* This is a solicited presentation and panel discussion session. Submitted contributions will be considered for a poster session

Hydrological process research and modelling play a key role in the management and sustainability of future water resources. This scientific session aims to explore the intricate interplay between scales of relevance in the context of future water resources management. By bringing together experts from surface hydrology, hydrogeology, eco-hydrology, and socio-hydrology, this session seeks to foster a dialogue on the pertinent scales that shape the availability, distribution, and utilization of future water resources.
The oral part of the session is composed of solicited presentations followed by a panel discussion. We solicit however poster contributions for a vibrating poster session.

Hydrology has significantly changed over the last few decades and is expected to continue evolving in the future due to climatic changes. With the increasing availability of observed streamflow data, remote sensing of evapotranspiration, water storage estimates, and the rapid advancement in global Earth system and land surface models, researchers now have a powerful toolset to understand hydrological changes on both regional and global scales.
However, numerous conflicting results exist between observational-based studies and global modeling results. These disparities highlight significant knowledge gaps in our understanding of hydrological processes in a changing climate. This session provides a valuable opportunity to address hydrological change topics in coordination with efforts from different regions worldwide to synthesize global-scale results.
We invite submissions covering a wide range of topics, including, but not limited to, the following topics:
1. Advanced techniques (ground observations and remote sensing) for more accurate estimation of hydrological components (precipitation, evapotranspiration, streamflow, and water storage changes) at catchment, regional, and global scales.
2. Responses and feedbacks of hydrological components to climate change and anthropogenic activities.
3. Projections of regional and global hydrological changes in the near and distant future.
4. Benchmarking hydrological modeling results using state-of-the-art observations.
5. Hydrological processes in hotspot regions such as the Tibetan Plateau, the Arctic, the Amazon and the regions with heavy irrigations.

Global climate change causes an increasing frequency and intensity of floods and droughts. Both are strongly linked and have the potential to reinforce each other. Floods and droughts cover the entire hydrological spectrum and share many similarities and links between the two types of extremes. Nevertheless, management strategies and technical compensation and mitigation measures are often thought only from one side of the extreme, like flood retention basins releasing the stored flood water within days instead of keeping it in the region. With the significant environmental, social and economic cost associated with such extremes, there is an increasing need for sustainable catchment management strategies that attenuate flow regimes, minimising the risk of flooding and ensuring a sustainable water supply and ecosystem resilience during drought periods.

This session addresses nature-based and technical strategies to mitigate the effects of hydrological extremes on the local water balance.

Nature-based solutions at the catchment scale work with or are inspired by nature to restore the natural functioning of our anthropogenically modified landscapes, providing both greater hydrological resilience to extreme events but critically also a host of additional benefits, particularly for biodiversity, climate, and society. As the popularity of nature-based solutions increases, trans-disciplinary research is required to: (1) determine what constitutes a nature-based solution; (2) maximise the effectiveness of such solutions and how they can be implemented alongside existing water management strategies; and (3) consider the social factors that are inherent in the successful implementation of nature-based solutions, overcoming the conflicts or barriers that may otherwise be associated with their implementation.

Technical solutions like managed aquifer recharge, mainly when applied in drinking water catchments, are often turned off during flooding events due to suspected contamination risks to the aquifer. In contrast, successful management of regional water resources seems to require approaches, tools, and management strategies that combine flood protection and drought prevention techniques, i.e., water retention, treatment, and infiltration in subsurface storage systems (ideally aquifers) for long-term, high-quality uses.

The effects of climate change highlight the importance of developing a resilient design approach for buildings, both in dense urban areas and rural communities. Nature-based solutions (NBSs) can help in this as an adaptation measure, providing multiple benefits at building scale. Increasing the applications of green walls and green-blue roofs can reduce heat stress, improve rainwater and wastewater management and drive the communities towards the concept of circular economy and self-subsistence.

This session aims to share and discuss the most recent advances in NBSs that increase building resilience and sustainability in the urban environment. Therefore, we aim for a session including researchers from different fields such as engineering and architecture, natural sciences such as microclimatology and meteorology, and social/psychological science. We encourage also those involved in policymaking to submit a contribution, to have an integrated approach to buildings development.

Our focus will primarily be on solutions that not only improve routine building management but also make meaningful contributions to the mitigation s of extreme events, like extreme urban heat stress (UHI/heat events) or extreme precipitation events and local flooding. Submissions may include (but not restricted to) contributions on:

- Laboratory, field measurements and numerical modelling studies (like microclimatic or hydrodynamic simulations) on green walls and green-blue roofs and other NBSs for rainwater management, wastewater treatment, thermal control, edible vegetation production, energy production
- Qualitative research like user- or agent-based approaches that investigate the potentials and effects of NBSs for climate change adaptation and improving thermal comfort, and further challenges of the water-energy nexus on this small/building scale.
- Urban areas mapping (e.g. GIS applications) or modelling for buildings urban management (BIM applications)
- Investment and cost return of NBS application to buildings
- Life-Cycle-Assessment (LCA) analysis
- Quantitative analysis on possible sanitary risks innovative wastewater treatment and reuse solutions at local scale
- Buildings retrofitting projects or real-scale applications
- NBS social acceptance

In essence, our session aims to explore the multifaceted aspects of NBSs in the context of building resilience, with particular emphasis on their impact, feasibility, and sustainability.

River deltas historically housed many of the Earth’s important ecosystems. The Anthropocene saw these grand terminals of the fluvial systems taking on a new role as now; they also support human lives while facing many intensifying pressures from natural systems, including floods, droughts, or salinity intrusion, that can heavily affect deltas’ Indigenous freshwater ecosystems while rendering the land inarable or even inhabitable. These negative impacts are exacerbated by human development and climate change induced sea level rise, increasing salinity and ground subsidence. Surface and groundwater resources for both domestic and agricultural purposes over overused, saline intrusion is increasing and land use for agriculture competes with nature and urbanization. How can we effectively meditate these impacts via Mitigation and adaptation? Or can we expect innovative strategies, such as using a water and food systems approach and Nature-based Solutions (NbS), to harvest the benefits of both effectively?
This session provides the opportunity for delta researchers to get updated on recent advancements of research related to adaptation and mitigation strategies in global mega deltas while providing ecosystem services (a.o. food supply) as they are taking on the rising climate hazards in the Anthropocene. We will discuss theoretical assessment studies, actual on-site interventions, innovative solutions and viewpoints of factors that may fuel/hamper the advancement of the delta research discourse. Contributions to addressing the following topics are welcome:
• Case studies reporting on-site observations.
• Theoretical assessments, including modelling of innovative mitigation and/or adaptation.
• Critical reviews of significant studies with clear focuses
• Reports of advancements in science-policy dialogues
• Innovative solution for adaptation and / or mitigation strategies

In an era of climate uncertainty and evolving human influence on natural environments, understanding the dynamics of long-term climatic and hydrologic change has become critical. This session has a focus on real-world case studies and applications, though which we seek to explore the multifaceted implications of climate change on water availability, aquatic environments, and the dynamics of socio-ecological riverine systems.

We invite tangible examples of climate change impact assessments on hydrological and related systems, including resource management, policy and adaptation. We hope to showcase research across diverse geographical regions and varied contexts to facilitate sharing of methods, insights and lessons learned.

Submissions are encouraged across the full spectrum of available techniques, including so-called “bottom-up” approaches to decision making under deep uncertainty. Studies applying novel modelling paradigms, innovative risk assessment frameworks, or characterising multiple (compound) sources of risk are particularly encouraged. By showcasing diversity, we aim to foster a practical understanding of the implications of long-term change, leading to better decision-making for an uncertain future.

The MacGyver session focuses on novel sensors made, or data sources unlocked, by scientists. All geoscientists are invited to present:
- new sensor systems, using technologies in novel or unintended ways,
- new data storage or transmission solutions sending data from the field with LoRa, WIFI, GSM, or any other nifty approach,
- started initiatives (e.g., Open-Sensing.org) that facilitate the creation and sharing of novel sensors, data acquisition and transmission systems.

Connected a sensor to an Arduino or Raspberri Pi? Used the new Lidar in the new iPhone to measure something relevant for hydrology? 3D printed an automated water quality sampler? Or build a Cloud Storage system from Open Source Components? Show it!

New methods in hydrology, plant physiology, seismology, remote sensing, ecology, etc. are all welcome. Bring prototypes and demonstrations to make this the most exciting Poster Only (!) session of the General Assembly.

This session is co-sponsered by MOXXI, the working group on novel observational methods of the IAHS.

Understanding the complex interactions between soil-plant-atmosphere compartments and human activities is critical for ensuring the sustainable management and preservation of ecosystem functions and services. Global climate change and human activities threaten the functions and services of our terrestrial ecosystems. The complexity and holistic nature of the consequences have been difficult to assess so far, as simplified experimental approaches and long-term observations have methodological constraints and often focus on a very limited set of response variables.
Larger and more realistic experimental systems such as in situ lysimeters or ecotrons can supply a wide range of high quality continuous and high-resolution data sets on ecosystem services and functions in the Earths critical zone. Individual facilities and larger networks such as TERENO-SOILCan (lysimeter) or ANAEE’s ecotron experimental infrastructures provide a unique platform for a variety of interdisciplinary research to better understand the dynamic of ecosystems.
The session will focus on ecosystem research based on lysimeters and ecotron experiments, including model application. Additionally, we want to address upscaling approaches from lysimeter to landscape scale or between several types of ecosystem experimental infrastructures (e.g., lab, field, or control environments), uncertainty assessments, representativeness of lysimeter-scale observations, and comparability of water, and greenhouse gases flux to in situ measurements. We welcome contributions that (1) assess and compare terrestrial ecosystems functioning and services, (2) focus on water and solute transport processes, as well as greenhouse gases within the soil-plant-atmosphere continuum, including processes such as non-rainfall water inputs (i.e., dew, fog, soil water vapor adsorption), (4) develop new techniques for analyzing lysimeter and ecotron observations, (5) including ecosystem or hydrological modelling approaches that use in-situ observations from lysimeters or ecotrons.

Ensuring safe water supply for human and environment, and protecting them from water hazards have become more challenging due to intensified impacts of climate change, globalization, and population growth. Hydrological knowledge is needed more than ever to address water security issues. However, scientific knowledge on resilience and water security is fragmented in discipline, people, and place. There is a substantial lack of synthesis and easily digestible scientific messages among hydrologists, across disciplines and from scientists to practitioners, decision-makers and the general public. Hence, there is a need for the hydrological research community to better link local hydrological research with global patterns of the water cycle, and further, to provide science-based water-centric decision support.

Therefore, the International Association of Hydrological Sciences (IAHS) is dedicating the next scientific decade to science for solutions. The short name is HELPING, and stands for Hydrology Engaging Local People IN one Global world. It aims to identify local water problems in holistic/system analyses (i.e., linking local and global scales, disciplines and needs, and connecting the dots), search for solutions, be bold and push boundaries to make an impact and connect people across and within regions (e.g., Global North, Global South, North-North, South-South) and provide synthesis to answer the needs of society for sustainable development, safety and security. The topic and vision of the new decade was an outcome of several on-line interactions and workshops during 2023 using a strategic planning approach, as summarised and documented at https://iahs.info/. To date, some 30 working groups have been suggested by the global hydrological community.

This session invites contributions related to the three major themes of HELPING, which all aim at understanding hydrological diversity and integrating knowledge across scales and regions to overcome the water crisis by:
(1) recognising global and local interactions;
(2) finding holistic solutions for water security;
(3) applying cross-cutting methods for facilitation, e.g. science communication, integrating new technology and fostering local co-creation processes.
In particular, we encourage submissions from early career scientists, suggested working groups, and studies that include transdisciplinary and applied experience for solving environmental and societal challenges related to water.

Many papers have advised on careful consideration of the approaches and methods we choose for our hydrological modelling studies as they potentially affect our modelling results and conclusions. However, there is no common and consistently updated guidance on what good modelling practice is and how it has evolved since e.g. Klemes (1986), Refsgaard & Henriksen (2004) or Jakeman et al. (2006). In recent years several papers have proposed useful practices such as benchmarking (e.g. Seibert et al., 2018), controlled model comparison (e.g. Clark et al., 2011), careful selection of calibration periods (e.g. Motavita et al., 2019) and methods (e.g. Fowler et al., 2018 ), or testing the impact of subjective modelling decisions along the modelling chain (Melsen et al., 2019). However, despite their very justified existence, none of the proposed methods have become quite as common and indispensable as the split sample test (KlemeŠ, 1986) and its generalisation to cross-validation.

This session intends to provide a platform for a visible and ongoing discussion on what ought to be the current standard(s) for an appropriate modelling protocol that considers uncertainty in all its facets and promotes transparency in the quest for robust and reliable results. We aim to bring together, highlight and foster work that develops, applies, or evaluates procedures for a trustworthy modelling workflow or that investigates good modelling practices for particular aspects of the workflow. We invite research that aims to improve the scientific basis of the entire modelling chain and puts good modelling practice in focus again. This might include (but is not limited to) contributions on:

(1) Benchmarking model results
(2) Developing robust calibration and evaluation frameworks
(3) Going beyond common metrics in assessing model performance and realism
(4) Conducting controlled model comparison studies
(5) Developing modelling protocols and/or reproducible workflows
(6) Examples of adopting the FAIR (Findable, Accessible, Interoperable and Reusable) principles in the modelling chain
(7) Investigating subjectivity and documenting choices along the modelling chain and
(8) Uncertainty propagation along the modelling chain
(9) Communicating model results and their uncertainty to end users of model results
(10) Evaluating implications of model limitations and identifying priorities for future model development and data acquisition planning

This session is dedicated to the comprehensive investigation of small-scale transport processes governing the movement of plastics (ranging from nano- to macroplastics) within the aquatic environment. While we aim to place special emphasis on laboratory experiments and modeling approaches, we also welcome presentations employing additional methodologies such as field work, and contributions focused on theoretical concepts.

The presentations will revolve around understanding and characterizing plastic movement, considering influential factors like particle size, shape, density, and environmental conditions such as temperature, salinity, flow velocities, water turbulence and suspended sediment concentrations. Additionally, relevant biological and chemical processes will be taken into account. Key processes to be addressed include sedimentation, resuspension, biofouling, aggregation and fragmentation, along with other interactions between plastics and the environment that may influence the transport and ultimate fate of plastic pollutants.

Beyond the presentation of research findings, this session will also focus on advancements in laboratory and modelling techniques, highlighting improvements in accuracy, complexity, and spatial-temporal resolution. Cutting-edge modelling approaches tailored to simulate the intricate transport dynamics of plastics in aquatic environments will be showcased.

Through engaging discussions, the session aims to enhance our comprehension and predictive capabilities, while also identifying unresolved questions and paving the way for future research endeavors in this vital area of study.

A large proportion of the global stream network ceases to flow periodically. These systems range from near-perennial streams with infrequent, short periods of zero flow to streams that experience flow only episodically after large rainfall events. The onset of streamflow in intermittent streams can affect the quantity and quality of water in downstream perennial rivers. Intermittent streams also support a unique and high biodiversity because they are coupled aquatic-terrestrial systems. However, non-perennial rivers and streams are usually unmonitored and often lack protection and adequate management. There is a clear need to study the hydrology, biogeochemistry and ecology of natural intermittent and ephemeral streams to characterize their flow regimes, to understand the main origins of intermittence and how this affects biogeochemistry and biodiversity, and to assess the consequences of altered flow intermittence due to climate change or other anthropogenic impacts.
This session welcomes all contributions on the science and management of non-perennial streams, and particularly those highlighting:
· current advances and approaches in monitoring and modelling flow intermittence,
· the effects of flow in non-perennial streams on downstream perennial stream water quantity and quality,
· the factors that affect the dynamics of the flowing stream network,
· land use and climate change impacts on flow intermittence,
· links between flow intermittence and biogeochemistry and/or ecology.
· public perceptions (and natural capital/ecosystem services) of non-perennial rivers,
· approaches to determine reference conditions on non-perennial rivers.

Despite only representing about 25% of continental land, mountains are an essential part of the global ecosystem and are recognised to be the source of much of the world’s fresh water supply. A considerable part of the world’s population depends on this water supply, around 26% live directly in the mountains and 40% live downstream of rivers originating in the mountains. The large elevation ranges and the heterogeneity of elevation-dependent hydro-meteorological conditions make mountains particularly sensitive to climate variability and change, but therefore also unique areas for identifying and monitoring the effects of global change.
This session aims to bring together the scientific community doing hydrology research on mountain ranges across the globe to share results and experiences. Therefore, this session invites contributions addressing past, present and future changes in mountain hydrology due to changes in either climate and/or land use, how these changes affect local and downstream territories, and adaptation strategies to ensure the long-term sustainability of mountain ecosystem services, with a special focus on water cycle regulation and water resources generation. Example topics of interest for this session are:
• Sources of information for evaluating past and present hydrological conditions (in either mountain surface and/or ground water systems).
• Methods for differentiating climatic and anthropogenic drivers of hydrological change in the mountains.
• Modelling approaches to assess mountain hydrological change.
• Evolution, forecasting and impacts of extreme events.
• Case studies on adaptation to changing mountain water resources availability.

Water is a strategic issue in drylands, where ecosystems and their inhabitants strongly rely on the scarce and often intermittent water availability or its low quality. The characteristics of drylands increase their vulnerability to climate change and susceptibility to the impact of short- to long-term extreme events and processes, such as floods, droughts, and desertification. These events can reshape the landscape through the mobilisation of surface sediments, deposits of which preserve archives of past Earth system states, including changes in the extent of deserts. Over the last century, anthropogenic modifications of all kinds and intensities have affected surface conditions. In drylands and Mediterranean hydrosystems, agricultural water use is constantly increasing threatening the sustainability of the surface and groundwater reservoirs, and their hydrology is then continuously evolving. Nevertheless, the study of hydroclimatic processes in drylands remains at the periphery of many geoscientific fields. A proper understanding of the hydrological, hydrometeorological and (paleo)climatic processes in these regions is a cornerstone to achieving the proposed sustainable development goals we set for the end of this century.

This session welcomes contributions from scientific disciplines addressing any of the drylands' full range of environmental and water-related processes. The purpose is to foster interdisciplinary research and expand knowledge and methods established in individual subdisciplines. We will address hydrological issues across global drylands, and devote a section of our session to a geographical focus on the Mediterranean region to analyse the changes in hydrologic processes and fluxes unique to that region.

Water stored in the snow pack and in glaciers represents an important component of the hydrological budget in many regions of the world, as well as a sustainment to life during dry seasons. Predicted impacts of climate change in catchments covered by snow or glaciers (including a shift from snow to rain, earlier snowmelt, and a decrease in peak snow accumulation) will reflect both on water resources availability and water uses at multiple scales, with potential implications for energy and food production.

The generation of runoff in catchments that are impacted by snow or ice, profoundly differs from rainfed catchments. And yet, our knowledge of snow/ice accumulation and melt patterns and their impact on runoff is highly uncertain, because of both limited availability and inherently large spatial variability of hydrological and weather data in such areas. This translates into limited process understanding, especially in a warming climate.

This session aims at bringing together those scientists that define themselves to some extent as cold region hydrologists, as large as this field can be. Contributions addressing the following topics are welcome:
- Experimental research on snow-melt & ice-melt runoff processes and potential implementation in hydrological models;
- Development of novel strategies for snowmelt runoff modelling in various (or changing) climatic and land-cover conditions;
- Evaluation of remote-sensing or in-situ snow products and application for snowmelt runoff calibration, data assimilation, streamflow forecasting or snow and ice physical properties quantification;
- Observational and modelling studies that shed new light on hydrological processes in glacier-covered catchments, e.g. impacts of glacier retreat on water resources and water storage dynamics or the application of techniques for tracing water flow paths;
- Studies on cryosphere-influenced mountain hydrology, such as landforms at high elevations and their relationship with streamflow, water balance of snow/ice-dominated mountain regions;
- Studies addressing the impact of climate change on the water cycle of snow and ice affected catchments.

The African continent is experiencing various impacts of climate induced sequential droughts, floods, heatwaves, and alteration between two extremes. These changes are causing water and food insecurity in the region. The advances seen in hydrological models in better reproductions of observed variables such as streamflow and water availability are improving predictions of socio-economic risks of floods, droughts, and water stress. However, in data-sparse regions the use of hydroclimatic models for disaster risk reductions still requires improvement.

This session aims to bring together communities working on different strands of African hydrology, climate risks, water and food security, and environmental risks. We welcome both fundamental and applied research in the areas of hydrological process understanding, monitoring, drought/flood forecasting and mapping, seasonal forecasting, water resources management, climate impact assessment and societal implications. Interdisciplinary studies that combine the physical drivers of water-related risks and their socio-economic impacts in Africa are encouraged. Case studies showcasing practical innovative solutions relevant for decision making under large uncertainty are welcomed.

Forests are primary regulators of water, energy, and carbon cycles. Maintaining forest functional integrity is fundamental to the sustainability of ecosystems, societies, and human development as described in the UN Sustainable Development Goals.
Global change and anthropogenic intervention are putting enormous pressure on forests, affecting the ecosystem services they provide through water quantity and quality, and biogeochemical cycles. The conventional wisdom that forest hydrology emphasizes the role of forests and forest management practices on runoff generation and water quality has expanded in light of rapid global change. Some of the largest pristine forest areas are in the tropics and have undergone drastic changes in land use in recent decades. Although novel modeling and observational techniques have been applied as alternatives to develop cutting-edge research, these tropical systems remain notably underrepresented in hydrological studies compared to temperate regions, especially concerning long-term experimental setups and monitoring networks.
Improving our understanding of how hydrological processes in forest ecosystems are determined by time-invariant factors and time-varying controls, as well as how forested catchments respond to dynamic environmental conditions and disturbances, will depend critically on understanding forest-water interactions. Building this knowledge requires interdisciplinary approaches in combination with new monitoring methods and modeling efforts.
This session brings together studies that will improve our understanding of water-forest interactions and stimulate debate on the impact of global change on hydrological processes in forest ecosystems at different scales.
We invite field experimentalists and modelers working in forests from boreal to tropical regions to submit contributions that:
1) Improve our understanding of forest (eco)hydrological processes using an experimental or modeling approach or a combination of both;
2) Assess the hydrology-related impacts of land use/cover change and environmental disturbances on forested ecosystems;
3) Feature innovative methods and observational techniques, such as optical sensors, tracer-based experiments, monitoring networks, citizen science, and drones, that reveal new insights or data sources in forest hydrology;
4) Include interdisciplinary research that supports consideration of overlooked soil-plant-atmosphere components in hydrological studies.

The Critical Zone (CZ) – the permeable near-surface layer of the Earth where the lithosphere, hydrosphere, atmosphere, and biosphere interact – is the place where cycles of carbon, nutrients, water and other biogeochemical processes intersect with ecosystems and society. Investigating the form and functioning of the CZ requires that insights from geology, hydrology, ecology, geochemistry, atmospheric science and other disciplines are integrated in a transdisciplinary manner. One successful approach to CZ research has been the development of intensively instrumented study areas, known as CZ observatories. Networks of observatories and interlinked thematically-focused projects have evolved to capitalize on advances possible through multifaceted collaborations across larger spatial scales. Processes that shape the critical zone also span wide ranges of temporal scales, from vegetation on seasonal timescales, to soil development and landscape evolution over thousands to millions of years. Because all of these processes together shape the critical zone and affect how it functions, bridging gaps between short term processes and longer-term environmental change is essential for understanding landscapes and maintaining their ability to sustain life.

This session will highlight the cutting edge of CZ science across spatial and administrative scales, from project, to observatory, to network levels. Submissions may also explore coupling across temporal scales, integrating relatively rapid processes with the longer-term evolution of the critical zone. Submissions are solicited that focus on integration of observations and modeling; hydrologic dynamics; geoecological interactions; biogeomorphology, mineral weathering and nutrient cycling; the rhizosphere; the societal relevance of CZ science; and other examples of how CZ research is evolving with new knowledge to face the challenges of our changing world. Contributions from early-career scientists are particularly encouraged.

Stable and radioactive isotopes as well as other natural and artificial tracers are useful tools (i) to fingerprint the sources of water and solutes in catchments, (ii) to trace flow pathways or (iii) to quantify exchanges of water, solutes and particulates between hydrological compartments. We invite contributions that demonstrate novel applications and recent developments of isotope and other tracer techniques in hydrological field studies and modelling in the areas of surface water-groundwater interactions, unsaturated and saturated zone, rainfall-runoff processes, cold-region hydrology, nutrient or contaminant transport, ecohydrology or other catchment processes.

Understanding and representing hydrological processes is the basis for developing and improving hydrological and Earth system models. Relevant hydrological data are becoming globally available at an unprecedented rate, opening new avenues for modelling (model parametrization, evaluation, and application) and process representation. As a result, a variety of models are developed and trained by new quantitative and qualitative data at various temporal and spatial scales.
In this session, we welcome contributions on novel frameworks for model development, evaluation and parametrization across spatio-temporal scales.

Potential contributions could (but are not limited to):
(1) introduce new global and regional data products into the modeling process;
(2) upscale experimental knowledge from smaller to larger scale for better usage in catchment models;
(3) advance seamless modeling of spatial patterns in hydrology and land models using distributed earth observations;
(4) improve model structure by representing often neglected processes in hydrological models such as human impacts, river regulations, irrigation, as well as vegetation dynamics;
(5) provide novel concepts for improving the characterization of internal and external model fluxes and their spatio-temporal dynamics;
(6) introduce new approaches for model calibration and evaluation, especially to improve process representation, and/or to improve model predictions under changing conditions;
(7) develop novel approaches and performance metrics for evaluating and constraining models in space and time

This session is organized as part of the grass-root modelling initiative on "Improving the Theoretical Underpinnings of Hydrologic Models" launched in 2016.

A multitude of processes contribute to the hydrologic functioning of catchments. Traditionally, catchment hydrology has been centered around surface runoff, which is readily observable. But at the same time, invisible below ground processes entailing the storage dynamics and flows of water are still underexplored. This includes subsurface runoff, as well as feedbacks of subsurface processes to the surface and the specific role of soil moisture in shaping these fluxes. This session aims to bring together contributions on the following topics and to address gaps in observations, models, and understanding of hydrologic systems:

- Identifying, tracing, and modeling subsurface runoff generation at the catchment scale.

- Factors and mechanisms controlling subsurface water storage and fluxes

- How soil moisture measurements at different scales can be used to improve process understanding, models, and hydrologic theory

- Interactions of surface and subsurface hydrologic processes

Large data samples of diverse catchments provide insights into the physiographic and hydroclimatic factors that shape hydrological processes. Such data sets enable research on topics such as extreme events, data and model uncertainty, hydrologic model evaluation and prediction in ungauged basins.

This session aims to showcase recent data and model-based efforts on large-sample hydrology that advance the characterization, organisation, understanding and modelling of hydrological diversity.

We specifically welcome abstracts that seek to accelerate progress on the following topics:

1. Development and improvement of large-sample data sets: How can we address current challenges on the unequal geographical representation of catchments, quantification of uncertainty, catchment heterogeneities and human interventions?
2. Catchment similarity and regionalization: Can currently available global datasets be used to define hydrologic similarity? How can information be transferred between catchments and to data-scarce regions?
3. Modelling capabilities: How can we improve process-based and machine learning modelling by using large samples of catchments?
4. Testing of hydrologic theories: How can we use large samples of catchments to test and refine hydrologic theories and asses their general validity?
5. Identification and characterisation of dominant hydrological processes: How can we use catchment descriptors available in large sample datasets to infer dominant controls for relevant hydrological processes?
6. Human impacts and non-stationarity: How can we (systematically) represent human influences in large sample datasets and use them to infer hydrological response under changing environmental conditions?

Water is our planet’s most vital resource, and the primary agent in some of the biggest hazards facing society and nature. Recent extreme heat and flood events underline the significance of water both as a threat and as an increasingly volatile resource.
The accurate and timely measurement of streamflow is therefore more critical than ever to enable the management of water for ecology, for people and industry, for flood risk management and for understanding changes to the hydrological regime. Despite this, effective monitoring networks remain scarce, under-resourced, and often under threat on a global scale. Even where they exist, observational networks are increasingly inadequate when faced with extreme conditions, and lack the precision and spatial coverage to fully represent crucial aspects of the hydrological cycle.

This session aims to tackle this problem by inviting presentations that demonstrate new and improved methods and approaches to streamflow monitoring, including:
1) Innovative methods for measuring/modelling/estimating river stream flows;
2) Real-time acquisition of hydrological variables;
3) Remote sensing and earth observation techniques for hydrological & morphological monitoring;
4) Measurement in extreme conditions associated with the changing climate;
5) Measurement of sudden-onset extreme flows associated with catastrophic events;
6) Strategies to quantify and describe hydro-morphological evolution of rivers;
7) New methods to cope with data-scarce environments;
8) Inter-comparison of innovative & classical models and approaches;
9) Evolution and refinement of existing methods;
10) Guidelines and standards for hydro-morphological streamflow monitoring;
11) Quantification of uncertainties;
12) Development of expert networks to advance methods.

Contributions are welcome with an emphasis on innovation, efficiency, operator safety, and meeting the growing challenges associated with the changing climate, and with natural and anthropogenically driven disasters such as dam failures and flash floods.

Additionally, presentations will be welcomed which explore options for greater collaboration in advancing river flow methods and which link innovative research to operational monitoring.

Anthropogenic activities like agriculture and wastewater discharge have resulted in the degradation of groundwater and surface water quality with severe implications for both human and environmental health. There is an urgent need to mitigate these impacts on water quality crucial for maintaining the ecological, recreational, and industrial functions of water resources.
Water quality is typically monitored and assessed at the catchment scale. Understanding the underlying processes and causal relationships remains challenging due to the complex interplay of hydrologic, biogeochemical, and temporal factors. This large scale of monitoring does not always match the scale of mitigation measures to minimize anthropogenic impact on water quality.
Data-driven statistical analysis of discharge and concentration time series at catchment outlets offers valuable insights into the scaling of processes and the effectiveness of measures. Long-term, high-frequency, and multi-site data sets can inform experimental and modeling studies, enabling us to move from patterns to processes and gain a deeper understanding of solute and particulate mobilization, retention, and export mechanisms.
This session brings together contributions on the assessment of mitigation measures, analysis of solute and particulate export dynamics, and catchment modeling to optimize mitigation placement.

Quantifying and understanding the impacts of environmental change on water quality and availability across space and time is critically important for ensuring that there is enough water of suitable quality to meet human and ecosystem needs now and in the future. Consequently, there is an urgent need for tools such as models, remote sensing, machine-learning and artificial intelligence algorithms that can anticipate these impacts and address the resulting environmental changes. These assessments in turn facilitate more effective water management that safeguards the critical ecosystem goods and services provided by freshwater resources. In addition, some of these tools, within both Bayesian and frequentist paradigms, enable consideration of prediction reliability, relating uncertainties to a decision makers’ attitudes and preferences towards risks, all while accounting for the uncertainty related to our system understanding, data and random processes. We seek contributions that apply modeling and other approaches to:
• investigate climate change impacts on water quality and quantity from local to global scales, including climate impact attribution studies
• quantify and couple supply and demand in support of water management including vulnerability assessment, scenario analysis, indicators, and the water footprint
• project future water supply and demand in the context of a changing climate, land use, population growth, and other potential drivers of change
• quantify the uncertainty of model predictions (due to data, model structure and parameter uncertainty)
• interpret and characterize uncertainties in machine-learning and data mining approaches that learn from large, possibly high-resolution data sets
• address the problem of scaling (e.g. disparity of scales between processes, observations, model resolution and predictions)
• test transferability and generalizability of findings
• assess water quality and quantity in either data-rich or data-sparse environments
• involve stakeholders in model development and maximise the use of expert knowledge to inform risk analysis and decision support, incl. monitoring, reporting and catchment management
• assess robustness in water quality and quantity hotspots

Land use and climate change as well as legal requirements (e.g. the EU Water Framework Directive) pose challenges for the assessment and sustainable management of surface water quality at the catchment scale. Sources and pathways of nutrients and other pollutants as well as nutrient interactions need to be characterized to understand and manage the impacts in river systems. Additionally, water quality assessment needs to cover the chemical and ecological status to link the hydrological view with aquatic ecology.
Models can help to optimize monitoring schemes and provide assessments of future changes and management options. However, insufficient temporal and/or spatial resolution, a short duration of observations and the widespread use of different analytical methods limit the potential for model application. Moreover, model-based water quality calculations are affected by errors in input data, model errors, inappropriate model complexity and insufficient process knowledge or implementation. In addition, models should be capable of representing changing land use and climate conditions to meet the needs of decision makers under uncertain future conditions Given these challenges, there remains a strong need for advances in water quality modeling.

This session aims to bring together scientists working on both experimental and modelling studies to improve the prediction and management of water quality constituents (e.g. nutrients, organic matter, algae, sediment) at the catchment scale. Contributions addressing the following topics are welcome:

- Experimental and modelling studies on the identification of sources, hot spots, pathways and interactions of nutrients and other, related pollutants at the catchment scale
- New approaches to develop effective water quality monitoring schemes
- Innovative monitoring strategies that support both process investigation and improved model performance
- Advanced modelling tools for integrating catchments and/or simulating in-stream processes
- Observational and modelling studies at the catchment scale that relate and quantify water quality changes to changes in land use and climate
- Measurements and modelling of abiotic and biotic interaction and feedback involved in the transport and fate of nutrients and other pollutants at the catchment scale
- Catchment management: pollution reduction measures, stakeholder involvement, scenario analysis for catchment management