Freshwater flow to the San Francisco Bay‐Delta estuary over nine decades (Part 1): Trend evaluation

Abstract

The San Francisco Bay-Delta estuary and its upstream watershed have been highly modified since exploration and settlement by Europeans in the mid-18th century. Although these hydrologic alterations supported the growth of California's economy to the eighth largest in the world, they have been accompanied by significant declines in native aquatic species and subsequent efforts to reverse these declines through flow management. To inform ongoing deliberations on management of freshwater flows to the estuary, we examined a recent nine-decade hydrologic record to evaluate seasonal and annual trends in reported Delta outflow. Statistically significant trends were observed in seasonal outflows, with decreasing trends observed in 4 months (February, April, May, and November) and increasing trends observed in 2 months (July and August). Trend significance in early-to-mid autumn (September and October) is ambiguous due to uncertainty associated with in-Delta agricultural water use. In spite of increasing water use over the period examined, we found no statistically significant annual trend in Delta outflow, a result likely due to large interannual variability. Linkages between outflow trends and changes in upstream flows and coincident developments such as reservoir construction and operation, out-of-basin imports and exports, and expansion of irrigated agriculture are discussed. To eliminate inter-annual variability as a factor, change attribution is explored using modelled flows and fixed climatology in a companion paper. --------------------------------------------------------------------------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Freshwater flow to the San Francisco Bay‐Delta estuary over nine decades (Part 2): Change attribution

“…This paper explains observed trends in freshwater flow to the San Francisco Bay‐Delta estuary as reported in a companion paper (Hutton, Rath, & Roy, ). We employ a historical hydrologic record spanning nine decades and define a set of idealized flow scenarios to identify drivers of change in delta outflow and consequent salinity regime.…”

“…Because of the importance of freshwater flow in the overall water management and regulatory structure in the San Francisco Bay‐Delta estuary, this work and a companion paper (Hutton et al, ) are focused on understanding how and why Delta outflow has changed over a nine‐decade period during which streamflow data have been systematically collected in the contributing watershed. This analysis acknowledges that the starting point of the record (October 1921) is not representative of pristine or “natural” conditions, and that numerous landscape changes had already occurred by this date.…”

Tidally‐averaged flows in the interior Sacramento–San Joaquin River Delta: Trends and change attribution

“…In contrast, this interior flow shows decreasing trends in nearly all months during the postproject period (July being the exception), with all nonsummer trends being significant. RIO: Over the full period of record, this interior flow shows significant decreasing trends in 2 months (April–May) and significant increasing trends in 4 months (July–October). These opposing trends result in annual flow trends (not shown here) that are nominally declining but not statistically significant, consistent with the analysis of Delta outflow in Hutton et al (). Summer flows exhibited a significant increase during the preproject period and a levelling off during the postproject period resulting in a net increase over the whole record.…”

“…Our analysis of the Sen's slope shows that CVP‐SWP miscellaneous operations (i.e., DCC gates) result in decreasing RIO flow, as these operations divert water from the main stem of the Sacramento River into the central Delta. Our analysis of the Sen's slope shows that CVP‐SWP net export result in small increases in RIO flow, as imports from the Trinity River increase flows in the Sacramento River (Hutton et al, ). The slopes suggest that changes associated with CVP‐SWP storage and nonproject operations are most significant. WEST: Decreases in this interior flow are primarily driven by CVP‐SWP exports in all seasons but spring, when nonproject operations are the primary driver of flow decreases.…”

“…The attribution analysis shows that interior flows respond to changes in Delta inflows (through drivers such as CVP‐SWP storage and upstream nonproject operations) as well as to facilities such as CVP‐SWP exports, DCC gates, and south Delta barriers that modify in‐Delta circulation. We note that trends in inflows to the Delta were not examined in this work and have been described previously (Hutton et al, ). These attribution results reflect specific operational changes that have occurred over recent decades in response to flow and salinity regulations.…”