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Existing satellite systems primarily observe individual components of the terrestrial water cycle, whereas drought is fundamentally an emergent, temporary process resulting from interactions among all terrestrial water storage compartments and human water use (AghaKouchak et al., 2015; AghaKouchak et al., 2021). Given the complex nature of monitoring drought across several temporal and spatial domains, remote sensing tools enable us to augment our limited in situ networks while also helping us to better detect rapid onset/intensification of drought. This technology frontier is ripe for flash drought detection and mapping (Otkin et al., 2018; Christian et al., 2024). These tools can also help us better support agricultural, environmental and water resource management and decision making. As such, to better understand and advance drought monitoring and mitigation, we recommend prioritizing measurements focused on multi-domain compartment water storage, subsurface moisture, human water use quantification and coupled water-energy budgets. This white paper is an outcome of a community-led, community-engaged effort to identify and elevate emerging opportunities in hydrologic science that can provide significant benefits for decision-making related to drought, floods, water quality, water supply and water demand.

We synthesize community priorities for advancing flood science and flood decision support over the next decade. We argue that useful information for flood monitoring and forecasting require integrated observations and models that quantify where water is, how much is stored, how fast it moves, how watershed conditions prime flood response, and how river, floodplain, and coastal geometry control inundation. We identify four priority science gaps: topo-bathymetry, surface water dynamics, antecedent land-surface conditions, and high-resolution precipitation forcing for flash floods. Progress will require sustaining trusted baseline observations while investing in targeted innovation, model-observation fusion, and decision-ready products developed with operational partners. Together, these advances will improve early warning, emergency response, infrastructure planning, recovery, insurance, zoning, and long-term flood-risk reduction. This white paper is an outcome of a community-led, community-engaged effort to elevate hydrologic science opportunities which would have significant benefit to support decision-making for drought, floods, water quality, water supply and water demand.

Water Supply refers to the freshwater resources generated through precipitation and snowmelt; stored in snowpack, soil moisture, groundwater, rivers, lakes, and reservoirs; lost through evaporation, transpiration, and sublimation; and redirected via human withdrawals. Characterizing the available water supply is critical for water management, agriculture and food security and long-term water resource planning. Many components of water supply are observable from space, however several critical processes are missing. Water stored in seasonal snowpacks is not currently measured from space at spatial and temporal resolutions necessary to understand its effective contribution to usable water supply. Precipitation measurements struggle in complex terrain, especially in capturing extreme events, quantifying snowfall, and partitioning between rain and snow. Total water storage is measured at very coarse resolutions rather than at watershed scales that would inform hydrological processes. And losses, such as through evapotranspiration, sublimation and consumption are not well captured. Future efforts to improve our ability to characterize water supply should focus on building missions to fill specific gaps, maintaining continuity of existing missions and building frameworks to leverage all data in a multi-sensor modeling approach. This white paper is an outcome of a community-led, community-engaged effort to elevate hydrologic science opportunities which would have significant benefit to support decision-making for drought, floods, water quality, water supply and water demand.

Water Demand focuses largely on terrestrial evapotranspiration (ET), including its drivers, uncertainties, feedbacks, and societal applications. ET refers to canopy transpiration, soil evaporation, and terrestrial open water evaporation and also includes ice and snow sublimation. Given the importance of soil water as both a source and a limiting factor for ET, our focus also includes the characterization of soil moisture. Methodologically, we leverage remote sensing while integrating modeling, data assimilation, data science, decision-support, and water cycle science more broadly. Synthesizing input from the community questionnaire, the Water Demand Focus Area identified four overarching gaps (or, conversely, goals) within this topic: 1. Causes and drivers of evaporation and ET variations are not understood; 2. Natural water stores (primarily surface water and groundwater) are not sufficiently monitored to inform basin-level land management; 3. Consumptive use (human use) for water resource management is not known at the field scale; and, 4. Water and energy budget observations are not adequately understood to balance water supply vs. water demand (across scales). This white paper is an outcome of a community-led, community-engaged effort to elevate hydrologic science opportunities which would have significant benefit to support decision-making for drought, floods, water quality, water supply and water demand.

Existing satellite systems primarily observe individual components of the terrestrial water cycle, whereas drought is fundamentally an emergent, temporary process resulting from interactions among all terrestrial water storage compartments and human water use (AghaKouchak et al., 2015; AghaKouchak et al., 2021). Given the complex nature of monitoring drought across several temporal and spatial domains, remote sensing tools enable us to augment our limited in situ networks while also helping us to better detect rapid onset/intensification of drought. This technology frontier is ripe for flash drought detection and mapping (Otkin et al., 2018; Christian et al., 2024). These tools can also help us better support agricultural, environmental and water resource management and decision making. As such, to better understand and advance drought monitoring and mitigation, we recommend prioritizing measurements focused on multi-domain compartment water storage, subsurface moisture, human water use quantification and coupled water-energy budgets. This white paper is an outcome of a community-led, community-engaged effort to identify and elevate emerging opportunities in hydrologic science that can provide significant benefits for decision-making related to drought, floods, water quality, water supply and water demand.

The use of satellite-derived water quality measurements can be used for decision support efforts in the areas of public health (e,g, harmful algae blooms) and environmental monitoring. In addition, satellite information helps support aquatic science efforts in the areas of primary productivity, sediment transport and biogeochemistry. To advance water quality information needs, a multi-satellite constellation providing daily, hyperspectral (or carefully selected ~20+band "superspectral") observations at ≤20-30m spatial resolution to resolve optically complex inland and coastal waters needs to be developed. Secondly, to build trust and confidence in the generated data, implement community-wide validation methods, shared equipment access, and multilingual training to ensure global data equity. Lastly, develop a “water quality suite” of integrated, multisource data sets along with community-based open-source models to support gap free data sets for multiple end user needs. The use of responsible AI may expand the list of constituents generated via satellites into indirect measures such as pathogens, dissolved oxygen, and toxic metals. This white paper is an outcome of a community-led, community-engaged effort to elevate hydrologic science opportunities which would have significant benefit to support decision-making for drought, floods, water quality, water supply and water demand. Water quality is critical for human and ecosystem health.