Uncertainty in Phosphorus Retention, Williams Fork Reservoir, Colorado

Uncertainties in the calculation of water and phosphorus budgets for a bottom‐withdrawal reservoir were determined for 1979–1982. Ungaged components of the water budget were estimated from the residual of measured terms. This residual accounted for less than 30% of total water input and for more than 45% of total phosphorus input in each year of the study. Uncertainty in phosphorus retention data was large because of uncertainty in the total phosphorus input, due to use of a residual term.