The Susquehanna River Basin Commission estimates flow adjusted concentration trends in nutrients, sediment, and trends in discharge as part of the Chesapeake Bay Program's annual trend update. Trends in discharge indicate the natural changes in hydrology. Changes in flow and the cumulative sources of flow (base flow and overland runoff) affect the observed concentrations and the estimated loads of nutrients and suspended sediment. The Flow Adjusted Concentration (FAC) is the concentration after the effects of flow are removed. Trends in FAC indicate that changes have occurred in the processes that deliver constituents to the stream system. This is the concentration that relates to the effects of nutrient-reduction activities and other actions taking place in the watershed.
Trends in FAC are calculated based on the USGS water year, October 1 to September 30, using the USGS 7-parameter, log-linear regression model (ESTIMATOR) for nutrient trends and the Seasonal Kendall test is used to find trends in discharge. Slope, p-value and sigma (error) values are taken from the DECTIME variable of the ESTIMATOR output. These values are then used to calculate flow adjusted trends using the following equations;
Trend = 100*(exp(Slope *(end yr – begin yr)) – 1)
Trend minimum= 100*(exp((Slope – (1.96*sigma)) *(end yr – begin yr)) – 1)
Trend maximum= 100*(exp((Slope + (1.96*sigma)) *(end yr – begin yr)) – 1)
The slope value determines whether trends are increasing or decreasing (positive slope means trends are increasing while negative slope means trends are decreasing). Trends are only reported when the p-value is < 0.05. If a parameter has more than 20% of its reported values below the method detection limit, no trend is reported and BMDL is listed.