Estimation of Annual Dissolved Ammonia Loads in an Urban Watershed

Estimations of the average annual dissolved ammonia load to a small, urban watershed in eastern Massachusetts were made using a variation of the ESTIMATOR regression model. Comparisons were made between different time-step regression models applying either instantaneous or daily average flow values to point concentrations. In addition, comparisons were made between models with and without detailed storm data. Estimates of the average annual dissolved ammonia load from the basin are approximately 29,500 (95% CI: 21,000 to 42,000) kg NH₃-N/yr and 29,200 (21,000 to 41,100) kg NH₃-N/yr for models excluding and including storm samples, respectively. Without including storm sampling data, little difference is seen in the fit and load estimate using the different time-step models. Smaller time-step models with intensive storm sampling provide more precise load estimates than daily average time-step models or models with fewer storm samples. When designing a sampling strategy to make the most precise annual load estimate, efforts should be made to collect detailed storm samples if flow data is available in intervals smaller than the daily average, otherwise collecting one to two samples per storm is adequate.