Assessing Hydrological Alteration Caused by Climate Change and Reservoir Operations in the San Joaquin River Basin, California

Mahesh L Maskey,Joshua H Viers,Josué Medellín-Azuara,Gustavo Facincani Dourado,David E Rheinheimer,Anna M Rallings

doi:10.3389/fenvs.2022.765426

Abstract

Freshwater aquatic ecosystems are highly sensitive to flow regime alteration caused by anthropogenic activities, including river regulation and atmospheric warming-induced climate change. Either climate change or reservoir operations are among the main drivers of changes in the flow regime of rivers globally. 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. 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. Results show that most indicators have pronounced departures from baseline conditions under anticipated future climate conditions given existing reservoir operations. For example, the high pulse count decreases during regulated flow conditions compared to increased frequency under unregulated flow conditions. Finally, we observed a higher degree of flow regime alteration due to reservoir operations than climate change. 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%. This study extends multi-dimensional hydrologic alteration analysis to inform climate adaptation strategies in managed river systems.

Highlights

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)
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 for the San Joaquin River Basins where hydropower generation is predominant do not exist to date
The highest flows are mostly removed by the river regulation, with a lower reduction in the Upper San Joaquin due to its rim dam’s lower storage capacity, mean while the lowest flows tend to be reduced in the Merced and Upper San Joaquin, and absent in the Tuolumne and Stanislaus basins

Summary

Introduction

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). Assessing Hydrological Alteration Using IHA (Poff et al, 1997; Bunn and Arthington, 2002; Grantham et al, 2010; Poff and Zimmerman, 2010; Yarnell et al, 2010). 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 for the San Joaquin River Basins where hydropower generation is predominant do not exist to date

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