Continuous sensor data is collected at 10 sites throughout New Hampshire. At each site data is collected every 15 minutes by the datalogger from a HOBO Stage logger (or site is paired with USGS site), Satlantic SUNA and YSI EXO2. Data is collected and transmitted to UNH by cell telemetry once a day where it is stored on the NH EPSCoR data server. The data that is collected from the SUNA are Nitrate in mg N/L and is corrected by grab sample NO3 analyzed by IC. Data that is collected by the YSI are Stream Temperature, pH, Specific Conductance, Dissolved Oxygen, and fDOM in QSU. The fDOM is corrected by temperature, turbidity (not included), and absorbance.

Solute concentrations from primarily weekly grab samples were used to train RF models, functioning as dependent variables in all models. Laboratory measured solutes included calcium (Ca2+), chloride (Cl−), dissolved organic carbon (DOC), potassium (K+), magnesium (Mg2+), sodium (Na+), nitrate (NO3−), and sulfate (SO42−). For the Hubbard Brook sites (W3 and W9) sample analysis occurred at the US Forest Service (Hubbard Brook Watershed Ecosystem Record (HBWatER), 2023). In brief, Cl−, NO3−, and SO42− at all sites and major cations in all but the Hubbard Brook sites were measured via ion chromatography (Pfaff & Hautman, 1999). For the Hubbard Brook sites, Ca2+, K+, Mg2+, and Na+ were measured by inductively coupled plasma atomic emission spectrometry (Pfaff & Hautman, 1999). Dissolved organic carbon and total dissolved nitrogen were measured by high temperature catalytic oxidation (Merriam et al., 1996; Potter & Wimsatt, 2005). For sites BDC, BEF, DCF, LMP72, MCQ, SBM, TPB, and WHB, water chemistry analyses were performed at the Water Quality Analysis Laboratory at the University of New Hampshire (Wymore et al., 2021). Anions (NO3−, Cl−, SO42−) and cations (Na+, Ca2+, Mg2+, K+) were analyzed using ion chromatography with suppressed conductivity detection (Dionex 1000 ICS with autosampler; Sunnyvale, CA). Units of NO3− and SO42− refer to the mass per unit volume of N and S, respectively. DOC (NDIR detection) and TDN (chemiluminescent detection) were measured by high-temperature catalytic oxidation with a Shimadzu TOC-LCSH with a TNM-1 nitrogen unit. Dissolved organic nitrogen (DON) was estimated via subtraction: DON = Total Dissolved Nitrogen [TDN] – Dissolved Inorganic Nitrogen [DIN]) where DIN equals the sum of NH4+ and NO3-.

Methods and findings are described in the associated manuscript.