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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5173

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Summary

In March 2003, 16,500 acres of salt evaporation ponds owned by the Cargill Corporation in San Francisco Bay were purchased using State, Federal, and private funds. Currently the California Coastal Conservancy is leading a collaborative planning effort with the U.S. Fish and Wildlife Service (USFWS) and the California Department of Fish and Game (DFG) to restore most of the ponds to tidal action while providing for flood management and public access and recreation.

The draft restoration plan envisions restoration of 50 to 90 percent of the acquired ponds within 50 years (South Bay Salt Pond Restoration Project, 2007). The ponds will be operated and maintained according to the South Bay Salt Ponds Initial Stewardship Plan (ISP) until they are restored. The goals of the ISP are to maintain existing habitat and to prevent a build up of salt in the ponds in a cost-effective manner until the long-term restoration plan is in effect (Life Science! Inc., 2003).

A salt pond box model (SPOOM) was developed by Lionberger and others (2004), written as a program in Visual Basic for Excel, to simulate water volume and salinity of a salt pond for use in estimating water and salinity budgets for ponds in the Napa–Sonoma Salt Pond Complex. The model uses the principle of conservation of mass to calculate daily pond volume and salinity and includes a salt crystallization and dissolution algorithm. Model inputs include precipitation, evaporation, infiltration, and water transfers. The SPOOM model was reconfigured to simulate volume and salinity for a portion of the purchased South Bay salt ponds, Alviso salt ponds A9–A17 (fig. 1), and a temperature subroutine was added.

SPOOM simulates each pond as one well-mixed box of saline water. Water enters the ponds from the adjacent sloughs and from rainfall and flows by gravity through the pond system and eventually is returned to the sloughs. Evaporation removes water from the pond, effectively increasing the salinity with time. The model user specifies pond-management operations such as screw-gate and combo-gate control, pumping rates, and winter flow controls to simulate pond function.

A new subroutine was added to SPOOM to calculate daily average, minimum, and maximum pond temperature from hourly calculations of net heat transfer. The temperature subroutine was validated by comparing simulated temperatures to pond temperatures measured on hourly and monthly timescales. Two data sets were used to validate the temperature subroutine; temperature data collected every 12 seconds over a 1-month period to validate hourly temperature simulations and data collected monthly over a 27-month period to validate monthly temperature simulations. Both simulations compared well to measured data.

SPOOM will be used by the USFWS for predicting how the pond system will respond under different management scenarios to achieve pond salinity and depths goals of the ISP. The user is able to specify how the ponds are connected, how screw gates and combination gates are controlled, and the vertical datum and unit systems used during the simulation. Historical meteorology data from 7 water years are included for pond simulations with an option for the user to enter a custom rainfall time series. An application of the model is presented, which compared two simulations for pond A14: (1) normal tidal outflow conditions and (2) no tidal outflow. The results were compared to determine how outflow affects water level, salinity, and temperature in pond A14.

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