Dayflow is a computer program designed to estimate daily average Delta outflow. The program uses daily river inflows, water exports, rainfall, and estimates of Delta agriculture depletions to estimate the “net” flow at the confluence of the Sacramento and San Joaquin Rivers, nominally at Chipps Island. It is a key index of the physical, chemical, biological state of the northern reach of the San Francisco Estuary.

The Dayflow program also estimates these flow and parameter estimates as daily averages:

1. Net flow through the Delta Cross Channel and Georgiana Slough (when measured flow is not available).
2. Net flow at Jersey Point (also called QWEST)
3. Position of X2, the 2 ppt salinity isohaline.

The Dayflow estimate of Delta outflow is referred to as the “net Delta outflow index” (NDOI) because it does not a account for tidal flows, the fortnight lunar fill-drain cycle of the estuary, or barometric pressure changes. It is a quantity that never actually occurs in real time. Rather it is an estimate of the net difference between ebbing and flooding tidal flows at Chipps Island ( ~ + / - 150,000 cfs), aliased to a daily average.  Depending on conditions, the actual net Delta outflow for a given day can be much higher or lower than the Dayflow estimate. An example comparison of Dayflow output with measured daily average outflow can be seen can be seen here.

Dayflow is computed once per year following the water year (October 1). At that time, we request official QA/QC’d data from several (Table 1) sources. Once all input data is received ( tried herding cats?), we compute the Dayflow estimate of Delta outflow. Our annual goal is to provide data for the previous water year by January 1.

Table 1: Responsible agencies and input data used as input to the Dayflow program.

Responsible Agency	Input data
USGS	Sacramento River at Freeport, Yolo Bypass at Woodland, Cosumnes River at Michigan Bar, San Joaquin River at Vernalis, Delta Cross Channel, Georgiana Slough
USACOE	Calaveras River
EBMUD	Mokelumne River at Woodbridge
DWR O&M	Precipitation at Stockton Fire Department, Clifton Court Forebay gate flow, Barker Slough export, Byron Bethany ID depletion, X2 (only when outflow is negative)
DWR Bay-Delta	Estimated Delta island consumptive use
DWR DPLA	Sacramento Weir spill, Lisbon Weir flow
USBR	Delta Cross-Channel gate status, Tracy export, Contra Costa export
SCWD	Lake Barryessa releases, Lake Solano inflow, Putah Creek

Over time, some inflow inputs have been lost because stream flow gages have been abandoned or discontinued due to lack of funding. The input data is further described in the Dayflow program documentation.

In 2000, the software used to perform Dayflow calculations was rewritten in Java. Input data is stored in a HEC-DSS file, and output is written to ASCII, excel, and DSS format files. Data are available in multi or single years.