San Francisco Estuary and Watershed Science

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. Vénant 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.