Hydrology is characterized by a wide variety of approaches and techniques used to describe hydrological processes. Can we derive a consistent theory of hydrology for developing parsimonious process descriptions using theories of multi-scaling, complex systems, self-organization, pattern dynamics as well as ecology and eco-hydrology? Such a theory should describe dominant controls on hydrological processes across spatio-temporal scales, hydro-climates and ecosystems.

What degree of accuracy and what precision is possible in the prediction of the response of hydrological systems? How much does this predictability depend on the availability of historical observations of the system response? How does predictability change with location and characteristics of a hydrologic system? These are crucial questions to define the current state of our science. However, we have very limited answers and recent reviews of the predictive capability of operational hydrological forecast showed no significant improvement over the last decade. Is there a theoretical limit to predictability in hydrology and how can we assess whether we have improved? Moderated by Markus Weiler (University of Freiburg).

Hydrology does not yet possess a generally agreed upon classification system for its most significant unit of integration, the catchment. Even only discussing the feasibility of and needs for such a system provide an assessment of the current state of our science. In particular, if we want to link climatic characteristics and structural characteristics of catchment form with functional characteristics of the catchment response. Achieving such a ‘mapping’ would not only provide an organizing structure, but also a predictive tool which would provide a first estimate of how we expect ungauged catchments to behave. In this session we will discuss the current state regarding the issue of catchment classification, ways forward and open questions. Markus Weiler (University of Freiburg) will also give a talk in this session.

Hydro-systems exhibit enormous complexity and heterogeneity, much of which is not easily observable. The complexity is driven by the interactions between water, biogeochemistry, ecology, soils, and human modifications, within the constraints set by the climate and the geologic history of the system. These interactions create complex, non-random patterns and structures in space-time. Current models have difficulty accounting for these interactions and unobserved structural complexity, and perform poorly without extensive calibration data, or under changing conditions. A new approach is needed. A new approach may ask: why do the complex structures exist at all? Taking the view that hydrology evolves along with soils, ecology, geomorphology, and human activities we may ask whether the organization of water pathways has a functional role in the maintenance of the overall system. This functional role may be expressed as an organizing principle – a constraint on possible ways that the water pathways may be organized such that the functional role is met. Examples might be maximizing net carbon profit, maintaining geomorphic form-function relationships, efficient resource use by biota, minimum entropy production etc. Formulating and testing organizing principles will necessarily require the synthesis of knowledge, data and concepts from many disciplines.

The Earth's surface is the ever-changing, dynamic interface between lithosphere, hydrosphere, cryosphere, and atmosphere. Challenges behind surface-dynamic modeling includes the identification and incorporation of features such as self-organization, localization, thresholds, strong linkages across interface boundaries, scale invariance, and the impact of biology and geochemistry on the landscape. Surface-dynamic modeling efforts cover applications as diverse as for: 1) conservation of natural resources; 2) characterization and mitigation of natural hazards; 3) geotechnical support of commercial and infrastructure development; 4) stewardship of the environment; and 5) terrestrial surveillance for global security. A diverse community of experts promoting the modeling of earth surface processes is thus invited to discuss their latest approaches in the prediction, erosion, transport, and deposition of sediment and solutes in landscapes and their sedimentary basins.

James Syvitski (james.syvitski@colorado.edu), Efi Foufoula-Georgiou (efi@umn.edu), Ben Hodges (hodges@mail.utexas.edu)

Recent research has demonstrated the importance of in-stream processing for watershed-scale nutrient export. Continued progress in understanding this important role of river networks will require integration of hydrologic, ecological, and biogeochemical processes into conceptual and quantitative models of stream ecosystems. The goal of this session is to identify the theoretical and methodological constraints to that integration, and to discuss and develop approaches that will facilitate improved understanding of interactions between hydrologic processes and nutrient dynamics at spatial scales ranging of hyporheic microsites to large watersheds. Of particular interest are the use of high-frequency water quantity and water quality measurements to understand biogeochemical processes, the incorporation of more sophisticated hydrologic models into nutrient spiraling approaches, and the use of modeling to scale up field measurements of biogeochemical processes to larger hydrologic units; however, we welcome all submissions that address interactions among biogeochemical, ecological, and hydrologic processes.

Environmental observatories are being discussed around the world. Invited speakers to this session will highlight three activities in Europe: the establishment of a hydrologic observatory in Denmark (Karsten Jensen, University of Copenhagen), the Terrestrial Environmental Observatory (TERENO) of the Helmholtz Center for Environmental Research (Steffen Zacharias, UFZ, Leipzig), and an artificial catchment being developed on a reclaimed strip mine by the Brandenburg University of Technology (Wolfgang Schaaf, TU-Cottbus). Abstracts are solicited on all aspects of observatory design, including test bedding activities from the WATERS Network, the development of Critical Zone Observatories and modeling exercises to inform observatory and network design. Contributions from experimental watersheds, including manipulation experiments, that seek an integrated understanding of hydrologic, biogeochemical, geological, and human engineered processes are also welcome.

The coupling of hydrological, biogeochemical, and ecological processes at catchment to landscape scale results in observable patterns in system structure, the amount and chemistry of stream discharge, and landsurface-atmosphere exchanges of energy and biogeochemicals. All of these patterns result from the close coupling of water, energy, carbon, and nutrient cycling, yet rarely are both the vertical (landsurface- atmosphere) and lateral (hydrological residence time and streamflow) exchanges associated with a particular ecosystem structure studied in concert. This session discusses recent efforts and opportunities to bridge this gap by focusing on how ecosystem structure, specifically vegetation, mediates four-dimensional (X,Y,Z, and time) fluxes of water, carbon, and nutrients in mountain catchments of the western United States.

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The amount and timing of seasonal snow cover in the western US is changing rapidly, presenting a need for multi-scale analyses of how the timing and amount of spring runoff may change. At small to medium scales vegetation die off has occurred and is accelerating, with unknown consequences for the partitioning of snow into sublimation, infiltration, runoff, soil moisture, and evapotranspiration. At larger scales of water resource management the simultaneous changes in climate and and land cover suggest that management models, developed under quais-stationary conditions, will need to be modified in the near future. This session discusses these issues from plot scale to regional water resources, highlighting needs for future integration of research and management tools.

Terrestrial hydrologic processes including infiltration, runoff, evapotranspiration, channel flow and groundwater transport translate precipitation forcing from the atmosphere into storages and fluxes of water that sustain ecosystems, transport material and provide water resources for human activities. Terrestrial processes, in turn, control the feedback of moisture and energy back to the atmosphere, thereby modulating regional climates and the global water and energy cycle. Rapidly changing land use and land cover patterns in conjunction with secular trends in climate are having a significant impact on the controls of energy and water exchanges between the land surface and atmosphere. To address the challenges of the coupled hydroclimate system, a new generation of terrestrial hydrologic and land surface models that account for process complexity is required. Investigators conducting studies in these arenas are invited to present their work, synthesize recent advances and identify emerging issues in multi-scale modeling of coupled land-atmosphere processes.

David Gochis (gochis@rap.ucar.edu), Enrique Vivoni (vivoni@nmt.edu), Venkat Lakshmi (vlakshmi@geol.sc.edu)

There is a large, international community of hydrologic and environmental modelers that have risen to the challenge of developing sophisticated, freely available models for tackling societal and research problems. Unfortunately, there is currently very little coordination between groups and this leads to a lack of interoperability, inefficient use of programmer time, incomplete documentation, inadequate testing and a fair amount of "re-inventing the wheel." These issues, along with success stories from the climate and ocean modeling communities, have given rise to a growing interest in community modeling and in "component-based" or "plug-and-play" approaches to modeling. A community, component-based approach to model development has numerous advantages. In particular, it promotes code re-use, interoperability, transparency, automated testing, rapid development, efficient cooperation between groups, recognition for contributors, clean separation of functionality and economy of scale. It can also help to facilitate a community-wide migration towards high-performance computing and adoption of data-sharing standards. The goal of this session is to share information about existing community models and to foster discussion between modelers on how best to coordinate and share their individual efforts.

Scott Peckham (scott.peckham@colorado.edu), Larry Murdoch (lmurdoc@clemson.edu)

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In spite of tremendous efforts and progress over several decades, our ability to predict solute transport through highly heterogeneous media remains extremely limited. However, recent advances in both characterization technologies and modeling approaches present us with an unprecedented opportunity to break the impasse through carefully designed field experiments. In this special session, we will discuss the current understanding and grand challenges in groundwater transport, examine the state-of-the-art methodologies and technologies for subsurface characterization and modeling, explore the key questions and hypotheses that have emerged from recent field tests, and identify novel field experiments that will allow us to directly test such hypotheses. The conclusions and insights from this special session will help CUAHSI develop ideas for a network of natural observatories for studying groundwater transport under different climatic, hydrologic, and geologic conditions.

Chunmiao Zheng (czheng@ua.edu)