Revision and proposed modification for a total maximum daily load model for Upper Klamath Lake, Oregon

This report presents Phase 2 of the review and development of the mass balance water-quality model, originally developed in 2001, that guided establishment of the phosphorus (P) total maximum daily load (TMDL) for Upper Klamath and Agency Lakes, Oregon. The purpose of Phase 2 was to incorporate a longer (19-year) set of external phosphorus loading data into the lake TMDL model than had originally been available, and to develop a proof-of-concept method for modeling algal mortality and the consequent decrease in chlorophyll a that had not been possible with the 2001 TMDL model formulation.

Using the extended 1991–2010 external phosphorus loading dataset, the lake TMDL model was recalibrated following the same procedures outlined in the Phase 1 review. The version of the model selected for further development incorporated an updated sediment initial condition, a numerical solution method for the chlorophyll a model, changes to light and phosphorus factors limiting algal growth, and a new pH-model regression, which removed Julian day dependence in order to avoid discontinuities in pH at year boundaries. This updated lake TMDL model was recalibrated using the extended dataset in order to compare calibration parameters to those obtained from a calibration with the original 7.5-year dataset. The resulting algal settling velocity calibrated from the extended dataset was more than twice the value calibrated with the original dataset, and, because the calibrated values of algal settling velocity and recycle rate are related (more rapid settling required more rapid recycling), the recycling rate also was larger than that determined with the original dataset. These changes in calibration parameters highlight the uncertainty in critical rates in the Upper Klamath Lake TMDL model and argue for their direct measurement in future data collection to increase confidence in the model predictions.