The position of the low salinity zone in the San Francisco Bay Delta-given its correlation with the abundance of several estuarine species-is used for water management in a system that supplies water to more than 20 million people and contains one of the most diverse ecosystems on the Pacific coast. This work consolidates legacy and modern salinity data to develop a reasonably complete daily record spanning nine decades. The position of the low salinity zone, which is effectively characterized by an empirical model that was developed to support data cleaning and filling, reveals statistically significant trends consistent with increasing water demands and introduction of upstream reservoirs, e.g., increasing salinity trends in wet months and decreasing salinity trends in dry months. Reservoir effects are particularly apparent in drier years, with greater seasonal variability in the early part of the record before major reservoirs operated in the watershed. These data provide a basis for further analysis of how and why the position of the estuary's low salinity zone has changed over time.

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.

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This paper explains observed trends in freshwater flow to the San Francisco Bay-Delta estuary as reported in a companion paper . 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. Flow changes are measured against a baseline scenario representing 1920-level land use and water management conditions. Additional scenarios are defined to represent the system absent state and federal water project reservoir and export operations, absent key non-project reservoir operations, and absent historicallyobserved sea level rise. These scenarios, in conjunction with the principle of superposition, are used to ascribe outflow and salinity trends to different anthropogenic and natural causes. We find that project and non-project water management are attributed similar responsibility for decreasing outflow trends in April and May and consequent increasing spring salinity trends. In contrast, we find that increasing July and August outflow trends (and lagged decreasing salinity trends) are attributed to flow contributions from project water management; these contributions more than fully attenuate impacts associated with non-project water management. By the mid-20th century, evolving societal values led to a growing awareness and concern for the adverse ecosystem effects that resulted from anthropogenic disturbances. In addition to the early hydrologic alterations described above, other evolving disturbances include out-of-basin exports, entry of invasive species and discharges and runoff of pollutants from a highly urbanized estuary margin. Restoration and environmental management efforts have been implemented in the region over the last four decades to address many of these stressors; however, hydrologic alteration (in general) and flow management (in particular) have been the stressor of primary focus. Freshwater flow (i.e., Delta outflow), which is essential for repelling salinity intrusion into the Delta and is critical to the ecosystem health of the estuary, is regulated to support both human uses and aquatic life (CSWRCB, 2006). Precipitation from the upstream watershed is --------------------------------------------------------------------------------------------------------------------------------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.

Large-scale climatic indices have been used as predictors of precipitation totals and extremes in many studies and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation. The authors show that the sea level pressure North Pacific high (NPH) wintertime anomaly, a component of the Northern Oscillation index (NOI), provides a superior covariate of interannual precipitation variability in Northern California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as the Southern Oscillation index (SOI), the multivariate ENSO index (MEI), Niño-3.4, and others. Furthermore, the authors show that the NPH anomaly more closely reflects the influence of Pacific basin conditions over California in general, over groups of stations used to characterize statewide precipitation in the Sierra Nevada range, and over the southern San Francisco Bay region (NASA Ames Research Center). This paper uses the term prediction to refer to the estimation of precipitation (the predictand) from a climate covariate (the predictor), such as a climate index, or atmospheric moisture. In this sense, predictor and predictand are simultaneous in time. Statistical models employed show the effectiveness of the NPH winter anomaly as a predictor of total winter precipitation and daily precipitation extremes at the Moffett Field station. NPH projected by global climate models is also used in conjunction with atmospheric humidity [atmospheric specific humidity (HUS) at the 850-hPa level] to obtain projections of mean and extreme precipitation. The authors show that future development of accurate forecasts of NPH anomalies issued several months in advance is important for forecasting total winter precipitation and is expected to directly benefit water resource management in California. Therefore, the authors suggest that investigating the lead-time predictability of NPH anomalies is an important direction for future research.