How does NAWQA assess concentrations in streams given multiple samples and effects of streamflow?
Statistical models that relate nutrient concentrations to streamflow and time were used to compute flow-weighted mean-annual concentrations of ammonia, nitrate, total nitrogen, orthophosphate, and total phosphorus. Use of flow-weighted values facilitate comparisons among sites and avoid the potential bias to assessment results caused by differences in the number of samples, sampling frequency, and flow conditions during the period of sampling. Models were developed to estimate nutrient concentrations for each day of the year. The models were calibrated using nutrient concentrations from stream samples and mean-daily streamflow for the date of sampling. These daily values were then used to calculate flow-weighted mean-annual concentrations. (See Mueller and Spahr, 2006, for details).
