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Inputs and Selected Outputs Used to Assess Spatial and Temporal Patterns in Streamflow, Water-Chemistry, and Aquatic Macroinvertebrates of Selected Streams in Fairfax County, Virginia, 2007-2018

October 8, 2021

Nitrogen (N), phosphorus (P), and suspended-sediment (SS) loads, in Fairfax County, Virginia streams have been calculated using monitoring data from five intensively monitored watersheds for the period from water year (October - September) 2008-2017. Nutrient and suspended-sediment loads were computed using a surrogate (multiple-linear regression) approach with lab analyzed N, P, and SS samples as the response variable and basic water-quality parameters (e.g. turbidity, specific conductance, pH, water temperature), streamflow, and time and seasonal terms that could be measured continuously as predictor (surrogate) variables. The load results represent the total mass of N, P, and SS that was exported from each of the Fairfax County watersheds. Trends in N, P, SS concentrations, as well as basic water-quality parameters including turbidity, water temperature, dissolved oxygen, specific conductance, and pH have been calculated using both non-flow-normalized and flow-normalized Seasonal Kendall tests at 14 monitoring stations in Fairfax County, Virginia for the period beginning April 2008 and ending March 2018. Flow-normalized trends were used to integrate out the effect of annual streamflow variability on concentration (or unit value). Streamflow is an important driver of water-quality conditions; therefore, several datasets are provided describing a suite of annual streamflow metrics: streamflow volume/yield, flashiness, peak stage, baseflow index, stormflow index, and runoff ratio. Depending on the metric, data are provided from the 5 intensively monitored Fairfax County, VA watersheds, the entire 20-station Fairfax County monitoring network (5 intensively monitored + 15 trend only monitoring stations), and/or 3 reference stations that are part of the USGS Chesapeake Bay Non-Tidal Network (USGS Station IDs 01646000, 01654000, 01658500). Streamflow analyses were conducted on data collected in water years 2008-2017 for all Fairfax County monitoring stations as well as station ID 01658500, 1948-2017 for station ID 01654000, and 1946-2017 for 01646000. Benthic macroinvertebrate data were collected annually (waters years 2008 - 2017) by Fairfax County staff from each of the 20 monitoring stations. These data were used to explore trends in 20 biological metrics; the data used to calculate these trends are provided.