Hydrology and Earth System Sciences

Major sources of groundwater recharge include rainfall, snow melt, and infiltration from surface water features. The rate and generic behavior of recharge lie at the interface of understanding between hillslope, vadose zone, groundwater, and ecohydrology. Catchment hydrologists may view 'subsurface runoff' as a simple loss term, while groundwater modelers may approximate 'recharge' merely as a fraction of rainfall. As groundwater recharge is one of the main drivers of the hydrological system, appropriate quantification is required for robust model predictions. Accurate measurement of recharge is merely impossible, however, which makes it difficult to assess the accuracy of recharge estimates, and as a resultant, the accuracy of model predictions. The objective of this special issue of HESS is to collect the most recent scientific work on Groundwater Recharge. Relevant issues include:

• Major processes influencing recharge, such as climate, soil, vegetation, plant/soil interactions, snow melt, and land-scape features.
• New and improved methods to measure recharge directly.
• Recharge signature in measured head fluctuations, isotope data, contaminants and nutrients, and other field measurements - Improved interpretation of surface measurements to constrain recharge estimates.
• The role of the unsaturated zone: heterogeneity, preferential flow paths, root water uptake.
• Conceptual modeling of the recharge process including alternatives to Richards' equation.
• Different sources of recharge: rain, snow, leaking pipes and canals, ephemeral streams, etc.
• Water planning considerations of recharge.
• Sustainable yield.

Changes in natural (e.g. climate change) and anthropogenic (e.g. land use) forcings will undoubtedly affect hydrological cycles and water availability at all scales. The mechanism and consequence of changes in hydrological processes on ecosystems and societies are still not well understood. These uncertainties hinder our ability to develop effective adaptation strategies to minimize the adverse effects of hydrological alternations on natural and human-dominated landscapes. Ecohydrological science has advanced rapidly in the past few decades in response to many of the immediate needs of solving modern environmental and resource issues, especially water and food shortages. We foresee that ecohydrologists will be increasingly called upon to address questions regarding large-scale vegetation water use and water security issues in the future. This special issue provides an opportunity for international experts in ecohydrology and global change hydrology to share recent advances in understanding the interactions between climate, water, carbon sequestration, biogeochemistry, and land management practices such as reforestation, ecological restoration, and bioenergy development. This special issue will address the dynamic interactions among climate, hydrology, vegetation, soil, and anthropogenic activities at watershed to regional sales. In particular, it will focus on the following three aspects of recent advances in ecohydrological science:

1. New understanding of the consequences of anthropogenic activities (e.g., deforestation, water management) and climate change on water cycles, water quality and biogeochemical dynamics under various geographical and socioeconomical conditions.
2. The advances in new technology applications in ecohydrological research such as integrated simulation models, remote sensing, GIS, isotopes, eddy flux techniques.
3. Case studies on the applications of ecohydrological principles in mitigating impacts of human disturbances and climate change on water resources.

Climate and global change impacts on the hydrological cycle, water resources and ecosystems pose great challenges for water and ecosystem management globally. The projected climate change scenarios clearly calls for development of new and improved integrated tools for the assessment of climate change impacts on the hydrological cycle, and for assessment of groundwater-surface water interaction. Coastal aquifers and ecosystems are currently under pressure globally from over-exploitation and saltwater intrusion. Population growth, climate change and sea level rise will enhance the pressures and the need for protection and sustainable management of water resources and ecosystems for coastal communities in the future.

This special issue of Hydrology and Earth Systems Sciences originates from the EU Interreg IVB project "CLIWAT" on climate change impact on the hydrological cycle and adaptive water management. The CLIWAT project develops and apply new innovative tools for mapping of the current status of groundwater and simulation of future changes to the hydrological cycle based on different climate scenarios and models. The papers of the special issue will focus on describing and demonstrating the applied methods and indicate where improvement and new innovative solutions are required to establish a full and efficient toolbox for evaluating current and future status and climate change impacts on water resources and ecosystems. The necessary tools fall in three groups:

• Tools for geological, geophysical and geochemical mapping and characterization of the subsurface including the distribution of freshwater and saltwater.
• Tools for assessing climate change impact on the evolution of water resources quantity and quality and ecosystems status (e.g. density dependent groundwater flow models and integrated hydrological models).
• Tools for efficient on-line visualization and dissemination of e.g. established models and climate change scenario simulations.

Teaching hydrology, at undergraduate level, graduate level and in a life-long learning context, has always been a challenge for educators (Nash et al., 1990), and many of the problems still remain (Wagener et al., 2007). Challenging aspects include the heterogeneity of the entities we study and of the students we teach. Students entering hydrology programs come from both engineering and science backgrounds with very different education foci and strengths as well as weaknesses. The educational system that supports the teaching of hydrology must undergo a paradigm shift away from the current practice of imparting a narrow set of basic concepts and a disciplinary set of skills to engineers and scientists with little considerations for the real needs of the area of hydrology, especially when considering the increasing impacts of global environmental change (Wagener et al., 2010). How do we balance the need for hydrology students to have strong disciplinary skills in basic subjects (like maths, physics, soil science) (Kavetski and Clark, 2011), with field and laboratory work (Nash et al., 1990; Kleinhans et al., 2010), while also developing the higher level skills of connecting across disciplines and across places? Given the great complexity of the water problems society faces in a changing world, the teaching of hydrology must adopt a more integrated view of the role of water in the natural and build-environment around us.

These issues call for the teaching of new skill sets, including the ability to read, interpret, and learn from patterns in the landscape; comparative studies to supplement place-based studies; learning through case studies; understanding the time-varying characteristics of hydrological systems, use of space for time substitutions; and modeling of interacting processes such as human-nature interactions and feedbacks. Above all, the new generation of hydrologists must be trained to become both analysts and synthesists. This will inevitably require dissolution of the historical separation between science and engineering in our approach to hydrology education. Teaching methods should be rooted in the scientific and quantitative understanding of hydrologic processes, providing flexible hydrologic problem-solving skills that can evolve if new insights become available, and which can be adapted to provide solutions for new problems and to understand new phenomena. Our hydrology textbooks generally do not contain in-depth treatments of how to predict the hydrologic response after changes in climate, degree in urbanization or land cover have occurred, despite the fact that such predictions will be fundamental for future research and practical hydrological applications. So, how should we teach that, considering that the methods for such prediction are subject to a current scientific debate, and, where is the teaching material coming from?

This special issue aims at addressing these challenges in hydrology education and will include both papers on general issues, such as the hydrological curriculum and professional competences required for the hydrologists of tomorrow, and experiences from concrete teaching approaches. Questions addressed in this special issue on education in hydrology include:

• How do we integrate quantitative and qualitative aspects of hydrology into a holistic approach to hydrology education?
• How does hydrology, and therefore hydrology education, change in a changing world?
• What constitutes a strong hydrology skill set that can evolve to study new phenomena and to solve new problems?
• What are the knowledge gaps we have to fill for teaching hydrology in a changing world?
• How much do hydrologists need to learn about different topics? Where do they have to be specialists and where can they be generalists?
• How do we balance place-based versus studies across gradients, numerical rigor versus lab-field experience, problem solving versus scientific inquiry etc.?
• How should field trips be designed for best learning experiences?
• Which new ideas are available for supporting student learning and understanding of hydrological systems? (lab experiments, exercises, …)
• How can continuing education support the life-long learning of hydrologists?
• What set of skills and competencies do hydrologists need to have to be effective in a changing and increasing complex world?
• How can we support the education of hydrologists in less developed countries, which are most vulnerable to environmental change, but who have the least resources for training and capacity building?

This special issue will feature the latest advances and developments in hydrologic data assimilation (DA) for operational hydrologic forecasting and water resources management as an outcome of the 2.5-day international workshop in Delft, the Netherlands, on November 1-3, 2010. Focusing on data assimilation for operational hydrologic forecasting and water resources management, this special issue will include high-quality papers on various related topics including:

• theoretical and mathematical aspects of hydrologic DA applications;
• objective utilization of new and existing sources of data (in-situ or remotely-sensed, quantitative or qualitative), for hydrologic DA applications;
• modeling and quantification of structural, parametric, observational, and anthropogenic uncertainties in DA applications;
• open-source and community-based tools for hydrologic DA in support of single-valued or ensemble analysis and prediction.

Hydrological research and practice have traditionally been concerned either with predictions of water related hazards such as foods and droughts, or with water resources management. This has motivated us to focus on prediction of integral systems responses – mostly stream flow – using hydrological model structures that represent the process patterns and redistribution of water and mass inside a hydrologcal system, based on parsimonious (and therefore simplified) concepts. This story of ongoing success seems to require a) a certain minimum catchment size so that errors arising from simplified process conceptualisations have the opportunity to aveage out, and b) stationarity of both the climate conditions and of the hydrological system itself. Even when ignoring the challenge of coping with hydrological system change (an increasingly optimistic endeavour these days), we still struggle to provide predictions for systems that exhibit organised complexity at the intermediate scale of 5–200 km2. Following Dooge (1986), these can be characterized as heterogeneous systems that display "some degree" of organisation, which is both too small to be treated with simple second order statistics and too large for the application of a reductionist, deterministic treatment based on the Darcy – Richards paradigm.

This special issue solicits contributions that address the following how, what, and why questions at catchment scales ranging from hillslope to lower mesoscale, thereby helping to link the bottom‐up and top‐down approaches and, in this way, help achieve movement towards unified theories at the catchment scale. Questions of interest include, but are not limited to:

• How to detect and quantify catchment structures, especially in the subsurface?
• How does structure control integral hydrological response at higher scales?
• How to infer model structures in a top‐down manner, building on representations of key landscape (or other) units in a more realistic manner?
• What model structures better allow reproduction of the current bio‐geo‐morphic system architectures (e.g., better reproduction of volumes of surface and subsurface stores and topologies of surface and subsurface flow paths)?
• How do ecological, pedological and geomorphological behaviours control processes and functioning of hydrological systems?
• How to account for the context dependence of process organisation within catchments?
• Why did a system evolve the way it did, in adaptive response to past hydro‐geo‐morphologic and biotic processes, and what can we learn from this to address future prediction challenges?
• What roles do feedbacks between biota and abiotic processes play in controlling structure formation and in stabilizing catchmen functions?