisciplinary synthesis. Novel insights can be ing new observational systems to test sensor robustness. gained through the integration of new observa- (2) Testing generality of understanding through cross- tions, existing data, theories, alternate methods site comparisons using measurement campaigns of of analysis, and broader interdisciplinary perspec- limited duration which build on information from tives. CUAHSI has described a synthesis facility fixed-place-based observations of longer duration. for hydrologic science in a white paper (CUAHSI And (3) understanding long-term response of hydrologic Technical Report #5) and two synthesis projects systems to climate and human impacts through the are underway to serve as demonstration projects. establishment of an expanded network of place- 3. A modern information system or cyberinfrastructure based observatories. These observatories will build for data access and modeling. The CUAHSI Hy- on existing environmental observing networks in drologic Information Systems project, now in its order to develop the inferential capacity to make re- third year, has been developing the prototype ap- liable predictions of water stores, fluxes, flowpaths, plications to federate multiple data sources includ- and residence-time distributions. The third stage is ing data from various federal and state agencies the most expensive and one whose merits must be as well as data from academic scientists, including carefully evaluated. Emerging placed-based envi- new observations. This project is also laying the ronmental observatory activities that include some groundwork for modern modeling frameworks hydrologic component, such as the Critical Zone that improve the efficiency of modeling and- en Observatories, the National Ecological Observing hance model comparison by allowing scientists to Network (NEON), and the WATer and Environ- contribute to a common library of interoperable mental Research Systems network (WATERS) test- modules. Computer simulation models, as precise bedding activities, are essential to inform the design quantitative statements of our hypotheses, will play of placed-based hydrologic observatories, their rela- a central role in this community research strategy. tion to shorter-duration measurement campaigns, 4. Advanced instrumentation. Addressing interdisciplin- and their relationship to other existing or emerging ary science requires data from all phases of the long-term environmental observational programs. hydrologic cycle. Consequently, the range of in- struments used by hydrologic scientists is vast and Each of these areas— observations, synthesis, in- rapidly growing. It ranges from geophysical ex- formatics, and instrumentation—provide outstand- ploration of the subsurface, to atmospheric scin- ing opportunities for education and outreach at tillation, to laser spectroscopy to measure stable all levels from school to citizen engagement. isotopes of water. It will embrace the new genera- Water is a substance we all need and something with tion of environmental sensors and sensor network which we are all familiar. Yet much of the hydrologic technology. No one can be an expert in cycle is hidden. Public understanding of ground- all areas of application, or with all instruments and water and , connections between differ- sensor technology. A pilot project for a Hydrologic ent phases of the hydrologic cycle, interaction with Measurement Facility has been underway since 2005 ecological systems, and the role of continental wa- exploring how to get sophisticated instruments ter dynamics in the global water and energy cycles, and sensor technology, along with the necessary is limited. The decadal research strategy includes traing on how to use them, into scientists’ hands. various outreach efforts, generally building on exist- - 4 - ing outlets such as science museums, but also sup- munity science plan will enable reconstruction of past porting the academic community to improve un- histories, promote important new measurements, de- dergraduate and graduate research experiences. ploy modern sensor technology, employ cyberinfra- The convergence of unprecedented natural and hu- structure to synthesize data and models, and integrate man perturbations on the hydrologic cycle, and the disciplines. It will combine knowledge from the past maturation of hydrology as an inter-disciplinary sci- with observations of the present to make possible pre- ence, makes this a crucial period. The proposed com- dictions of the future of continental water dynamics.

- 5 - 1. Continental Water Dynamics 1.1 Definition theories needed for prediction of continental water Continental freshwater is part of the -wide dynamics. First, spatial heterogeneity in the land sur- hydrologic cycle also involving the , the at- face complicates measurement strategies; unlike the mosphere, the biosphere, and the . But ocean and atmosphere, the earth is not a continu- continental freshwater plays a special role; it is ous fluid. Second, temporal dynamics of hydrologic the component of the hydrologic cycle that links response to precipitation are non-linear and exhibit most other terrestrial environmental sciences, sus- threshold behaviors requiring that measurements be tains life on land, and directly affects the pros- made at sufficiently high frequency and accuracy to perity, safety and economic viability of a nation. capture this response. Third, accessing the subterra- Continental water dynamics describes the dynamic interac- nean portions of the hydrologic system is difficult; tion of water with the atmosphere, , ecosystems, even deploying instruments can alter the properties and humans, including human-built water infrastructure. that we seek to measure. Finally, and most impor- Research of continental water dynamics crosses all tantly, the continental environment itself is changing spatial scales, examining water-related states and flux- from natural processes and human activities; water es on hillslopes and headwater , in river basins both responds to and is an agent of those changes. and regional aquifers, and across the nation. It exam- Each of these challenges present opportunities for ines temporal variation and change, spatial patterns and advancing scientific knowledge. To realize those op- heterogeneity, and the emergence of new behavior at portunities, CUAHSI recommends an integrated larger scales. Research also seeks to quantify the phys- strategy of observations, synthesis, informatics, and ical, chemical, and biological processes that influence instrumentation. This strategy will enable reconstruc- water resource through the collective use of theory, tion of past histories, will promote important new manipulative experiments, observations and modeling. measurements, will deploy modern sensor technol- Compared to oceanic water dynamics (see sidebar), ogy, will employ cyberinfrastructure to synthesize continental water dynamics is more strongly con- data and models, and to integrate disciplines. It trolled by biological and geologic processes and ex- will combine knowledge from the past with obser- hibits many nonlinear feedbacks. This difference vations of the present to make possible predic- presents a series of challenges in developing the tions of the future of Continental Water Dynamics.

Surficial Processes on the Ocean and the Continents.The complexities of the Continental Water Dynamics are illustrated by comparing the hydro- logic processes operate over the ocean surface with those of the terrestrial environment. Over any one patch of ocean there are essentially two hydro- logic processes of importance: precipitation and evaporation (top figure). In this environment hydrology is an entirely physical system and estimating rates for processes are relatively straight forward because the ocean has a free-water surface at 100% relative humidity. In contrast, these same two processes are greatly complicated in the ter- restrial environment (bottom figure) due to the topographic, soil, vegetative, and heterogeneity of the land surface. For example topographic gradients control precipitation amounts and variably saturated surfaces strongly control the rates of evaporation. Partitioning of water at the land surface is controlled by these and other physical processes, including , groundwater recharge and discharge and , and by biological processes, notably plant transpiration. Many of these processes, because they occur below the land surface are difficult to directly measure. Finally human water use, through groundwater pumping, surface water di- versions, and agricultural and urban development and demands, have greatly altered the hydrology across the continent and around the world. These complexities of the terrestrial environment, especially these heterogeneities, represent the challenge of terrestrial hydrology. (Contributed by James Hogan, University of Arizona) - 6 - 1.2 Advancing Continental Water Dynamics cipitation. Long-term shifts in the hydrologic cycle, To understand the opportunities for advancing our with implications for continental water dynamics, understanding of Continental Water Dynamics, the especially floods and , are considered a pri- challenges presented by the continental environment mary aspect of these climate-related changes. While must first be presented more completely. There are estimates of global temperature rise are consistent three key areas. The first two include the temporal among researchers, especially those using climate and spatial heterogeneity of the continental environ- models, predictions of precipitation responses are ment. The third is a challenge of modeling and pre- highly variable and uncertain. Even if average pre- diction in an observational science where our ability cipitation doesn’t change, any shifts in temperature to perform controlled experiments is quite limited. are expected to change the intensity, distribution, 1.2.1 Variability, Change and Adaptation and even the type of precipitation. We’ve already Continental Water Dynamics describes the dynamic detected less precipitation as snow and earlier snow- temporal variation of processes and responses, from melt in the Pacific Northwest, with important impli- minutes to daily, seasonal, and inter-annual swings cations for water resources management. Landscape of water-related fluxes and stores, to rare and epi- and vegetation are also changing, due to both natu- sodic events. But it also must grapple with long term ral geomorphological and ecological processes, and change of average conditions, of the magnitude of human engineering of for living space, swings, and of the propensity for rare events. Conti- agriculture or other purposes. Whether abrupt or nental Water Dynamics focuses on detecting, attrib- smooth, episodic or rare, these changes will impact uting, understanding and predicting variability and continental water dynamics, the environment, and change. Progress in this area will allow science and people, across a wide range of space and time scales. society to discriminate change from variability, and to Adaptation. Although transpiration, a biological pro- adapt to and manage both of these under uncertainty. cess, constitutes a major flux of the hydrologic cy- Rare and Episodic Events. Many of the most im- cle, this process, like many other biologic processes, portant dynamical processes occur during episodic or was always considered driven by water and energy extreme events. These include rare pulses of water, availability. Only with the ability to directly measure sediment, contaminants, and nutrients that often are transpiration fluxes through the use of sonic an- unobserved during average conditions. Limnologists emometers and large aperture scintillometers, has it currently cannot predict the transport of car- become apparent that vegetation play a much more bon and associated with major hydrologic active role in the hydrologic cycle. adopt strat- events, when biogeochemical action is most intense. egies to improve their competitiveness in water- Geomorphologists realize that most landscape evolu- stressed environments such as translocating water tion occurs during some of these same events, but between soil horizons and altering root distribution the majority of their understanding is based only on in response to hydrologic conditions. Therefore, studies in pristine headwater catchments. Ground- any hydrologic predictions of response to differ- water hydrologists in the semi-arid Southwest sus- ent climate conditions must consider the possibil- pect that it is a rare series of precipitation events that ity that vegetation will adapt to the new regime and sufficiently wet the soil to escape capture by thirsty alter transpiration from what is currently observed. plants and recharge aquifers, but have not directly ob- 1.2.2. Spatial Heterogeneity, Scaling and Emer- served this process. Water resource specialists know gent Behavior is the major limit to human development in Hydrologic and related processes exhibit natural spa- water-stressed , but cannot predict drought or tial heterogeneity, as well as structured variability, its consequences without considerable uncertainty. such as that imposed by river networks and gradients Change. The broader scientific community has pro- in the landscape, and in , , and vegetation. duced convincing evidence that, because of the ef- Spatial patterns and heterogeneity need very dense fects of natural processes and human civilization, observations to be accurately characterized. While the Earth is experiencing environmental changes of products may provide adequate in- a historically unprecedented magnitude. In the next formation on land cover and land use, the state of hundred years, most areas will likely undergo major vegetation, and even stream network morphology, changes in temperature, and changes in regional pre- they provide highly uncertain estimates of precipita- - 7 - tion, soil moisture, vegetation transpiration, and aqui- the ability of the model to match observations of fer recharge. These remote sensing estimates provide runoff or chemical concentrations at the watershed little information on geological heterogeneity in un- outlet does not mean that the hypothesis contained derlying aquifers and the amount of flow or nutri- within the model are the only possible explanation ent load in streams. Moreover, even remote sensing for the observations. The relatively low informa- products must be assisted by land-based observations tion content of the calibration signals means that for “ground-truthing” and by process understanding one can be right for the wrong reason—that is, the for extrapolation to ungauged or poorly monitored hypothesis may appear to be supported by the data basins and also to future, yet unobserved, conditions. but have low predictive power under different con- Research in continental water dynamics crosses all ditions. Ideally, multiple working hypotheses should terrestrial scales. Horizontally, it ranges from hill- be explored to identify environmental conditions slopes and headwater streams (1 – 1,000 m), through when they yield different predictions to permit dis- river basins and regional aquifers (1 – 10,000 km), to crimination among the competing hypotheses. This the whole continent. Vertically, it peers downward at has been impractical until recently given the effort the microscale of a single soil pore, leaf stomata, or required to develop complex simulation models. biogeochemically active microbial community, and Getting the right answer for the right reason is not a upward at the entire continent. As scale increases simple undertaking (Horton, 1931; Kirchner, 2006). new levels of complexity are introduced, to the soil Scientific understanding of hydrologic systems rests profile, to the atmospheric boundary layer. Processes on physically based “laws” that have been developed and properties can no longer be explained by simple at the microscale. Most simulation models are built upscaling methods from the finer to the coarser scale; on the assumption that these laws can be directly up- instead, at each level of complexity new processes scaled to predict water dynamics at the watershed, ba- and properties emerge. Without a proper understand- sin, or even global scales. An active area of research ing of the physical basis and scales of these emerging is whether this upscaling assumption is justified (e.g., properties, our predictive ability will remain limited Seibert, 2003). Kirchner et al. (2000) and Kirchner et both in scale and in accuracy. On the other hand, the al. (2001) provide an example of how new data (high presence of patterns and emergent behavior, if prop- frequency measurements of a conservative tracer) erly understood, can enhance predictability even with combined with an analysis technique developed in limited observations. This presents a unique opportu- astronomy (spectral analysis of irregularly spaced nity in the hydrologic and related sciences. To take ad- time series) can yield new theoretical understanding vantage of this opportunity however, a systematic ob- of water transport (the lack of a characteristic flow- serving system is needed that will collect observations path length in a hillslope) that wouldn’t have been de- of a suite of environmental interacting variables over rived from a direct upscaling of microscale theory. a large range of space and time scales and subject them Therefore, progress in continental water dynamics to rigorous analysis and comparison with physical requires an active debate about upscaling properties. process-based understanding and model predictions. Community support of this debate includes provid- 1.2.3 Modeling and Prediction in Continental ing both novel data sets and analytical approaches. Water Dynamics Another prediction challenge for hydrologic science Like all earth sciences, hydrologic science must infer is assessing the generality of findings. In a place-based mechanism primarily from observational data rather science, how do we transcend the uniqueness of place? than from controlled experiments. Digital simulation Given the spatial heterogeneities of the continental models have been used extensively in hydrology, but environment and each place’s unique geologic and often as tools to design infrastructure or human history, it is difficult to know to what extent to predict the effects of management actions, such the findings depend on idiosyncrasies of the particu- as pumping an or harvesting a . To ad- lar field site. Testing hypotheses in multiple locations vance the hydrologic science, as opposed to opera- is currently very difficult and expensive because com- tional hydrology, simulation models are used to test parable data sets do not generally exist. Determining hypotheses. In observational systems where we can- a more efficient way to conduct multi-site compari- not control (or even measure) all parts of the system, sons is a community challenge for hydrologic science.

- 8 - 1.3 Opportunities of research results to water resource management. Despite the challenges posed by the continental en- These technologies promise to revolutionize the study vironment, the opportunities for collecting spatially of continental water dynamics, especially when these dense and temporally frequent data afforded by new technologies are deployed in a coordinated manner. sensor and networking technologies are immense. At the other end of the spectrum, remote sens- Examples include the ability to directly measure ac- ing techniques, including from aircraft (see tual at the scale of hundreds of Sidebar) as well as -based sensors provide square meters through eddy covariance techniques data at an unprecedented scale. These data provide and, at even larger scales, through large aperture scin- a new view of the earth that facilitate development tillometers. Temperature-based sap-flow measure- of new hypotheses and modeling approaches. Ac- ments enable estimation of water use by individual cess and use of these data pose important chal- trees at sub-daily frequencies. Measurement of soil lenges, including processing and interpreting massive moisture through time-domain reflectometry has data sets (McLaughlin, 2002). Only a small portion been simplified through miniaturization. Geophysi- of the hydrologic sciences community takes ad- cal techniques have been adapted to measure prop- vantage of such data today because of these barri- erties of the near-surface that give unprecedented ers; making such data accessible and interpretable insight into aquifer structure and water dynamics. to the broader community is a goal of CUAHSI. Some solutes can be measured at high frequency Ground-based sensors will be deployed in fixed-place and high precision with field-deployable “labs in a observatories and in shorter-duration campaigns. In can” and stable isotopes of water can be measured both cases, ground-based sensor networks will be de- using laser spectroscopy in the field for one-ten- signed to take maximum advantage of spatial data, thousandth of the price of lab analysis. Wireless particularly from . Highly specialized instru- communications make field deployment, even in ments used for shorter-term deployments will be main- complex , more feasible than ever before, and tained by CUAHSI’s Hydrologic Measurement Facility. at larger scales that permit more direct application

Hydrologic Science Hydrologic science studies the occurrence, distribution, circulation and properties of wa- ter, and its interaction with a wide range of physical, chemical and biological process- es, acknowledging also the added complexity of social and behavioral sciences (NRC, 1991). In the 15 years since 1991 NRC report, the efforts of individuals and small collaborations have signifi- cantly advanced understanding of individual components of continental freshwater in both natural and human engineered systems. For example, motivated by societal concerns with groundwater , im- pressive strides were made toward characterizing geological heterogeneity and predicting its influence on subsurface fluxes and behavior of water, chemicals, and microbiota. The boundary between theatmo- sphere and the land surface became the focus of intense research efforts aiming to understand the influ- ence of soil moisture and surface inhomogeneities on atmospheric processes, and the nature of wa- ter, energy and carbon fluxes from and to the atmosphere. The theoretical and modeling developments were aided by a spectacular suite of land and space-based observing platforms and campaigns (see Box) which resulted in innovative land-atmosphere modeling approaches such as Large Eddy Simulation (LES). Geomorphologists have long recognized that stream channel networks, which control the space-time behavior of many water-related fluxes, form spatial patterns that look similar across a wide range of environments and scales. Over the last fifteen years there has been a revolution developing quantitative tools to simulate, measure, and explain these networks, their similarities, and their controls of fluxes of water, and more recently, and nutrients. Without solid underlying theories of the interconnectedness of the physical, chemical and biologi- cal processes, the use of observations in constraining and guiding future improvements in predictive models, and development of generalized theories that can transcend place and time, the complex water problems of today cannot be addressed. Continental Water Dynamics aims to develop the scientific ba- sis for a holistic and comprehensive approach to solving complex water problems in service to society.

- 9 - Concurrent with the development of sensor technol- From the user’s point of view, CUAHSI HIS will pro- ogy, information technologies have also advanced vide a seamless environment for accessing data from rapidly. CUAHSI’s Hydrologic Information System multiple providers directly from within the model- (HIS) project has been applying web services technol- ing environment of their choice (Figure 1). The re- ogies to enable federation of data from various feder- mote data sources act as if they are local data bases. al agencies (including the US Geological Survey, Na- These web services also serve as the basis for the tional Climate Data Center, Environmental Protection development of a community library of mod- Agency and others). Eventually, academic scientists el modules and analytical techniques. Providing can be included in this federation by registering their a community modeling framework that simpli- data sets with a central catalog. This approach is being fies model development and encourages model tested with 10 projects currently being funded by NSF comparison is an important goal for CUAHSI. as “test beds” for the proposed WATERS Network.

Figure 1. CUAHSI Web Services permit extraction of data from multiple providers, transforma- tion of that data into various formats and loading data directly into various analysis environments.

- 10 - New Observational Techniques: LIDAR. However, despite the extreme spatial and temporal Geosciences is witnessing an era of rapid develop- variability and the large range of scales of interact- ment of new sensor technology and observational ing processes, one cannot sample everywhere and techniques that are poised to revolutionize our un- all the time. Thus the challenge exists to use even derstanding, and thus predictive modeling capabili- rudimentary knowledge of the underlying vari- ties, of earth surface dynamics. High resolution to- ability, of cause-effect relationships, and of pos- pography (e.g., 1 m topography data from LIDAR) sible scaling relationships to optimize sampling net- and wireless sensor technology with embedded work design. To that effect, detailed knowledge of networked sampling (e.g., concurrent and adaptive topography is expected to play a significant role. sampling over large spatial coverage and short time intervals; and particle-tracking techniques) provide The newly available high resolution LIDAR topo- an opportunity to bridge the gap between the small graphic data [Carter et al., 2001] provide the oppor- scales at which bio-geochemical processes occur tunity to resolve channel and floodplain morphology, and the larger scales at which organizing patterns stream corridor geometry (including geomorpho- are observed. Wireless technology, smart sensors logic disturbances at ), and vegetation with controlled activation capabilities, e.g., during ex- characteristics throughout the basin. Having such treme floods or high temperatures, small sensors that high-resolution channel morphology continuously can be attached to moving (“talking stones”) available along stream reaches and over the whole [McNamara and Borden, 2004], isotopes for dat- watershed offers the potential of understanding ing, and imaging, can all work synergistically cause-effect relationships between channel attributes to sample processes at scales ranging from a few and biological and geochemical processes. Such rela- mm and seconds to planetary length and time scales tionships can guide efficient design of environmen- . tal observatories and also guide efforts to upscale local processes, e.g., algal production [Hondzo and Wang, 2002], and denitrification potential to stream reach averages, and ultimately to indices character- izing the state of the whole watershed. However, existing methods for defining channel heads from 90 m or 30 m DEMs do not perform well when applied to the extraction of topographic features from 1 m LIDAR data. New “geomorphologically- informed” image processing techniques are needed to take advantage of the rich information provided by these sensors, including the automatic mapping Image created from LIDAR data. Note excavation on of service and skid trails created during log- right (orange and black outlines)caused by a land- slide, and the resulting deposits. ging that are large contributors of sediment to the streams. –Contributed by E. Foufoula and W. Dietrich

Synthesis of existing data sets has been identified disciplinary nature of continental water dynamics, as a third opportunity to advance continental water the large amount of existing data, and the develop- dynamics. The ecology community has pioneered ment of services from HIS, the hydrologic science the use of a physical facility, that National Center community has called for the creation of a synthe- for Ecological Analysis and Synthesis (NCEAS), as sis facility (CUAHSI Technical Report #5, 2003). a venue for self-forming working groups to come Access to these three community services—in- together with post-doctoral research associates formation systems, instrumentation, and syn- and sabbatical visitors. The result has been a series thesis—are the foundation for advancing our of highly influential publications. Given the multi- understanding of continental water dynamics.

- 11 - 2. Community Science Goals CUAHSI has chosen three community science The final area is the ultimate goal for the hydrologic goals around which to organize our activities: science community. Achieving this goal requires prog- 1. linking the hydrosphere and biosphere, ress on the first two goals. Community support is re- 2. upscaling hydrologic, biogeochemical, and geo- quired primarily in the modeling area, including data morphic processes, and assimilation and benchmarking of current understand- 3. predicting the effects of human development ing. Liaisons with professional communities from at- and climate change on water resources. mospheric sciences, environmental engineering, and The first area reflects a paradigm shift during the ecology are also required for the development of the last decade with the emergence of ecohydrology as a comprehensive models required to explore this area. subdiscipline that explores how Darwinian concepts Synthesis activities will be required to make progress of competition for water by vegetation can inform Each of these goals is important to society. Al- our understanding of hydrologic processes. Although though seemingly disparate, progress on each re- vegetation controls on the hydrologic cycle (and the quires a more comprehensive understanding of the intimate linkage of the carbon and hydrologic cycle) hydrologic cycle and its four fundamental proper- are central to this theme, understanding the link- ties of stores, fluxes, flowpaths and residence time. age between the hydrosphere and biosphere is also For reference in the following discussion, a simpli- important to biogeochemical cycling of elements fied, one-dimensional schematic of the hydrologic because of the close linkage between water and the cycle is presented. Four stores are delineated (atmo- microbial communities that control processes such spheric water, surface water, , and ground- as and nutrient transformation. Com- water) and the fluxes between them. This simplified munity support is required to bring the multidisci- view ignores that, in a three-dimensional world, ex- plinary teams together to make advances in this area. changes among these stores occur in both directions, The second community goal, by contrast, focus- such as a river that can alternate between gaining wa- es on a process of discovery rather than a specific ter from the saturated zone and losing water to the scientific area. Observations are typically made at saturated zone depending upon its geologic setting. plot to catchment scale (from a few square meters Thus the flowpaths can be far more complex than to a square kilometer), and mechanisms of hydro- are apparent from this diagram. As a result, deter- logic transport and biogeochemical transformations mining the residence time within a store is also more can be inferred from these observations. Whether complicated; residence time is best considered as these findings can be directly scaled to the- water a distribution of rather than a single value. Finally, shed or river basin scale (thousands to millions of such a diagram ignores that these stores are dynam- square kilometers) is an area of active research. ic, with interfaces between the stores (areas gener- Community support in modeling tools, synthesis, ally of intense biologic activity) changing in time. and field infrastructure at larger scales is needed. Figure 2. Schematic diagram of the terrestrial hydrologic cycle. Five major stores of water are shown: atmospheric water, snow and , surface water (e.g., and rivers), soil water and groundwater. These stores are linked by 9 major fluxes: 1. Precipitation, 2. Transpiration through vegetation, 3. Evaporation from free water sur- faces, 4. Infiltration from the land surface to the unsaturated zone, 5. Runoff from land surface to surface water, primarily from unvegetated, impervious surfaces, 6. Recharge of ground- water from unsaturated zone, 7. Discharge of groundwater to surface water, 8. Recharge of groundwater from surface water, 9.Transport of groundwater down gradient within or be- tween aquifers, 10.Human withdrawals, such as groundwater pumping, and 11. Human return flows, such as irrigation, and discharge.

- 12 - 2.1 Linking the Hydrosphere and the water flow, geomorphology, biogeochemistry)? Biosphere • Can the structure of porous media in the rooting The representation of vegetation simply respond- zone be predicted as competing physical processes, ing to rainfall has given way to an appreciation of where a self-organizing system of pores develop to dynamic interaction among climate, soil, and water drain water as quickly as possible and ecological pro- (Rodriguez-Iturbe, 2000). The field of ecohydrol- cesses where vegetation seeks to maximize carbon ogy has emerged, defined by Newman et al (2006) production by maintaining a mesic environment? as a discipline that seeks “to elucidate (1) how hydro- 2.2 Upscaling hydrologic, biogeochemical, logical processes influence the distribution, structure, and geomorphic processes function, and dynamics of biological communities Hydrologic science seeks physically based “laws” and (2) how feedbacks from biological communities that will have wide applicability and high predictive affect the hydrologic cycle.” Newman et al. (2006) power. Traditionally, hydrology has applied findings further suggest that ecohydrology could lead to a from fluid mechanics, together with the necessary synthesis of Newtonian and Darwinian approaches constitutive relations to develop sets of governing to science [e.g., Harte, 2002]. Newtonian principles equations, much the same as atmospheric and ocean of simplification, ideal systems, and predictive un- sciences have done. However, because the derstanding (typically used by hydrologists) can be is not a continuous fluid, hydrologic science has had combined with Darwinian principles of complexity, to contend with heterogeneities in porous media, in contingency, and interdependence (typically used by surface roughness or in channel geometries. Typi- ecologists). Can such a combination lead to a pro- cally, these heterogeneities have been captured in so- found and more rapid advance in our understanding called ‘effective’ parameters, which are determined of environmental processes? Harte [2002] identifies by calibration of the model. In biogeochemistry three “ingredients” for how such a synthesis can be and geomorphology, similar approaches have been realized: (1) development of simple, falsifiable mod- adopted, such as the application of thermodynam- els, (2) identification of patterns and laws (e.g., scal- ics to the chemistry of natural waters, and mechan- ing laws), and (3) embracing the science of place. ics to sediment particle interactions. In each case, a The soil-water store (Figure 2) figures prominently highly idealized representation of the natural sys- in the linkage between the hydrosphere and bio- tem is necessary to construct governing equations. sphere as it mediates between transpiration and With advances in computing power and in instru- recharge and is a key factor in soil-forming pro- mentation, we have the ability for developing com- cesses (). Yet, this store of water is puter models and to collect data with greater spatial the most poorly measured in the hydrologic cycle. and temporal resolution than ever before. Two com- Where water is limiting in arid and semi-arid en- peting approaches have emerged in the hydrologic vironments, the feedbacks between the bio- science community. The first seeks to use these ad- sphere and the atmosphere are expected to vances to directly upscale the current governing equa- be strong. See sidebar by Newman (2006). tions to larger areas using, for example, finer finite Example questions that emerge in link- element meshes or cellular automata techniques. The ing the biosphere and hydrosphere are: second approach identifies patterns at the catch- • How does the biosphere mediate the interaction ment or watershed scale and seeks to understand between the slower sub-surface moisture dynamics how these patterns arise (Sivapalan, 2003; Kirchner and faster above-surface atmospheric dynamics? et al., 2001). Organizing principles are sought that • What controls the partitioning of precipita- seek to understand how heterogeneities arise rather tion between recharge, runoff, and evapo- than attempting to measure them. Flood scaling is transpiration, and how does this con- an example of this second approach (see sidebar). trol differ with environment and scale? Example questions in this community goal are • What are the spatial patterns and temporal vari- • How do spatial patterns and temporal variabil- ability of snowpack, soil moisture and veg- ity of different physical, chemical and biological etation, and how do they respond to and/ processes link up to create stream networks and or control patterns of precipitation, recharge, pathways, and what new pro- runoff, and evapotranspiration (and ground- cesses emerge from this watershed network? - 13 - • Over what time and space scales, and dur- • How do spatially organized patterns of vegetation, ing what seasons, are macropores and other such as biomes, emerge as a result of feedback be- short-circuit pathways dominant, and what tween sub-surface and atmospheric water dynam- is the role of disturbance in these pathways? ics and will they change under changing climate?

How does vegetation interact with the hydrologic cycle in arid and semi-arid environments? Adapted from Newman et al (2006). The distribution, growth, and mortality of vegetation plant thresholds for xylem cavitation (Holbrook and are more sensitive to the hydrologic cycle than to any Zwieniecki, 1999; Sperry et al., 2002; Tyree and Sper- other factor (e.g. nutrients, ) on a global aver- ry, 1988) based on branch-level conductivity measure- age. The growth and biomass accumulation of vege- ments, and on stomatal regulation of transpiration tation is strongly correlated with total annual precipi- (Bond and Kavanaugh, 1999; Cowan, 1977; Jarvis tation (Knapp and Smith, 2001; Waring and Running, and Morison, 1981; Oren et al., 1999) from leaf-level 1998). In the Southwest, seasonality of precipitation measurements. This work has identified some funda- input is also critically important because the monsoon mental mechanisms by which plants regulate water precipitation typically arrives in mid-summer; a time loss, mechanisms which are commonly incorporated of relatively hot weather that can support high growth into ecosystem process models. However, technology rates, but can also cause temperature stress and signif- to measure the response of carbon gain to hydrologic icant cavitation and subsequent mortality if a drought variation has lagged behind water flux measurements. occurs. Such seasonality has dramatic impacts on Stable carbon isotope ratios of plant organic matter vegetation (Fernandez-Illescas and Rodriguez-Iturbe, have demonstrated species adaptation to water avail- 2004; Schwinning and Ehleringer, 2001; Huxman et ability over the lifespan of plants (Ehleringer et al., al., 2004; Smith et al., 2000) of arid and semiarid eco- 1993). On shorter timescales, eddy covariance mea- systems. The current drought is already showing dra- surements of ecosystem carbon exchange are now matic effects on the vegetation; the current mortality allowing us to determine the short-term (daily) re- of piñon and ponderosa pine is widespread through- sponse to water pulses (Huxman et al., 2004). Eco- out Utah, Colorado, New Mexico and Arizona. In system-scale stable isotope measurements are now contrast, vegetation along riparian corridors is histor- showing regional and temporal response of ecosys- ically accustomed to flooding as a source of nutrients tem water use efficiency to water availability (Bowl- and water. Such flooding has been minimized through ing et al., 2002; McDowell et al., 2004). Incorporating engineering efforts to control river flows. Overall, this knowledge into an ecohydrological framework direct anthropogenic manipulations of river flows is essential for predicting vegetation response to along with indirect alteration of the climate that shifts changes in water inputs and for predicting how veg- the timing, frequency, and magnitude of precipitation etation will affect water fluxes and water storage. At are likely to have profound impacts on vegetation larger scales, changes in species abundance and com- survival and productivity in this where vegeta- position resulting from climatic fluctuation and dis- tion is already coping with minimal water availability. turbance must be taken into account (Neilson and Predicting vegetation response to changes in the hy- Marks, 1994; Neilson, 1995). Measurements at this drologic regime requires models based on first prin- scale have truly lagged behind the models. However, ciples of plant carbon- (Landsberg and new technologies (e.g., advances in satellite remote Waring, 1997; Running and Coughlan, 1988; Williams sensing capabilities) show promise for improving et al. 1996). Understanding the response of plant our ability to quantify biogeographic responses to carbon gain (photosynthesis) to water availability is changes in the hydrologic cycle. Understanding the necessary because plant productivity and survival are carbon balance response of terrestrial ecosystems to dependent on carbon acquisition. We have detailed changes in the hydrologic cycle is essential for future understanding of whole-plant transpiration (Granier, prediction of terrestrial carbon sequestration un- 1987) based on continuous sapflow measurements, on der various climate change scenarios (IPCC, 2001).

- 14 - The effect of network topology and channel-floodplain morphology on the scaling of floods, sediment and nutrient fluxes, and ecosystem dynamics (after Paola et al., 2006) Scaling of floods has been the subject of consider- flux relative to available discharge drive the dynamic able research in hydrology starting with the simple evolution of the network structure itself? normalization methods, e.g., the index flood method, Recent research has shed new light on how the spa- to the recent statistical multiscaling theories [Gupta tial structure of ecosystems interacts with the spatial et al., 1994]. A key question concerns the variation structure of the landscape [Caylor et al., 2005; Cay- of flood intensity and frequency with the drainage lor et al., 2004; Porporato et al., 2004; Porporato and area of the basin (scale). Analysis of observations Rodriguez-Iturbe, 2002]. Channel networks provide from several regions has supported the inference an organizing template for the ecohydrological and that floods exhibit a multiscaling structure (i.e. the biogeochemical interactions that determine the veg- statistical moments scale as power laws with drain- etation patterns and ecosystem dynamics in a river age area with an exponent that depends nonlinearly basin. Important questions remain to be answered: on the order of the moment) with a scaling break What are the feedbacks between flow regime and at a characteristic scale. Although such an approach dynamics of riparian vegetation? What is the rela- yields a concise statistical model which can be use- tive role of large-scale determinants of vegetation ful for regional flood quantile estimation of design patterns, e.g., optimal response to water stress, and events, a number of open questions remain. For ex- smaller-scale controls mediated by the network struc- ample, what is the physical origin of the observed ture? What is the relative role of space-time rainfall scaling and what determines the scale of the break? variability versus channel network topology in deter- What is the relative role of space-time precipitation mining the spatial patterns and dynamics of vegeta- variability versus geomorphologic controls, e.g., sys- tion? How does the physical structure of the land- tematic variability of hydraulic geometry with scale scape influence habitat quality and diversity, and how and dynamic channel-floodplain interactions, in de- does it control sources and flows of organisms and termining the scaling of floods and streamflow hy- limiting nutrients [Power et al., 2005; Power et al., drographs [Dodov and Foufoula-Georgiou, 2004; , 1995]? In turn, how do organisms shape the land- 2005; Menadbe and Sivapalan, 2001]? Bringing sedi- scape through microbial weathering, the stirring and ment into the picture, what controls the size distribu- diffusion of soil, flow baffling and the stabilization tion of sediments produced and delivered to channel of bars, banks and floodplains [Dietrich and - Per networks by hillslopes? How are the size distribution ron, 2006]? What are the coupled dynamics of hill- and flux rates of bed material affected by the drainage slope-floodplain-stream interactions and what is their network structure? And how does bedload sediment role in biogeochemical cycling [Green et al., 2005]?

2.3 Predicting the Effects of Human De- what are the ecological, economic, and social trade- velopment and Climate Change on Water offs for providing water to a growing population? Resources Challenges also exist at shorter time scales. and Human development and predicted climate change Lettenmaier (2006) note that seasonal river forecast- may force the continental environment into condi- ing in the West has not improved since the 1960’s due tions that have not been experienced before. An inte- to a combination of greater climate variability and grated understanding of continental water dynamics the inability for operational techniques to is required to convert a prediction of an n degree in- incorporate new sources of data, such as satellite ob- crease in temperature into a prediction of the effects servations of the extent of snow cover. Current op- of such an increase on water resources and vegetation erational models are based upon regression equations communities. Will agriculture still be feasible? How that assume a static relationship between climatic will an increase in severe events impact the hydro- and hydrologic independent variables and predicted logic cycle? As populations shift to coastal and arid stream discharge. These models can’t incorporate areas, water resources become stressed. What is the that these relationship are apparently changing. Dif- hydrologic “carrying capacity” of a river basin and ferent approaches, such as ensemble streamflow

- 15 - predictions, a form of , are needed. ity, type, and intensity of precipitation, and what Sample questions in this area include effect will it have on partitioning of precipitation • How does human development change the par- at the land surface and on watershed response? titioning of precipitation, the behavior of drain- • Will continued human development couple with age networks, the pathways for subsurface climate change to modify patterns and amplify flow, and the movement of pollutants and how variability, especially the propensity for rare or does this differ with environment and scale? episodic events like major floods and droughts? • How will climate change alter the pattern, variabil-

- 16 - 3. Implementation Plan These and other questions, and the severity, complexity ment of prototype and collaborative observatory and scale of water-related challenges facing the nation networks, such as WATERS Network, NEON, and today, requires a more structured approach to advance Critical Zone Observatories. Although these will not our understanding of Continental Water Dynamics in themselves constitute a hydrologic observatory than we have at present. New observations, informat- program, they will provide valuable community data ics, instrumentation, and synthesis will be combined for synthesis and planning, as well as hypothesis test- in a three-step approach, beginning with benchmark- ing. Second, it will initiate benchmarking of these ing current understanding, followed by cross-site prototype observatories, along with long-term other comparison and culminating in long-term studies. long-term facilities with a hydrologic component (e.g. Phase 1. Benchmarking Current Understanding USGS, USFS, and ARS experimental watersheds). and Prototyping New Observational Systems Informatics. The CUAHSI Hydrologic Informa- (2008-2012) tion System promises to streamline access to data To achieve community goals, we must both en- from multiple government and academic provid- gage in synthesis activities and continue to progress ers. This effort to date has focused on simply time- on observational and informatics goals. CUAHSI series data collected at a point. Assembling data strongly supports continuation of the development from just a few agencies has shown the breadth of and testing of prototype observatories, both as monitoring activities that contribute to our under- stand-alone facilities and through partnerships with standing of continental water dynamics (Figure 3). other observatory initiatives within NSF. By way Although this map includes a subset of points of synthesis, CUAHSI envisions a series of work- where measurements are made, and doesn’t in- shops, organized by science goal and subdiscipline clude dynamic fields, such as precipitation , within hydrologic science that presents the state of a picture emerges of the virtual earth in which the science and prepares quantitative benchmarks data from multiple sources can be displayed in a that can be used to measure progress. Accomplish- common geographical framework and be read- ing this effort will require informatics support. ily accessible for analysis, as shown in Figure 1. Observations. Considerable effort is underway The current HIS project is prototyping the tech- across the hydrologic community in the U.S. and nology for federating multiple sources of point elsewhere to build both virtual and prototype real data; this effort will be expanded to consider geo- observatories that will provide the short- and long- graphic coverages, and plan for dynamic fields, such term measurements that are absolutely critical for as precipitation radar and remotely sensed data. advances in the field. This building of on-the- The CUAHSI HIS team is also committed to serve data ground observatories, which have ground-based in- specific to the virtual and prototype hydrologic obser- struments linked with satellite and other remotely vatories. In many ways this is a greater challenge than sensed data, is beginning to occur despite the lack the efforts to streamline access to data from govern- of an organized hydrologic observatory program. ment agencies. Support for observatory science will Hydrologists are actively involved in planning for a involve managing data at multiple levels, from raw sen- WATERS observatory initiative, and have contrib- sor data through processed products. During Phase I uted to planning the NEON observational network. the CUAHSI HIS team will build prototypes systems During this phase, CUAHSI will engage in two main that can be scaled to serve a larger observatory effort. activities. First, it will continue to support develop-

- 17 - Figure 3. Map showing federation of surface water and groundwater monitoring locations (USGS National Water Information System, small blue dots), climate monitoring stations (NWS-Automated Surface Observing Stations, small magenta dots; and NWS-Climate Reference Stations, large orange dots), and carbon/ sites (Ameriflux, large purple dots).

The current HIS project covers only a portion of the a context for many other environmental disciplines. informatics needs of hydrologic science. The virtual Assembling the data is one part of informatics needs. earth, mentioned above, can be expanded into three A community modeling framework must also be de- dimensions, as shown in Figure 4. Development and veloped to enable better communication among sci- use of 3-dimensional “geovolumes” and “hydrovol- entists and to improve the efficiency of this effort. umes” will bring together hydrologic, atmospheric, and Many modeling environments have been developed by solid-. Sufficient progress has been made other disciplines, such as Kepler (http://kepler-proj- on three-dimensional geographic information system ect.org), D2K (http://alg.ncsa.uiuc.edu/do/tools/ structures to bring this technology to bear on our data. d2k) and NCSA’s CyberIntegrator (http://isda.ncsa. Activity #1. Development of three-dimensional uiuc.edu/ecid/ECID_cyberintegrator.htm) that may dynamic virtual earth. This activity will require contribute to this effort. One “workflow” technol- workshops to scope pilot activities. Many of the en- ogy developed specifically for hydrologic models by abling technologies are in place and have been evalu- the European Community is OpenMI (http://www. ated. The creation of virtual earth will permit synthe- openmi.org), whose conceptual structure is consis- sis among surface-earth disciplines as well as provide tent with the data structure adopted by CUAHSI HIS.

Figure 4. HIS can be expanded to consider three-dimensional data as well as two- dimensional data through geovolumes and hydrovolumes. - 18 - Activity #2. Development of Community Mod- Phase 2. Testing Generality of Understanding eling Framework. A workshop is required to (2008-2017) review potential technologies for establishing a With benchmarks already in place for many sites, hy- community modeling framework and to scope a potheses can be developed to test understanding along pilot project to explore the potential of such a gradients represented by multiple sites. The informat- framework for the hydrologic science community. ics needs for this phase have largely been defined by ac- tivities in Phase 1. However, additional measurements Activity #3. Simulation models of the integrat- and measurement systems will be needed to make ed hydrologic cycle. A workshop is required to meaningful intersite comparisons to test substantive explore alternate approaches to a comprehensive hypotheses. In some cases, sufficient data exist now model of the hydrologic cycle that would handle or will be developed during Phase 1 to move directly the vastly different dynamics between the atmo- to intersite comparisons. These will be presented first sphere, and land subsurface and surface environ- as example synthesis activities. A solicitation for in- ments. This workshop could form the basis for a tersite comparisons should be developed based upon solicitation for development of multiple comprehen- information gained in Phase 1 to generate proposals sive models that could take place over 2 to 3 years. for intersite comparisons. Meaningful comparisons Synthesis. The informatics activities described must control for obvious differences in climate, soil, above can support a number of observational and and geology to yield scientifically interesting results. synthesis activities that will lead to benchmark de- Observations. This phase will feature intensive velopment organized around the community sci- measurement campaigns of limited duration to ence goals. For Linking the Hydrosphere and Bio- test hypotheses. Such campaigns are common in sphere, an initial workshop would scope potential the atmospheric sciences community, but rare in synthesis activities for interdisciplinary teams of hydrologic science, in part, because an organizing scientists to pursue that could form the basis for a entity such as CUAHSI was only recently found- program solicitation for synthesis activities. For the ed. Campaigns will build on and compliment the Scaling goal, workshops would be organized around prototype observatory activities. Candidates for a range of scales (e.g. plot to catchment, or catch- place-based “observatories” will be evaluated, ment to watershed) to determine the state of the first by reconstruction using digital watersheds, science and the most effective benchmarks. For the and then by a limited duration planned campaign. Prediction goal, workshops would be organized to Synthesis. One example of a topic ready for inter- assess the ability of the current generation of mod- site comparison is the Slope InterComparison Ex- els to predict system response to climate and human periment (SLICE), a working group under the Inter- perturbations. Again, progress can be made most national Association of Hydrologic Science’s PUB effectively by a targeted solicitation for this task. initiative, where groups from around the world who CUAHSI will also expand its efforts to assess have instrumented trenched hillslopes are working how both historical and future data and infor- at comparing results. These studies began with a mation from its sister observational activities single design, originating in Japan, that was imple- noted above can compliment that from hydro- mented elsewhere so that comparable data have logic observatory efforts. This will be an espe- been collected. Other examples of studies ready for cial challenge given the heterogeneous proto- cross-site comparison include Evaporation-Tran- cols, data systems and missions of these facilities. spiration-Recharge- arrays that have been Instrumentation. CUAHSI will work to de- deployed at many sites around the country. (See velop a sustainable model for its hydrolog- sidebar). As with SLICE, ETRS arrays are collecting ic measurement facility programs, follow- comparable data around a uniform modeling con- ing the plans now in place. At a minimum, cept, and, hence, the results are ready for synthesis. efforts will be made to develop a prototype facility

- 19 - ETRS Arrays. The atmosphere-land-surface-subsurface components of the hydrologic cycle represent a complex system of forcings and feedbacks across time scales which range from catastrophic events (flood and drought) to seasonal, interannual and longer time scales of land use and climate change. Understanding the dynamic couplings of soil, ground and surface waters will provide new information and understanding for the efficient development, management and prediction of the nation’s surface and groundwater resources.

EvapoTranspiration Recharge Snow (ETRS) arrays are a new generation of sensor platforms that will al- low nested observations of -to-boundary layer at the plot, stream-reach, and hillslope scales or- ganized by landscape control volumes within the watershed unit. The deployment of ETRS arrays in an integrated 3-D domain is illustrated in the first figure. Nesting these observations extends plot scale the- ory to watershed scale estimation of water, energy and solute balances. ETRS arrays employ turbulence formulation for atmospheric boundary layer processes and Richards/Darcy formulation for subsurface flow. These new generation platforms will provide ground validation for remote sensing land- use/land cover data sets as well as provide the basic landscape volumes to close water, energy and mass balances.

Measurement of water, energy and mass flux includes direct estimation of (K)at the plot scale and as a gradient across the hillslope through a nested array, finite element approach. Co- herent data to inform the approach is necessary to achieve reliable estimation of flux across the land- scape control volume. A viable system of observations will also emphasize the leveraging of in-place in- strumentation and access at existing reference sites of federal, state, local and academic institutions. Contributed by Kevin Dressler and Chris Duffy, PSU

- 20 - Instrumentation. Historically, intersite compari- both pair-wise and broader network comparisons. sons to test hypotheses have been complicated by Efforts, such as Critical Zone Observatories and the lack of comparable data. Access to instrumen- test-beds for the proposed WATERS Network, will tation through the CUAHSI Hydrologic Measure- contribute to our ability to operate these facilities ment Facility nodes and through the USGS Hy- in the community interest. The informatics, in- drologic Instrumentation Facility (which has been strumentation, and synthesis infrastructure devel- obtained by CUAHSI for its member universities) oped during the first two phases will all be need- will be critical in developing comparable data sets. ed for the operation of long-term observatories. Phase 3. Understanding Long-term Response It is recognized that the timeline for implemen- (2012-) tation of this phase will depend on the avail- With results from intersite comparisons, we will have ability of support through NSF’s Major Re- gained a more systematic understanding of the factors search Equipment and Facilities Construction controlling hydrologic response to human and cli- (MREFC) program. It is assumed that the ear- mate perturbations. The primary motivation for long- liest date for access to that program is late 2012. term fixed sites is the development of coherent data Synthesis. It is expected that a permanent synthesis sets through a range of environmental conditions. center will be established under this phase, to continue Observations. A national network of hydrologic the community synthesis activities in phases I and II. observatories, on the order of 10,000 to 50,000 Informatics. Major investments in informat- km2 in size, will be established based on Phase 1 ics will be needed to scale the prototype sys- and Phase 2 activities. These will complement and tems developed under Phases I and II. It is in some cases incorporate existing placed-based estimated that perhaps half of any MREFC proj- facilities of sister sciences, like ecology’s Long ect would be dedicated to cyberinfrastructure. Term Ecological Research and operational agency Instrumentation. It is expected that a permanent experimental watersheds of the U.S. Geological hydrologic measurement facility will be established Survey, Agricultural Research Service, and For- under this phase, to continue support of community est Service. Observatories will form continental- and individual investigator measurement campaigns. scale gradients, and support research involving

- 21 - 4. Education and Outreach K-12. CUAHSI is developing hydrologic literacy tive Management, while the University of Maryland standards (see sidebar) and plans to use it will fos- Baltimore County has a new program in Urban Water. ter K-12 learning in part through existing relation- Outreach to scientific community and the public. ships among its member institutions. For example, a CUAHSI will encourage and seek partnerships to dis- member institution or watershed-based group would seminate hydrologic science information to citizens. use data and research both to populate instruction- Currently, CUAHSI has developed an educational al modules, focusing on hydrologic science, and to video about watershed-based science and the state of educate instructors regarding activities and data. In- hydrologic science in general. Showcased at the 2006 strumentation from the HMF would be used to de- Fall AGU in San Francisco, scientists of all disciplines velop state-of-the-art examples of data collection, were able to view and respond to the piece. Such a processing and hypothesis testing to give students video is important for public knowledge at public lo- actual experience in doing science. Students would cations such as museum kiosks, university and mission have access to real-time data, research reports and agency websites, and in state and national parks. Re- field sites (whenever possible) to participate in ongo- lationships with news outlets, such as National Public ing projects as they happen. Using CUAHS data and Radio, in the form of public service announcements tools, students will be able to communicate their ideas or interviews of hydrologic science researchers and back to the scientists and the general public through representatives of operational agencies would reach a CUAHSI supported web portal and media outlets. diverse audience. CUAHSI will also support existing Higher Education. The CUAHSI Synthesis Proj- outlets such as Southwest Hydrology magazine, pro- ect will be a unifying connection that facilitates and duced by SAHRA ( of semi-Arid Hy- coordinates interdisciplinary and inter-institutional drology and Riparian Areas), the NSF-funded science exchanges of knowledge, and leads to innovative new and technology center at the University of Arizona. modes of leaning for hydrologic science. CUAHSI Hydrologic observing system information, researcher will work through existing self-formed academic profiles, data archive sites and others will be shared groups with the mission to improve the predictive through this and other publications, for the benefit understanding of Continental Water Dynamics so of the science community and the general public. that society can make better choices concerning local, Outreach to the decision makers. CUAHSI will regional, and even global water-related issues. Many also support knowledge transfer to decision-making of these groups exist and have forged working rela- agencies including operational facilities (e.g. Regional tionships with federal, state and local agencies. These River Forecast Centers and reservoir operators, etc) are leading to information and resource sharing, and and government activities such as forest land man- to other sources of financial support for research agement. Transfer will occur in two parts. First, the and related infrastructure. University courses will also cyberinfrastructure will support an open modeling utilize products from the hydrologic observing sys- system that includes the development of decision- tem. An example is a joint internet course, developed support tools and real-time module testing (see Box between University of Texas Austin and Utah State on Computer Aided Dispute Resolution). The de- University, on Geographic Information Science for velopment and testing will be done with user group water resources that uses existing CUAHSI cyberin- input and in support of individual user needs. Sec- frastructure (HIS data services). CUAHSI Member ond, CUAHSI technical staff and those associated Universities are currently supporting integrated hy- with HIS will perform workshops to train manage- drologic science education through NSF’s program ment on use of evolving tools. Explicit linkages to on Integrated Graduate Education and Research decision-making groups will foster a necessary ability Training (IGERT). The University of Florida has a to adapt to and manage Continental Water Dynamics. program on Water, Wetlands, and Watersheds: Adap-

- 22 - Principles of Hydrologic Literacy A water-literate citizen will have a broad understanding of these overriding principles: • Water is crucially important in creating all the familiar surface characteristics that make our “Blue Planet” unique in the . • Water exists in discrete reservoirs that are connected to one another; water is continually exchanged among these reservoirs through the hydrologic cycle. • Water is essential to life on Earth. A water-literate citizen is able to make informed and responsible contributions to public policy formula- tion related to water availability and quality. Water literacy requires understanding of the following specific principles: 1. Water exists in three states: Solid, liquid and gas. 2. Water in its different states is found in nearly every place on Earth 3. Water moves among the atmosphere, ocean, rock layers and living things in a continuous cycle 4. Water is essential to life on Earth 5. A watershed is the unit of land that drains a single contiguous area 6. Water shapes the land around us and in turn the land shapes the distribution of water 7. Human actions have an impact on water quality and quantity 8. Water in the ocean, atmosphere and on land moderates weather and climate. Computer Aided Dispute Resolution. The watersheds in which we live comprise a complex set of physical and social systems that interact over a range of spatial and temporal scales. These systems are continually evolving in response to changing climatic patterns, land use practices and the increasing intervention of humans. Efficient management of these watersheds defies myopic, piecemeal approaches driven by political whim. Rath- er, planning benefits from the fusion of knowledge and experience widely distributed across physical and social scientists, , resource managers, decision-makers, stakeholders, and the general public. Ideally, an envi- ronment is established that promotes shared learning leading to cooperative and . Success requires a process for inclusive and transparent sharing of ideas complemented by tools to structure, quantify, and visualize the collective understanding and data, providing an informed basis of dialogue and exploration. System dynamics provides a unique mathematical framework for integrating the physical and social processes important to watershed management, and for providing an interactive environment for engaging the public. System dynamics models are predi- cated on the classical formalisms of physical and social science; albeit, at reduced spatial and temporal res- olution. This tradeoff in complex- ity and thus computational burden, allows real-time analysis over an ex- tended decision space. As with any model, their ultimate value depends on the appropriate synthesis and in- tegration of data, science, and scale. Of particular challenge to these systems models is the coupling and interaction that occurs at the inter- section between disparate disciplines and scales of analysis. Exploration of Continental Water Dynamics provides a unique basis for understanding science at the “frontiers”. Alternatively, such system models provide the vehicle for informed dialogue and shared learning across the diversity of disciplines engaged in Continental Water Dynamics. Finally, broadly vetted sys- tems models provide a unique basis for communicating Continental Water Dynamics to the public and for placing it in a meaningful context for decision-makers. Contributed by Vincent Tidwell, Sandia National Laboratories. - 23 - 5. Linkages and Partnerships Continental Water Dynamics integrates the foundly affected by the space-time dynamics of water oceans, land and atmosphere, as well as link- and transported constituents. Terrestrial and riparian ing humans and other biota to their environ- vegetation respond to the space-time variation of soil ment. As such, advancements in hydrologic sci- and atmospheric moisture, but they also play a criti- ence benefit other disciplines and hydrologic cal function in hydrologic cycle dynamics, control- science requires other disciplines to make progress. ling the temporal and spatial variations of soil mois- ture, evapotranspiration, recharge, runoff, and even Water and the Critical Zone. Water is both a trans- groundwater. Persistent patchiness and hot spots of port vector for solutes and sediments, as well as a sol- aquatic life and biogeochemical processes suggest an vent. Because continental water dynamics concerns important but unexplained role for fluid flow, in some the fluxes, stores and residence times of water and cases apparently due to episodic turbulent events, transported constituents in soil, streams, lakes and while in other cases gradients are preserved despite aquifers, this topic provides the foundation for hydro- observed turbulence. Water’s role in ecology also in- logic science. Water and the Critical Zone also deals cludes the dynamic interaction of vegetation, aquatic with the dynamic interaction of the hydrologic cycle life, and water with other physical and chemical as- with other physical and chemical aspects of the envi- pects of the environment, particularly the landscape ronment, particularly the climate, the landscape, and and climate. Vegetation utilizes nutrients and modifies even geology. Continental Water Dynamics is driven soil, affecting the nutrient cycle and soil weathering, by larger scale climate signals; through its influence and shaping the landscape from hillslope to basin and on land surface processes and latent heat fluxes, and continental scales. Vegetation also affects the land- delivery of to the oceans, it in turn influ- surface energy balance, influences the atmospheric ences climate. Continental Water Dynamics responds boundary layer, recycles atmospheric moisture, and to the shape of the landscape and stream network sequesters carbon. Small lakes and wetlands also store pattern, which essentially transforms precipitation carbon dioxide, that can be released to the atmosphere into recharge, runoff, other fluxes, and stores; yet the when atmospheric or water dynamics change, with hydrologic cycle is a major geomorphological shaper implications for climate impacts if the release occurs of that landscape. Continental Water Dynamics is on a regional scale. CUAHSI’s main science partners particularly sensitive to geology, indirectly through in addressing these issues are ecology and limnolo- the effects of geology on landscape and climate (e.g., gy. Agency partners include U.S. Geological Survey, due to mountain building and related mountain rain NOAA, NASA, and the Department of Agriculture. shadows), and directly through the effects of subsur- face geological heterogeneity on groundwater and re- Water and Humans. There is a highly dynamic re- lated chemical movement; geology is sensitive to the lationship of water and people. Ten-thousand years hydrologic cycle, for example due to glaciation or, on ago water played a central role in the development much shorter time scales, due to the development of of agriculture and the explosion of human civiliza- caves and karst. CUAHSI’s main science partners in tion, and now it is essential to the sustainability of addressing these issues are atmospheric and ocean a modern industrialized America. As recent disasters sciences, and other Earth-surface sciences, includ- remind us, water is not always beneficial to civiliza- ing, geomorphology, weathering and , and tion; it is often the most destructive vector of natu- abiotic aspects of . Agency partners in- ral disasters. It’s also not a one-way transaction. How clude the U.S. Geological Survey, NOAA, and NASA. water circulates, where it is stored, and what quality it takes are continuously stressed and engineered by Water and Ecology. Water plays a fundamental people. CUAHSI’s main science partner in address- role in sustaining plant and animal life in terrestrial, ing these issues is the engineering community, with riparian, and aquatic environments, and in turn it whom CUAHSI is designing the proposed WA- also deals with their influences on Continental Water TERS network. Agency partners include the En- Dynamics. Ecosystem function and health are pro- vironmental Protection Agency, the Agriculture

- 24 - Research Service, the Forest Service, and NOAA. ics. All share similar attributes, including sensitivity to episodic and rare events, significance of spatial pat- Partnerships CUAHSI terns, heterogeneity, and gradients in the landscape, will strengthen links to other sciences by working importance of scaling leading to thresholds and closely with other NSF-sponsored and complemen- emergent behavior, and predictive uncertainty and tary environmental observing-system initiatives, such limits of predictability. With these shared attributes, as ecology’s NEON, and geoscience’s other Earth- hydrologic and other environmental observing- surface science initiatives in geomorphology and system initiatives can develop and implement joint- weathering science. Like the hydrologic observing infrastructure components, while coherently pursu- system, each of these other environmental observ- ing separate but complementary science objectives. ing systems integrates across disciplines, promotes Working together yields not only financial and scien- important new measurements, deploys modern sen- tific economies of scale, but also connects communi- sor technology, and employs cyberinfrastructure to ties intellectually and operationally so that together synthesize data and models. Water plays an impor- they can address even more complex scientific issues tant role in all of these other environments, each of than the individual communities have yet imagined. which also feedbacks to Continental Water Dynam-

- 25 - 6. Why Now? There are three arguments for why now related research. For example, enhancing the ability is the time for hydrologic science to pur- to “predict changes in freshwater resources and the sue a community-based observing strategy. environment caused by floods, droughts, sedimenta- The community is ready. Hydrologic science first tion and contamination in the context of growing organized as an interdisciplinary effort only fifteen water demand” was identified as one of four grand years ago. Since then the community has grown challenges in environmental sciences by the NRC enormously in size and the science made remark- (2001). The USGCRP’s Hydrologic cycle Study able advances along several narrow fronts, but prog- Group’s Plan for a New Science Initiative on the ress toward the original interdisciplinary vision was Global Hydrologic cycle (2001) asserts, “despite im- slow and fragmented. Progress was also out of bal- pressive advances in the models of the Earth system, ance with that in sister sciences, especially ecological, processes central to the global hydrologic cycle are atmospheric and ocean sciences, who find connec- still poorly understood,” and, “clearly, data must be tions to Continental Water Dynamics to be increas- developed to support better management decisions ingly important for understanding their portions of and improve model predictions of hydrologic cycle the Earth system. The hydrologic-science commu- variations”. More recently, NSF’s Advisory Com- nity has matured and is ready to become truly inter- mittee on Environmental Research and Education disciplinary. Hydrologic science is now challenging (2005) called for integrated, multidisciplinary, and traditional boundaries within its own research and multi-scale water-related research to meet the chal- reaching out to these sister sciences, as evidenced by lenge of ensuring “an adequate quantity and quality the growth of new fields such as ecohydrology and of freshwater for sustaining all forms of life” in the . Building on the legacy of past presence of “continued human population growth successes, hydrologic science is ready to broaden re- and the uncertain impacts of environmental change.” search agendas, coordinate observations across dis- The technology is ready. The advance of com- ciplines and locations, employ coherent protocols, putational, data analysis and visualization tools (in- provide strategies for integration of data and models, cluding GIS, object-oriented modeling approaches develop consistent theory and language, and imple- and the vast increase in computing power), coupled ment a thriving interdisciplinary knowledge transfer. with new sensors and data acquisition technology, The science is ready and the science is needed. enables the collection, processing, and modeling Hydrologic science has evolved to the point where its of data that was inconceivable as little as five years disparate parts can now be brought together to focus ago. However, the prospect of an individual scien- on specific questions, places and times, to improve tist mastering the broad range of disciplines and their predictive understanding. In this context water is the allied data collection and interpretation techniques unifying link within environmental sciences and be- is daunting. A more practical and productive ap- tween them and the social and behavioral sciences. proach to addressing these opportunities is the de- There are unpredicted and sometimes abrupt chang- velopment of common programs and infrastructure es to continental water-related systems, coupled to to be shared by the hydrologic science community. changes in climate and landscapes, ecological systems, The future. The intellectual depth and rigor of and populations, and that past approaches to predic- hydrologic science, as well as its breadth, will grow tions and problem solving are no longer adequate. with the shift from research limited to individu- Both reductionist and emerging behavior approaches als and small collaborations, to community based to hydrologic science offer insights and predictive abil- and supported interdisciplinary efforts. 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