Abstract
doi: http://dx.doi.org/10.15447/sfews.2016v14iss3art3 Outflow from the Sacramento–San Joaquin Delta is a key parameter used in the management of the San Francisco Bay–Delta system. At present we can estimate this by assuming a steady state balance of inflows and outflows (Dayflow) or by direct measurement. In this paper, I explore differences between observed sub-tidal variations in measured outflow and Dayflow values using water level and flow data taken during the summer of 2015 and an analytical framework based on the sub-tidally filtered St. Venant equations. This analysis shows that flows associated with sub-tidal water level variations in the Delta explain most of the difference between the two flow measures. These variations largely result from low-frequency variations in sea level in the coastal ocean and to wind stresses acting on Suisun Bay, with spring–neap variations in tides playing a lesser role. Overall, a comparison of Dayflow and the direct flow measurement for water years 2008 to 2014 shows that the two flow measures are in good agreement, although the root mean square difference between the two values (ca. 5,000 cfs) is comparable to—or larger than—typical low flow values of Dayflow.
Highlights
The 2013–2015 drought, which has seen California’s water supply storage brought down to historic lows, has focused attention on the management of the flow of water from the Sacramento–San Joaquin Delta (Delta) into San Francisco Bay
Current practice is to estimate Delta Outflow through the use of a hydrologic balance known as Dayflow that is calculated by the California Department of Water Resources (CDWR c2016a)
These sources contain data collected by the California Department of Water Resources (CDWR), the U.S Bureau of Reclamation, the U.S Geological Survey (USGS) and National Oceanic and Atmospheric Administration (NOAA)
Summary
The 2013–2015 drought, which has seen California’s water supply storage brought down to historic lows, has focused attention on the management of the flow of water from the Sacramento–San Joaquin Delta (Delta) into San Francisco Bay. Current practice is to estimate Delta Outflow through the use of a hydrologic balance known as Dayflow (hereinafter denoted QDF ) that is calculated by the California Department of Water Resources (CDWR c2016a) This balance includes some quantities that are measured with a relatively high degree of precision (e.g., export pumping rates), some quantities that are measured with justifiably less precision, e.g., sub-tidal (net) flows at Freeport (FPT) on the Sacramento River and Vernalis (VNS) on the San Joaquin River, and some that because they must be estimated, have a high degree of uncertainty.
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