The hydrological response to precipitation at the catchment scale is the result of the interplay between the space-time variability of precipitation, the catchment geomorphological / pedological / ecological characteristics and antecedent hydrological conditions. Therefore, (1) accurate measurement and prediction of the spatial and temporal distribution of precipitation over a catchment and (2) the efficient and appropriate description of the catchment properties are important issues in hydrology. This session focuses on the following aspects of the space-time variability of precipitation:
- Novel techniques for measuring liquid and solid precipitation at hydrologically relevant space and time scales, from in situ measurements to remote sensing techniques, and from ground-based devices to spaceborne platforms.
- Novel approaches to better identify, understand and simulate the dominant microphysical processes at work in liquid and solid precipitation.
- Applications of measured and/or modelled precipitation fields in catchment hydrological models for the purpose of process understanding or predicting hydrological response.

The assessment of precipitation variability and uncertainty is crucial in a variety of applications, such as flood risk forecasting, water resource assessments, evaluation of the hydrological impacts of climate change, determination of design floods, and hydrological modelling in general. Within this framework, this session aims to gather contributions on research, advanced applications, and future needs in the understanding and modelling of precipitation variability, and its sources of uncertainty.
Specifically, contributions focusing on one or more of the following issues are particularly welcome:
- Novel studies aimed at the assessment and representation of different sources of uncertainty versus natural variability of precipitation.
- Methods to account for different accuracy in precipitation time series, e.g. due to change and improvement of observation networks.
- Uncertainty and variability in spatially and temporally heterogeneous multi-source precipitation products.
- Estimation of precipitation variability and uncertainty at ungauged sites.
- Precipitation data assimilation.
- Process conceptualization and modelling approaches at different spatial and temporal scales, including model parameter identification and calibration, and sensitivity analyses to parameterization and scales of process representation.
- Modelling approaches based on ensemble simulations and methods for synthetic representation of precipitation variability and uncertainty.
- Scaling and scale invariance properties of precipitation fields in space and/or in time.
- Physically and statistically based approaches to downscale information from meteorological and climate models to spatial and temporal scales useful for hydrological modelling and applications.

Hydroclimatic conditions and the availability of water resources in space and time constitute important factors for maintaining an adequate food supply, the quality of the environment, and the welfare of inhabitants, in the context of sustainable growth and economic development. This session is designed to explore the impacts of hydroclimatic variability, climate change, and the temporal and spatial availability of water resources on: food production, population health, the quality of the environment, and the welfare of local ecosystems. We particularly welcome submissions on the following topics:

Complex inter-linkages between hydroclimatic conditions, food production, and population health, including: extreme weather events, surface and subsurface water resources, surface temperatures, and their impacts on food security, livelihoods, and water- and food-borne illnesses in urban and rural environments.

Quantitative assessment of surface-water and groundwater resources, and their contribution to agricultural system and ecosystem statuses.

Spatiotemporal modeling of the availability of water resources, flooding, droughts, and climate change, in the context of water quality and usage for food production, agricultural irrigation, and health impacts over a wide range of spatiotemporal scales

Intelligent infrastructure for water usage, irrigation, environmental and ecological health monitoring, such as development of advanced sensors, remote sensing, data collection, and associated modeling approaches.

Modelling tools for organizing integrated solutions for water, precision agriculture, ecosystem health monitoring, and characterization of environmental conditions.

Water re-allocation and treatment for agricultural, environmental, and health related purposes.

Impact assessment of water-related natural disasters, and anthropogenic forcings (e.g. inappropriate agricultural practices, and land usage) on the natural environment; e.g. health impacts from water and air, fragmentation of habitats, etc.

Hydroclimatic variability is an emerging challenge with increasing implications on water resources management, planning, and the mitigation of water-related natural hazards. This variability, along with the continuous development of water demands, and aging water supply system infrastructure make the sustainability of water use a high priority for modern society. In fact, the Global Risk 2015 Report of the World Economic Forum highlights global water crises as being the biggest threat facing the planet over the next decade.

To mitigate the above concerns we need to shed light on hydroclimatic variability and change. Several questions and mysteries are still unresolved regarding natural fluctuations of climate, anthropogenic climate change and associated variability, and changes in water resources. What is a hydroclimatic trend? What is a (long term) cycle? How can we distinguish between a trend and a cycle? Is such discrimination technically useful? How do human activities affect rainfall, hydrological change and water resources availability? How to set priorities and take action to ensure sustainability in light of variability and change?

The objective of this session is to explore hydrological and climatic temporal variability and their connections and feedbacks. More specifically, the session aims to:

1. investigate the hydrological cycle and climatic variability and change, both at regional and global scales;

2. explore the interplay between change and variability and its effect on sustainability of water uses;

3. advance our understanding of the hydrological cycle, benefiting from hydrological records and innovative techniques; and

4. improve the efficiency, simplicity, and accurate characterization of data-driven modeling techniques to quantify the impacts of past, present and future hydroclimatic change on human societies.

This session is sponsored by the International Association of Hydrological Sciences (IAHS) and the World Meteorological Organization – Commission for Hydrology (WMO CHy) and it is also related to the scientific decade 2013–2022 of IAHS, entitled “Panta Rhei - Everything Flows”.

Extreme hydro-meteorological events drive a number of hydrologic and geomorphic hazards, (such as floods, landslides and debris flows) which pose a significant threat to modern societies on a global scale. The continuous increase of population and urban settlements in hazard-prone areas in combination with evidence of changes in extreme weather events lead to a continuous increase of the risk associated with weather-induced hazards. To improve resilience and to design more effective mitigation strategies, we need to better understand the aspects of vulnerability, risk, and triggers that are associated with these hazards.

This session aims to gather contributions dealing with various hydro-meteorological hazards that address the aspects of vulnerability analysis, risk estimation, impact assessment, mitigation policies and communication strategies. Specifically, we aim to collect contributions from the academia, the industry (e.g. insurance) and government agencies (e.g. civil protection) that will help identify the latest developments and ways forward for increasing the resilience of communities at local, regional and national scales, and proposals for improving the interaction between different entities and sciences.

Contributions focusing on, but not limited to, novel developments and findings on the following topics are particularly encouraged:

- Physical and social vulnerability analysis and impact assessment of precipitation-related hazards.
- Advances in the estimation of socioeconomic risk from precipitation-induced hazards.
- Characteristics of hydro-meteorological patterns leading to high-impact events.
- Evidence on the relationship between hydro-meteorological patterns and socio-economic impacts.
- Hazard mitigation procedures.
- Communication strategies for increasing public awareness, preparedness, and self-protective response.
- Impact-based forecast and warning systems.

Keywords: vulnerability analysis, risk estimation, impact assessment, mitigation strategies, precipitation induced hazards, pluvial floods.

This PICO session addresses three sub-topics :

Precipitation variability: from drop scale to lot scale:
The understanding of small scale (sec – drop scale to min -km) spatio-temporal variability of precipitation is essential for larger scale studies, especially in highly heterogeneous areas (mountains, cities). Nevertheless grasping this variability remains an open challenge. An illustration of the range of scales involved is the ratio between the effective sampling areas of point measurement devices (rain gauges and disdrometers) and weather radars, which is greater than 10^7! This session aims at bridging this scale gap and improving the understanding of small scale precipitation variability, both liquid and solid, as well as its hydro-meteorological consequences at larger scales.

Hydroclimatic and hydrometeorologic stochastics: Extremes, scales, probabilities:
The departure of statistical properties of hydrometeorological processes from the classical statistical prototype has been established. This session aims at presenting the latest developments on:
- Coupling stochastic approaches with deterministic hydrometeorological predictions;
- Stochastic-dynamic approaches;
- Variability at climatic scales and its interplay with the ergodicity of space-time probabilities;
- Linking underlying physics and scaling stochastics of hydrometeorological extremes;
- Development of parsimonious representations of probability distributions of hydrometeorological extremes over a wide range of scales and states; as well as their applications in risk analysis and hazard predictions
The session is co-sponsored by the ICSH-IAHS, former STAHY.

The atmospheric water cycle under change: feedbacks, land use, hydrological changes and implications :
Traditionally, hydrologists have always considered precipitation and temperature as input to their models and evaporation as a loss. However, more than half of the evaporation globally comes back as precipitation on land. Anthropogenic pressure through land-use changes (and greenhouse gasses) alter, not only, the local hydrology, but through atmospheric water and energy feedbacks also effect the water cycle in remote locations. This session aims to:
- investigate the remote and local atmospheric feedbacks from human interventions, based on observations and coupled modelling approaches.
- explore the implications of atmospheric feedbacks on the hydrologic cycle for land and water management (ex. changing land cover)

Urban hydrological processes are characterized by high spatial variability and short response times resulting from a high degree of imperviousness. Therefore, urban catchments are especially sensitive to space-time variability of precipitation at small scales. High-resolution precipitation measurements in cities are crucial to properly describe and analyses urban hydrological response. At the same time, urban landscapes pose specific challenges to obtaining representative precipitation and hydrological observations.

This session focuses on high-resolution precipitation and hydrological measurements in cities and on approaches to improve modeling of urban hydrological response, including:

- Novel techniques for high-resolution precipitation measurement in cities and for multi-sensor data merging to improve the representation of urban precipitation fields.
- Novel approaches to hydrological field measurements in cities, including data obtained from citizen observatories.
- Precipitation modeling for urban applications, including convective permitting models and stochastic rainfall generators.
- Novel approaches to modeling urban catchment properties and hydrological response, from physics-based, conceptual and data-driven models to stochastic and statistical conceptualization.
- Applications of measured precipitation fields to urban hydrological models to improve hydrological prediction at different time horizons to ultimately enable improved management of urban drainage systems (including catchment strategy development, flood forecasting and management, real-time control and proactive protection strategies aimed at preventing flooding and pollution).
- Strategies to deal with upcoming challenges, including climate change and rapid urbanization.

Hydro-meteorological extremes such as floods, droughts, storms, or heatwaves often affect large regions therefore causing large damages and costs. Hazard and risk assessments, aiming at reducing the negative consequences of such extreme events, are often performed with a focus on one location despite the spatial nature of extreme events. While spatial extremes receive a lot of attention by the media, little is known about their driving factors and it remains challenging to assess their risk by modelling approaches. Key challenges in advancing our understanding of spatial extremes and in developing new modeling approaches include the definition of multivariate events, the quantification of spatial dependence, the dealing with large dimensions, the introduction of flexible dependence structures, the estimation of their probability of occurrence, the identification of potential drivers for spatial dependence, and linking different spatial scales. This session invites contributions which help to better understand processes governing spatial extremes and/or propose new ways of describing and modeling spatial extremes at different spatial scales.

Target audience: hydrologists, climatologists, statisticians, machine learners, and researchers interested in spatial risk assessments.