Modeling procedures for the implementation of dissolved oxygen criteria to

protect the Fish and Wildlife Propagation beneficial use [REVOKED] Additional technical guidance for modeling procedures may be found in the CPP. (1) Model types. Many types of water quality models for DO are available. Some are extremely complex, some are extremely simple. Professional judgment is exercised to determine the appropriate modeling strategy for a particular situation. A simple model requires little or no field data but its applications are limited. A simple model should only be used for small, non-complex systems with discharge flows under 1 MGD or as an initial screening tool for larger systems. Typical simple models include desktop or spreadsheet-based formulae. A complex model requires a large amount of field data and should be calibrated and/or verified with observed conditions and then used to make predictive decisions. Complex models are appropriate for complex systems, multiple discharges, or large systems involving point source discharges of 1.0 MGD or more. Typical models include various versions of QUAL2, RIVERMOD, HSPF, and the BASINS system. (2) Target DO concentrations. Some of the DO criteria in the OWQS are minimum values while others allow for a 1.0 mg/l excursion from the criterion for up to 8 hours during any 24 hour period. Dynamic models can predict these time-dependent conditions and the OWQS criteria may be used directly. Since steady state water quality models cannot represent such a time-based excursion, appropriate average DO target values are used to protect the minimum DO criteria. The following average DO target concentrations will be used for steady state models when the numerical criteria apply: for streams designated HLAC, 5 mg/l for the early life stage season, 4 mg/l for the summer season, and 4 mg/l for the winter season; for streams designated WWAC, 6 mg/l for the early life stage season, 5 mg/l for the summer season, and 6 mg/l for the winter season; for streams designated CWAC, 7 mg/l for the early life stage season, 6 mg/l for the summer season, and 7 mg/l for the winter season. When site specific data are available, the target concentrations shall be adjusted to account for the magnitude of the actual diurnal variation. When numerical criteria do not apply, the OWQS require that DO concentrations be maintained at a level to prevent "nuisance conditions". A target average DO concentration of 2 mg/l will be utilized to prevent nuisance conditions. (3) Margin of safety. To compensate for uncertainty, a margin of safety is required for all models. The CPP provides guidance for recommended margins of safety for various types of models. As model complexity and use of actual data increase, the recommended margin of 30 safety decreases. The margin of safety is determined by comparing the unallocated load to the maximum assimilative capacity of the system as predicted by the water quality model.