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Reservoirs play a significant role in modifying the spatiotemporal availability of surface water to meet multi-sector human demands, despite representing a relatively small fraction of the global water budget. Yet the integrated modeling frameworks that explore the interactions among climate, land, energy, water, and socioeconomic systems at a global scale often contain limited representations of water storage dynamics that incorporate feedbacks from other systems. In this study, we implement a representation of water storage in the Global Change Analysis Model (GCAM) to enable exploration of the future role (e.g., expansion) of reservoir water storage globally in meeting demands for, and evolving in response to interactions with, the climate, land, and energy systems. GCAM represents 235 global water basins, operates at 5-year time steps, and uses supply curves to capture economic competition among renewable water (now including reservoirs), non-renewable groundwater, and desalination. Our approach consists of developing the GLObal Reservoir Yield (GLORY) model, which uses a Linear Programming (LP)-based optimization algorithm, and dynamically linking GLORY with GCAM. The new coupled GCAM-GLORY approach improves the representation of reservoir water storage in GCAM in several ways. First, the GLORY model identifies the cost to supply increasing levels of water supply from reservoir storage by considering regional physical and economic factors, such as evolving monthly reservoir inflows and demands, and the levelized cost to construct additional reservoir storage capacity. Second, by passing those costs to GCAM, GLORY enables exploring future regional reservoir expansion pathways and their response to climate and socioeconomic drivers. To guide the model toward reasonable reservoir expansion pathways, GLORY applies a diverse array of feasibility constraints related to protected land, population, water sources, and cropland. Finally, the GLORY-GCAM feedback loop allows evolving water demands from GCAM to inform GLORY, resulting in an updated supply curve at each time step, thus enabling GCAM to establish a more meaningful economic value of water. This study improves our understanding of the sensitivity of reservoir water supply to multiple physical and economic dimensions, such as sub-annual variations in climate conditions and human water demands, especially for basins experiencing socioeconomic droughts. [ABSTRACT FROM AUTHOR]

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 planning, design, and construction of hydropower dams are occurring at a rapid pace in the Mekong/Lancang River Basin in Southeast Asia. For years the river and its tributaries transported an average of 160 million metric tons of sediment per year to the South China Sea. Reservoirs are expected to trap significant fractions of this sediment, rendering much of it unavailable to transport nutrients and maintain the river's geomorphic structure and ecological habitats. This paper introduces and applies a method for identifying and evaluating alternative dam siting, design, and operating (SDO) policy options that could help maintain more natural sediment regimes and for evaluating the effect of these sediment-management strategies on hydropower production. This paper focuses on the planned Lower Sesan 2 (LSS2) Dam in Cambodia. This dam would prevent a significant source of sediment from reaching critical Mekong ecosystems, such as Cambodia's Tonle Sap Lake and the Mekong Delta in Vietnam. The concept of replacing LSS2 as planned with several smaller alternative dams with SDO modifications to enable reservoir sediment flushing is evaluated. These modifications could increase sediment discharge from this site by as much as three to four times compared to current plans but reduce short-term annual energy production by a third or more. Results demonstrate that most of the loss in energy production associated with SDO modifications (at least 94% in the cases considered) results from reduced reservoir size as opposed to modified reservoir operations. It is also found that, from an operational standpoint, sediment flushing is likely best conducted during the transition period between the dry and wet seasons. [ABSTRACT FROM AUTHOR]

This paper introduces PySedSim, an open-source, object-oriented, Python-based, daily time step river basin simulation screening model for flow, sediment, and hydropower in networks of reservoirs and river channels. The model enables users to explore representations of four key concerns relevant to the selection and evaluation of alternative reservoir configurations: 1) management approaches to improve the passage of sediment through and around reservoirs to avoid storage capacity loss and downstream ecological impacts; 2) search for flexible and adaptive reservoir operating policies designed to achieve multiple objectives; 3) alternative design features such as dam gates, which are necessary to enable ecologically-focused reservoir features; and 4) uncertainties associated with hydroclimatic drivers and sediment processes. PySedSim is intended to support deliberative decision-making and design processes. We highlight PySedSim's functionality by demonstrating its use in a real decision context focused on identifying siting, design, and operation alternatives for the proposed Sambor mega Dam in Cambodia. Image 1 • Reservoir design and operation alternatives improve sediment passage. • Flexible problem formulation improves understanding of tradeoffs and alternatives. • Evaluating sediment, ecology, and hydropower together yields synergistic insights. • PySedSim supported iterative deliberation of alternatives to Sambor Dam (Cambodia). [ABSTRACT FROM AUTHOR]