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This study enhances an existing global hydrological model (GHM), Xanthos, by adding a new water management module that distinguishes between the operational characteristics of irrigation, hydropower, and flood control reservoirs. We remapped reservoirs in the Global Reservoir and Dam (GRanD) database to the 0.5 ∘ spatial resolution in Xanthos so that a single lumped reservoir exists per grid cell, which yielded 3790 large reservoirs. We implemented unique operation rules for each reservoir type, based on their primary purposes. In particular, hydropower reservoirs have been treated as flood control reservoirs in previous GHM studies, while here, we determined the operation rules for hydropower reservoirs via optimization that maximizes long-term hydropower production. We conducted global simulations using the enhanced Xanthos and validated monthly streamflow for 91 large river basins, where high-quality observed streamflow data were available. A total of 1878 (296 hydropower, 486 irrigation, and 1096 flood control and others) out of the 3790 reservoirs are located in the 91 basins and are part of our reported results. The Kling–Gupta efficiency (KGE) value (after adding the new water management) is ≥ 0.5 and ≥ 0.0 in 39 and 81 basins, respectively. After adding the new water management module, model performance improved for 75 out of 91 basins and worsened for only 7. To measure the relative difference between explicitly representing hydropower reservoirs and representing hydropower reservoirs as flood control reservoirs (as is commonly done in other GHMs), we use the normalized root mean square error (NRMSE) and the coefficient of determination (R2). Out of the 296 hydropower reservoirs, the NRMSE is > 0.25 (i.e., considering 0.25 to represent a moderate difference) for over 44 % of the 296 reservoirs when comparing both the simulated reservoir releases and storage time series between the two simulations. We suggest that correctly representing hydropower reservoirs in GHMs could have important implications for our understanding and management of freshwater resource challenges at regional-to-global scales. This enhanced global water management modeling framework will allow the analysis of future global reservoir development and management from a coupled human–earth system perspective. [ABSTRACT FROM AUTHOR]

The current 'Deployable Output' approach for assessing water resources system performance in England and Wales is a practical, communicable means for assessing the adequacy of a water supply system and determining the relative benefits of proposed system enhancements. A recognised flaw with this approach is that it fails to characterise the severity of potential supply shortfalls, leading to mischaracterisation of risks and benefits associated with alternative candidate investments. Here, we propose a Monte Carlo procedure that could supplement the existing process by exposing the magnitude (% water demand unserved) and duration (number of days) of supply curtailments under a range of drought scenarios. The method is demonstrated using a realistic, stylised water resources system and a discrete number of infrastructure investments. Results demonstrate that vulnerability assessments can expose previously unidentified risks that might radically alter a planner's estimate of the cost‐effectiveness of a particular investment. [ABSTRACT FROM AUTHOR]

The Monsoon Asia region is home to ten of the world's biggest rivers, supporting the lives of 1.7 billion people who rely on streamflow for water, energy, and food. Yet a synthesized understanding of multicentennial streamflow variability for this region is lacking. To fill this gap, we produce the first large scale streamflow reconstruction over Monsoon Asia (62 stations in 16 countries, 813 years of mean annual flow). In making this reconstruction, we develop a novel, automated, climate‐informed, and dynamic reconstruction framework that is skillful over most of the region. We show that streamflow in Monsoon Asia is spatially coherent, owing to common drivers from the Pacific, Indian, and Atlantic Oceans. We also show how these oceanic teleconnections change over space and time. By characterizing past and present hydroclimatic variability, we provide a platform for assessing the impact of future climatic changes and informing water management decisions. Plain Language Summary: Ten of the world's biggest rivers are located entirely within the Asian Monsoon region. They provide water, energy, and food for 1.7 billion people. To manage these critical resources, we need a better understanding of river discharge—how does it change over a long time? Are there common variation patterns among rivers? To answer these questions, we use information derived from tree rings to reconstruct average annual river discharge history at 62 gauges in 16 Asian countries. Our reconstruction reveals the riparian footprint of megadroughts and large volcanic eruptions over the past eight centuries. We show that simultaneous droughts and pluvials have often occurred at adjacent river basins in the past, because Asian rivers share common influences from the Pacific, Indian, and Atlantic Oceans. We also show how these oceanic teleconnections change over space and time. Our findings can inform big decisions made on water‐dependent infrastructure, thus benefiting the riparian people of the Asian Monsoon region. Key Points: Climate‐informed dynamic streamflow reconstruction is skillful over most of Monsoon AsiaStreamflow in Monsoon Asia is spatially coherentReconstruction reveals spatial and temporal variability in streamflow‐ocean teleconnections [ABSTRACT FROM AUTHOR]

Medium- to long-range forecasts often guide reservoir release decisions to support water management objectives, including mitigating flood and drought risks. While there is a burgeoning field of science targeted at improving forecast products and associated decision support models, data describing how and when forecasts are applied in practice remain undeveloped. This lack of knowledge may prevent hydrological modelers from developing accurate reservoir release schemes for large-scale, distributed hydrology models that are increasingly used to assess the vulnerabilities of large regions to hydrological stress. We address this issue by estimating seasonally varying, regulated inflow forecast horizons used in the operations of more than 300 dams throughout the conterminous United States (CONUS). For each dam, we take actual forward observed inflows (perfect foresight) as a proxy for forecasted flows available to the operator and then identify for each week of the year the forward horizon that best explains the release decisions taken. Resulting "horizon curves" specify for each dam the inferred inflow forecast horizon as a function of the week of the water year. These curves are analyzed for strength of evidence for contribution of medium- to long-range forecasts in decision making. We use random forest classification to estimate that approximately 80 % of large dams and reservoirs in the US (1553±50 out of 1927 dams with at least 10 Mm 3 storage capacity) adopt medium- to long-range inflow forecasts to inform release decisions during at least part of the water year. Long-range forecast horizons (more than 6 weeks ahead) are detected in the operations of reservoirs located in high-elevation regions of the western US, where snowpack information likely guides the release. A simulation exercise conducted on four key western US reservoirs indicates that forecast-informed models of reservoir operations may outperform models that neglect the horizon curve – including during flood and drought conditions. [ABSTRACT FROM AUTHOR]

• First repository of operating policies for all large reservoirs in U.S. • Operating policies are inferred for 595 data-rich reservoirs. • Policies are extrapolated to 1,335 data-scarce reservoirs. • Policies improve accuracy of reservoir simulation over current best practice. • Policies unlock continental scale hydrology for a range of new applications. Large-scale hydrological and water resource models (LHMs) require water storage and release schemes to represent flow regulation by reservoirs. Owing to a lack of observed reservoir operations, state-of-the-art LHMs deploy a generic reservoir scheme that may fail to represent local operating behaviors. Here we introduce a new dataset of bespoke water storage and release policies for 1,930 reservoirs of conterminous United States. The Inferred Storage Targets and Release Functions (ISTARF-CONUS) dataset relies on a new inventory of observed daily reservoir operations (ResOpsUS) to generate reservoir operating rules for 595 data-rich reservoirs. These functions are developed in a standardized form that allows for extrapolation of operating schemes to 1,335 data-scarce reservoirs—leading to the first inventory of empirically derived reservoir operating policies for all large CONUS reservoirs documented in the Global Reservoir and Dams (GRanD) database. Evaluation of the new scheme in daily simulations forced with observed inflow demonstrates substantial and robust improvement for both release and storage relative to the popular Hanasaki method. Performance of the extrapolation approach for data-scarce reservoirs is evaluated with leave-one-out validation and is shown to also offer modest gains on average over Hanasaki. ISTARF-CONUS may be readily adopted in any LHM featuring large reservoirs of the conterminous United States. [ABSTRACT FROM AUTHOR]

The Asian Monsoon region is home to a quarter of the world's population, most of whom relies on rivers for water supply. Water management in this region would benefit from an improved understanding of long-term hydrologic variability, made possible with streamflow reconstruction studies. Yet, few such studies have been conducted here. In this work, we produce the first large-scale streamflow reconstruction over the last four centuries for various gauges in five large river basins (Mekong, Yangtze, Chao Phraya, Ganges and Brahmaputra) using a Linear Dynamical Systems approach and the Monsoon Asia Drought Atlas (MADA) as the paleoclimate proxy. We complement this gauge-based reconstruction by applying the same methodology to the outputs of a global hydrological model (FLO1K), which enables spatial analyses that are otherwise difficult with the limited gauge network. The gridded reconstruction also indicates potential skills for future reconstruction works. Both reconstructions reveal a history of regime shifts with prolonged droughts exceeding the lengths of those found in instrumental records and show the spatial footprints of the Asian megadroughts. Analyses of the dominant modes of variability suggest that relationships between streamflow in Asia and climate drivers (ENSO and IOD) vary significantly through space and time. Overall, the findings presented advance understanding of regional hydrologic variability and can help improve water resource management practice in many countries. [ABSTRACT FROM AUTHOR]