Department of Water Resources Home

Michal Koller
Environmental Planning and Information Branch
Dayflow Program
3500 Industrial Blvd.
West Sacramento, CA 95691
(916) 376-9728

Mailing Address:
P.O. Box 942836
Sacramento, CA 94236-0001

Net Delta Outflow vs Net Delta Outflow Index

A primary output of Dayflow is an estimate of Net Delta Outflow, often referred to as Net Delta Outflow Index (NDOI). This is an arithmetic summation of river inflows, precipitation, agricultural consumptive demand, and project exports. The USGS has established a network of flow monitoring stations in the Delta. Using flow monitoring data from Rio Vista, Three Mile Slough, Jersey Point, and Dutch Slough, a direct estimate of Net Delta Outflow (NDO) is now available. The plots below compare NDO and NDOI:

NDOI is designated by the Dayflow parameter QOUT, which is defined as


NDO, which is not a Dayflow parameter, is a direct estimate of daily average flow based on 15 minute USGS ultrasonic velocity meter (UVM) flow data from the IEP tributaries database. NDO was calculated as the sum

  • NDO = Rio Vista + Three Mile Slough + Jersey Point + Dutch Slough
    • Rio Vista = Sacramento River at Rio Vista UVM
    • Three Mile Slough = Three Mile Slough at San Joaquin River UVM
    • Jersey Point= San Joaquin River at Jersey Point UVM
    • Dutch Slough = Dutch Slough at Jersey Island UVM

Figure 1 shows NDO vs NDOI for water years 1996-2000.  Note that NDO is not shown where Rio Vista, Three Mile Slough, Jersey Point, and Dutch Slough UVMs were not in simultaneous operation.

Figure 2 shows a small portion of Figure 1 (17JUL1997 - 21SEP1997). Moon phases are included to show the effects of the spring-neap cycle. The period averages of NDOI and NDO are nearly the same. However, the astronomical forcing of the sun and moon cause the Delta to fill and drain over approximately 14 days. When superimposed on the effect of riverine inflow and project operations as estimated by Dayflow NDOI, the spring - neap cycle can lead to short periods of negative NDO. Users of Dayflow output should be aware of this limitation.

Figure 1: Comparison of NDO and Dayflow NDOI: WY 1996-2000

Figure 2: Comparison of NDO and Dayflow NDOI, 17JUL1997-21SEP1997
Effects of Spring-Neap Cycle

moon graphics provided by U.S. Naval Observatory: