A conceptual chart showing how the NDOI-based estimate of Delta outflow may be seasonally biased due to its inherent conflation of in-Delta consumptive use and channel depletions. Image: Paul Hutton.
An examination of 50 years of records reveals an apparent seasonal bias in estimates of freshwater flow from the Delta to the San Francisco Estuary.
The seasonal bias suggests flows were overestimated during the summer months and underestimated during the winter. Estimates of Delta outflow use a measurement called net Delta outflow index (NDOI), which is determined by taking the amount of Delta inflow, from sources such as the Calaveras, Sacramento, and San Joaquin rivers, and subtracting Delta exports, a direct measurement, and net Delta channel depletions. A team led by TetraTech’s Paul Hutton compared NDOI estimates against measured Delta outflow at four points, as well as against the Estuary’s historical salinity record. The study
, which appears in the December 2021 issue of San Francisco Estuary and Watershed Science, focused on net Delta channel depletions, which are determined indirectly and involve considerations such as land and water use, crop evapotranspiration, and soil moisture. The seasonal bias was an (estimated) average of 1,000 cubic feet of water per second over or under the NDOI estimates, a significant percentage of typical summer outflow. Accurate accounts of freshwater flow to the Estuary, also called the Delta outflow, are important because they influence water management. “The Central Valley Project and State Water Project operations, for example, rely on real-time estimates of Delta outflow to guide regulatory compliance of reservoir releases and Delta exports,” explains Hutton. While the Delta Watermaster is currently collaborating to improve one area of net channel depletion estimations, better crop evapotranspiration assessments, ut Hutton believes more needs to be done, such as accurate assessments of the changes in local soil moisture and other aspects of net Delta channel depletion, to obtain more overall accurate NDOI estimates. Reported by Dianna Bautista