SacPAS: Central Valley Prediction & Assessment of Salmon

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OMR Tidally Filtered Flow and Index Calculations as performed by SacPAS

The Sacramento-San Joaquin Delta is a complex hydrosystem with numerous conditions controlled in part by human activities. These activities impact the ecological functioning of the delta with respect to critically endangered species. One of the hydrosystem indicators used to assess the functional ecology of the delta is the flow at the Old and Middle rivers (OMR).

The Central Valley Operations office provides two flow indicators calculated from various hydrology parameters. They provide a time-delayed USGS Tidally Filtered OMR flow computed from available USGS tidally filtered data at USGS sites 11312676 (Middle River) and 11313405 (Old River). They also calculate a highly correlated OMR Index based on delta hydrology for each day (DWR 2017). These two flow indices are published on a daily basis in PDF files on the CVO website: .

The USBR has requested that SacPAS provide tabular and graphic displays of the OMR indices to facilitate Delta management. Because published PDFs are not reliably read by machine tools, SacPAS calculates the OMR tidally filtered flow and the OMR index based on methods as described in DWR (2017).

SacPAS obtains the daily USGS tidally filtered discharge values (parameter 72137 Discharge, tidally filtered, cubic feet per second (Mean)) at the Middle River and Old River sites: . These two daily values are added together to provide a OMR daily discharge value. SacPAS then calculates a running 5- and 14-day mean for the tidally filtered flows.

To mitigate the effects of delayed and missing data, DWR (2017) calculates an OMR Index based on the MWD model, originally developed by Dr. Paul Hutton with Metropolitan Water District, with the form:

OMR Index(cfs) = A * [ Prior day flows on San Joaquin River @ Vernalis ] + B * [ South Delta Diversions and Exports ] + C

where South Delta Diversions and Exports are defined as:

South Delta Diversions and Exports (cfs) = Clifton Court Forebay Intake + Jones Pumping Plant Exports + Total CCWD Diversions - CCWD Contra Costa Canal Diversions + (1/4 * Delta Net Channel Depletions)

SacPAS obtains:

The values of the coefficients A, B, & C are determined by the status of the HOR and GCL barriers as well as the flow at Vernalis (DWR 2017, Table 1). Changes to the barrier status are obtained from .

The SacPAS value for the OMR Index is calculated on a daily basis as well as a running 5- and 14-day average for 28 October 2016 to the present.


California Department of Water Resources (DWR). 2017. Old and Middle River Flow Review: USGS Gauge Readings versus Index Calculations for 2011 through 2017. Online:

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SacPAS: Central Valley Prediction & Assessment of Salmon, University of Washington, Columbia Basin Research,

Friday, 28-Feb-2020 16:39:53 PST