Vulnerability and risk: climate change and water supply from California’s Central Valley water system

Abstract

Water allocation institutions globally must operate within legal and political contexts established by precedent and codified in operating rules, even as they flex and adjust to climate change. California's Central Valley Water System (CVS) is a prime example. Recent global, national, regional, and local climate change assessments have highlighted climate-change-driven impacts on the CVS; however, these previous studies have not discussed the relative likelihood of performance decline, making it difficult to use the information for planning. In response, this paper presents a systematic climate change stress test that utilizes a physically based hydrologic model linked with a water resources system model representing the infrastructure, operations, and policy constraints of the interconnected system of natural river channels and man-made facilities that comprise the CVS. The results provide a summary of the sensitivity of the system to climate change, indicating the specific climate changes that cause performance of the system to decline below historical norms, and an estimation of the General Circulation Model (GCM) informed probability of those changes by 2050. Degraded performance is especially likely for State Water Project (SWP) deliveries (> 85%), and September carryover/ drought storage in the Oroville Reservoir (the SWP's largest reservoir,~95% likely to degrade). A decline in Net Delta Outflow is likely in all seasons except summer and early fall (when regulations require supplemental releases to combat salinity from sea level rise). For most of these metrics, the modeled performance drop is more severe in dry years than in wet years.

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

“…4). The resulting reduction in annual average storage is aligned with other studies that show similar reductions in Oroville carryover storage under future climate conditions (Ray et al 2020).…”

“…The results presented here have important implications for securing water supplies under changing climate conditions and encourage increased interaction between the climate and water management enterprises broadly. As we show here in the context of California's hydroclimate, despite extensive research indicating substantial projected hydroclimate shifts, lingering perceptions of uncertainty and a historic mismatch between the type of water analysis conducted by climate modelers and by regional water managers have limited quantitative inclusion of climate change projections in regional water planning, though promising efforts are emerging (e.g., Ray et al 2020;Steinschneider et al 2019). Our analysis demonstrates that, while inter-model agreement for future total annual precipitation changes is low, there is strong inter-model agreement in response to both more and less severe climate change scenarios for key non-mean state shifts that bear more directly on the viability of regional water management strategies.…”

“…1). Though use of climate data is increasing in the water sector, this uncertainty in gross regional precipitation metrics has been interpreted in certain planning and policy contexts as precluding meaningful quantitative integration of climate projection data into water planning, as discussed for instance by Schwarz et al (2018) and Ray et al (2020) study. For example, one groundwater sustainability plan submitted under SGMA cites the "conflicting results" on total precipitation changes in coarse-resolution GCMs as motivation to use an unmodified historical precipitation record as its expectation for future basin-wide precipitation conditions (IWVGA 2020), similar to issues identified in other plans submitted (Christian-Smith et al 2017).…”

“…The future behavior of these management-relevant statistics can be-in a number of important contexts-entirely obscured by analysis of climate model projections narrowly focused on annual or even monthly total or average precipitation changes (e.g., Swain et al 2018). Detailed efforts at quantitative climate data incorporation are present in the water sector (e.g., Groves et al 2015;Park et al 2019;Ray et al 2020;Steinschneider et al 2019) and represent an important advancement over decision-making based on the historical record, but, with few exceptions, have not yet resulted in integration of climate projection data that accounts for hydroclimatic shifts beyond mean annual or monthly precipitation and temperature shifts into water decision-making. Operating rules of many key California surface reservoirs continue to be based on statistics of reservoir inflow intensity and timing from early twentieth-century observations (Eum et al 2012).…”

Possibilistic response surfaces combining fuzzy targets and hydro-climatic uncertainty in flood vulnerability assessment

“…Response surfaces have been illustrated by many case studies (e.g. Nazemi et al, 2013, Turner et al, 2014, Whateley et al, 2014, Herman et al, 2015, Steinschneider et al, 2015, Spence et al, 2016, Pirttioja et al, 2019, Ray et al, 2020, expanded to many-objectives or stakeholder systems (Poff et al, 2016;Culley et al, 2016, Kim et al, 2019 and sometimes officially adopted in management processes (Moody and Brown, 2013, Weaver et al, 2013.…”