![]() The fourth Climate Change Assessment suggests global temperatures will increase between 3.1 and 4.9☌ (5.6–8.8☏) while the water supply from snowpack will decline by two-thirds by 2,100 ( Pachauri and Reisinger, 2007). Understanding the harmful impacts caused by regulation and climate change on flow alteration are equally important to maintain healthy ecosystems. Although several efforts have been made to manage environmental flows and establish the scientific foundation to understand the mechanism of flow alteration ( Richter et al., 1996 Poff et al., 1997 Bunn and Arthington, 2002 Poff and Zimmerman, 2010), adequate studies that have investigated the effect of both reservoir operations and climate change specifically for the San Joaquin River Basins where hydropower generation is predominant do not exist to date. Many rivers have already lost their natural conditions, due to reservoir operations such as hydropower, irrigation, and other human activities ( Poff and Zimmerman, 2010 Fong et al., 2016 Hayes et al., 2018 Grill et al., 2019 Kuriqi et al., 2019). Recently, there has been increased attention on how reservoir operations and water diversions affect the natural flow threatening riverine ecosystem ( Grantham et al., 2010 Rheinheimer and Viers, 2015 Yang et al., 2018). Altered hydrology can result in perturbed sediment dynamics, limited habitat formation, poor biogeochemical cycling, invasion of non-native species, and disruption of phenological cues ( Poff et al., 1997 Bunn and Arthington, 2002 Grantham et al., 2010 Poff and Zimmerman, 2010 Yarnell et al., 2010). Consequently, changes in the flow regime have a potential negative impact on aquatic ecosystems and functions ( Bunn and Arthington, 2002), not limited to biodiversity and productivity. The natural flow regime heavily influences river geomorphological and ecosystem processes, affecting diverse abiotic and biotic components ( Poff et al., 1997 Poff and Zimmerman, 2010). This study extends multi-dimensional hydrologic alteration analysis to inform climate adaptation strategies in managed river systems. The degree of alteration ranges from 1.0 to 9.0% across the basin among all future climate scenarios, while reservoir operations alter the flow regime with a degree of alteration from 8.0 to 25%. Finally, we observed a higher degree of flow regime alteration due to reservoir operations than climate change. ![]() For example, the high pulse count decreases during regulated flow conditions compared to increased frequency under unregulated flow conditions. ![]() Results show that most indicators have pronounced departures from baseline conditions under anticipated future climate conditions given existing reservoir operations. We selected a set of Indicators of Hydrologic Alteration (IHA) to evaluate historical and projected future trends of streamflow dynamics: rise and fall rates, durations and counts of low and high pulses, and the magnitude of extremes. Using modeled unregulated and simulated regulated streamflow under historical and future climate scenarios, this study evaluated potential changes to the flow regime due to climate change and reservoir operations for the major tributaries of the San Joaquin River Basin, California United States. Either climate change or reservoir operations are among the main drivers of changes in the flow regime of rivers globally. 3Center for Water for Latin America and the Caribbean, School of Engineering and Science, Tecnológico de Monterrey, Monterrey, Mexicoįreshwater aquatic ecosystems are highly sensitive to flow regime alteration caused by anthropogenic activities, including river regulation and atmospheric warming-induced climate change.2Water Systems Management Lab, School of Engineering, University of California, Merced, Merced, CA, United States.1School of Engineering, University of California, Merced, Merced, CA, United States.Rheinheimer 1,3 Josué Medellín-Azuara 2 Joshua H. Maskey 1,2 * Gustavo Facincani Dourado 1 Anna M.
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