Abstract
Abstract. Major multi-reservoir cascades represent a primary mechanism for dealing with hydrologic variability and extremes within institutionally complex river basins worldwide. These coordinated management processes fundamentally reshape water balance dynamics. Yet, multi-reservoir coordination processes have been largely ignored in the increasingly sophisticated representations of reservoir operations within large-scale hydrological models. The aim of this paper is twofold, namely (i) to provide evidence that the common modeling practice of parameterizing each reservoir in a cascade independently from the others is a significant approximation and (ii) to demonstrate potential unintended consequences of this independence approximation when simulating the dynamics of hydrological extremes in complex reservoir cascades. We explore these questions using the Water Balance Model, which features detailed representations of the human infrastructure coupled to the natural processes that shape water balance dynamics. It is applied to the Upper Snake River basin in the western US and its heavily regulated multi-reservoir cascade. We employ a time-varying sensitivity analysis that utilizes the method of Morris factor screening to explicitly track how the dominant release rule parameters evolve both along the cascade and in time according to seasonal high- and low-flow events. This enables us to address aim (i) by demonstrating how the progressive and cumulative dominance of upstream releases significantly dampens the ability of downstream reservoir rules' parameters to influence flow conditions. We address aim (ii) by comparing simulation results with observed reservoir operations during critical low-flow and high-flow events in the basin. Our time-varying parameter sensitivity analysis with the method of Morris clarifies how independent single-reservoir parameterizations and their tacit assumption of independence leads to reservoir release behaviors that generate artificial water shortages and flooding, whereas the observed coordinated cascade operations avoided these outcomes for the same events. To further explore the role of (non-)coordination in the large deviations from the observed operations, we use an offline multi-reservoir water balance model in which adding basic coordination mechanisms drawn from the observed emergency operations is sufficient to correct the deficiencies of the independently parameterized reservoir rules from the hydrological model. These results demonstrate the importance of understanding the state–space context in which reservoir releases occur and where operational coordination plays a crucial role in avoiding or mitigating water-related extremes. Understanding how major infrastructure is coordinated and controlled in major river basins is essential for properly assessing future flood and drought hazards in a changing world.
Highlights
The cumulative impacts of reservoir cascades on river flows has been recognized and demonstrated worldwide by earlyPublished by Copernicus Publications on behalf of the European Geosciences Union.C
Our results focus on the reservoir cascade on the main stem of the Upper Snake River (Table 2)
The historical record, and experiments based on offline water balance models of the reservoir cascade, demonstrate that, in both instances, coordinated reservoir management avoided the occurrence of these events
Summary
The cumulative impacts of reservoir cascades on river flows has been recognized and demonstrated worldwide by earlyC. Rougé et al.: Diagnostics of reservoir in hydrological models global hydrological models (Dynesius and Nilsson, 1994; Vörösmarty et al, 1997) Since these findings, frequently corroborated in the literature (e.g., Nilsson et al, 2005; Adam et al, 2007; Döll et al, 2009; Biemans et al, 2011; Grill et al, 2019), have taken a new significance with the planned or ongoing construction of more than 3700 major dams, most of them in the Global South (Zarfl et al, 2015). A striking illustration of this fact is the cumulative consequences of building multiple dams on river flow regimes, ecosystem benefits or sediment transport in previously relatively undammed major river basins such as the Amazon (Latrubesse et al, 2017; Timpe and Kaplan, 2017) or the Mekong (Schmitt et al, 2018)
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