Responses of Unimpaired Flows, Storage, and Managed Flows to Scenarios of Climate Change in the San Francisco Bay‐Delta Watershed

Abstract

Projections of meteorology downscaled from global climate model runs were used to drive a model of unimpaired hydrology of the Sacramento/San Joaquin watershed, which in turn drove models of operational responses and managed flows. Twenty daily climate change scenarios for water years 1980–2099 were evaluated with the goal of producing inflow boundary conditions for a watershed sediment model and for a hydrodynamical model of the San Francisco Bay‐Delta estuary. The resulting time series of meteorology, snowpack, unimpaired flow, reservoir storage, and managed flow were analyzed for century‐scale trends. In the Sacramento basin, which dominates Bay‐Delta inflows, all 20 scenarios portrayed warming trends (with a mean of 4.1 °C) and most had precipitation increases (with a mean increase of 9%). Sacramento basin snowpack water equivalent declined sharply (by 89%), which was associated with a major shift toward earlier unimpaired runoff timing (33% more flow arriving prior to 1 April). Sacramento basin reservoirs showed large declines in end‐of‐September storage. Water‐year averaged outflows increased for most scenarios for both unimpaired and impaired flows, and frequency of extremely high daily unimpaired and impaired flows increased (increases of 175% and 170%, respectively). Managed Delta inflows were projected to experience large increases in the wet season and declines in the dry season. Changes in management strategy and infrastructure can mitigate some of these changes, though to what degree is uncertain.

Potential Changes in Runoff of California’s Major Water Supply Watersheds in the 21st Century

“…Earlier peaks are also projected for Sacramento River above Bend Bridge during late-century. Earlier peak of Sacramento basin runoff is also reported in a recent study [59] which looks at daily runoff projections derived from those 20 climate projections applied in the current study. For San Joaquin Valley watersheds, however, the peak timing is expected to largely remain in the same month.…”

“…Earlier peaks are also projected for Sacramento River above Bend Bridge during late-century. Earlier peak of Sacramento basin runoff is also reported in a recent study [59] which looks at daily runoff projections derived from those 20 In terms of peak timing, the study indicates that for Feather River, Yuba River, and American River watersheds, earlier (than historical) peaks are projected. Earlier peaks are also projected for Sacramento River above Bend Bridge during late-century.…”

Inter-model agreement on projected shifts in California hydroclimate characteristics critical to water management

“…As an illustrative case study of the effects of change in hydroclimate statistics on surface water operations, we characterize the influence of shifting monthly hydroclimate statistics on Oroville Reservoir, the SWP's largest reservoir, based on outputs published and validated by Knowles et al (2018). They project future unimpaired and impaired hydrology, reservoir storage, and downstream managed flows in the Sacramento and San Joaquin basins based on the same climate projection data used in this study, but aggregated at certain analysis steps to relatively coarse (monthly) temporal resolution (see Supplementary materials for additional details).…”

“…We use these outputs to assess monthly storage behavior at Lake Oroville at end-of-century (2070-2099) under RCP 8.5 versus historical conditions and compare it with changes in reservoir outflows. Note that this historical baseline differs from that used to assess the intraannual and inter-annual metrics within the LOCA dataset (sections 2.1 and 2.2), and reflects the historical definition used by Knowles et al (2018). Publicly available hydrologic and resource planning models available for the state of California are currently limited to monthly time steps, precluding quantitative assessment of the impact of predicted changes in daily hydroclimate statistics (e.g., change in maximum 3-day precipitation) that we assess as part of this study.…”

“…Publicly available hydrologic and resource planning models available for the state of California are currently limited to monthly time steps, precluding quantitative assessment of the impact of predicted changes in daily hydroclimate statistics (e.g., change in maximum 3-day precipitation) that we assess as part of this study. We therefore qualitatively combine the modeled shifts in monthly Oroville Reservoir operations with information on operational limitations and sub-monthly climate shifts identified in our LOCA analysis (but not included in the model setup used by Knowles et al (2018)) to assess how the operations changes and the outlook of future reservoir function would be further impacted by expected sub-monthly hydroclimate shifts.…”