This data set includes nitrate concentration, water temperature, and discharge collected every 15 minutes during Dec. 2011 - Nov. 2013 from the USGS gage at the Potomac River near Wash, DC, Little Falls Pump Sta (01646500). Additional data includes day length, and photosynthetically active radiation from USDA research station in Beltsville, MD. Several calculations derived from these data are included in the data set. These data accompany a paper titled "Patterns of Diel Variation in Nitrate Concentrations in the Potomac River".