Supporting Restoration Decisions through Integration of Tree-Ring and Modeling Data: Reconstructing Flow and Salinity in the San Francisco Estuary over the Past Millennium

Abstract

This work presents updated reconstructions of watershed runoff to San Francisco Estuary from tree-ring data to AD 903, coupled with models relating runoff to freshwater flow to the estuary and salinity intrusion. We characterize pre-development freshwater flow and salinity conditions in the estuary over the past millennium and compare this characterization with contemporary conditions to better understand the magnitude and seasonality of changes over this time. This work shows that the instrumented flow record spans the range of runoff patterns over the past millennium (averaged over 5, 10, 20 and 100 years), and thus serves as a reasonable basis for planning-level evaluations of historical hydrologic conditions in the estuary. Over annual timescales we show that, although median freshwater flow to the estuary has not changed significantly, it has been more variable over the past century compared to pre-development flow conditions. We further show that the contemporary period is generally associated with greater spring salinity intrusion and lesser summer–fall salinity intrusion relative to the pre-development period. Thus, salinity intrusion in summer and fall months was a common occurrence under pre-development conditions and has been moderated in the contemporary period due to the operations of upstream reservoirs, which were designed to hold winter and spring runoff for release in summer and fall. This work also confirms a dramatic decadal-scale hydrologic shift in the watershed from very wet to very dry conditions during the late 19th and early 20th centuries; while not unprecedented, these shifts have been seen only a few times in the past millennium. This shift resulted in an increase in salinity intrusion in the first three decades of the 20th century, as documented through early records. Population growth and extensive watershed modification during this period exacerbated this underlying hydrologic shift. Putting this shift in the context of other anthropogenic drivers is important in understanding the historical response of the estuary and in setting salinity targets for estuarine restoration. By characterizing the long-term behavior of San Francisco Estuary, this work supports decision-making in the State of California related to flow and salinity management for restoration of the estuarine ecosystem.

Inferring Trends and Change Attribution Using Regression-Based Unimpaired Runoff to San Francisco Estuary, 1872-2022

“…This finding suggests that -at least over this study period -the effect of warming temperatures on annual unimpaired runoff is not apparent and the most noticeable driver of annual runoff changes is drought. Drought events, especially multi-year events, are features of the study area's climate as observed in the millennial-scale paleoclimate record (Meko and Woodhouse, 2005;Hutton et al, 2021). However, in recent decades, these departures have primarily been caused by precipitation variability.…”

“…Future-oriented research typically utilizes global climate model projections of temperature and precipitation coupled with watershed modeling frameworks (e.g., Maurer and Duffy, 2005;Huang et al, 2012;Burke and Ficklin, 2017). Independent of these analyses, paleoclimate research using tree-ring chronologies has been used to understand millennial-scale runoff patterns, such as severe historical long-term droughts spanning decades that would have major consequences if they occurred in the modern era (Graumlich, 1993;Cook et al, 2004;Meko and Woodhouse, 2005;Hutton et al, 2021).…”

Climate Change Impacts on San Francisco Estuary Aquatic Ecosystems: A Review

“…2. An uptrend in salinity intrusion is greater in the Spring than in the Summer-Fall (July-October vs. February-June; Hutton et al 2021).…”

“…A decrease in the duration of floods can lead to an increase in flood intensity, making extreme flood events like the "Great Flood of 1862" more common (Swain et al 2018). Extreme flood events larger than those in recent history are possible based on tree-ring chronologies that date back 2,000 years (Meko et al 2014) in the western region, and 1,000 years in the San Francisco Estuary (Hutton et al 2021).…”

Decision Support Tools for Water Quality Management

“…The eleventh paper in this Special Issue-"Supporting Restoration Decisions through Integration of Tree-Ring and Modeling Data: Reconstructing Flow and Salinity in the San Francisco Estuary over the Past Millennium" [36] (https://www.mdpi.com/2073-4 441/13/15/2139 (accessed on 20 August 2022)) by Paul Hutton, David Meko and Sujoy Roy-also focuses on salinity, although this application is further downstream in the Sacramento-San Joaquin River Delta. This paper presents updated reconstructions of watershed runoff into the Estuary from tree-ring data coupled with models that associate rainfall runoff with freshwater flow to the estuary and salinity intrusion.…”