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ABSTRACT Machine‐learning models have been surprisingly successful at predicting stream solute concentrations, even for solutes without dedicated sensors. It would be extremely valuable if these models could predict solute concentrations in streams beyond the one in which they were trained. We assessed the generalisability of random forest models by training them in one or more streams and testing them in another. Models were made using grab sample and sensor data from 10 New Hampshire streams and rivers. As observed in previous studies, models trained in one stream were capable of accurately predicting solute concentrations in that stream. However, models trained on one stream produced inaccurate predictions of solute concentrations in other streams, with the exception of solutes measured by dedicated sensors (i.e., nitrate and dissolved organic carbon). Using data from multiple watersheds improved model results, but model performance was still worse than using the mean of the training dataset (Nash–Sutcliffe Efficiency < 0). Our results demonstrate that machine‐learning models thus far reliably predict solute concentrations only where trained, as differences in solute concentration patterns and sensor‐solute relationships limit their broader applicability. 

Air temperature is measured at seven locations in rain gauge clearings throughout the experimental watersheds and at Headquarters. The oldest air temperature record dates back to October 20,1955 at Station 1. From 1955 - 2014, temperature measurements were made continuously using hygrothermographs housed in standard shelters. Beginning in 2014, digital sensors housed in solar radiation shields collected daily minimum and maximum temperature measurements. Measurements made by the hygrothermographs have been corrected for screen bias to match the current aspirated radiation shields. These data are gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Precipitation has been measured at the Hubbard Brook Experimental Forest using rain gauges located in or around each watershed since 1956. Three types of rain gauges have been used: standard, mechanical weight recording, and electronic weight recording. Between 1956 and 2014, precipitation was measured weekly at standard gages located at 24 stations in or near gauged watersheds and at the headquarters building. Weight-recording gauges were located at 7 of the 24 stations and capture a continuous strip-chart record. Weekly totals were prorated using daily totals from the nearest recording gauges. Beginning in 2011, electronic weighing rain gauges were implemented to measure 15-minute precipitation. The number of precipitation stations was reduced to 10, when each station was fully converted to an electronic gauge for measuring 15-minute and daily precipitation beginning in 2015. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

From 1956 to 2014, watershed precipitation was estimated using the Thiessen Means weighting method. Daily watershed precipitation values were a weighted average of daily precipitation from standard gauges in or near the watershed. The weighting factor for each gauge was the fraction of the watershed area nearest that gauge. Beginning in 2015, a similar system was implemented using the 10 remaining rain gauges, however Inverse Distance Weighting was used to estimate the weighting of each gauge instead of the Theissen polygon approach. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

From 2011 to 2017, ten electronic weighing rain gauges were progressively implemented at the Hubbard Brook Experimental Forest to measure precipitation at 15-minute intervals. 15-minute resolution watershed precipitation values for nine research watersheds are calculated as a weighted average of precipitation using Inverse Distance Weighting. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Air temperature is measured at 15-minute intervals in six clearings throughout the Hubbard Brook experimental watersheds and at the Headquarters Station. Beginning in 2014, digital sensors housed in aspirated solar radiation shields collected air temperature measurements. These data are gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Beginning in 2011, ten electronic weighing rain gauges were progressively implemented at the Hubbard Brook Experimental Forest to measure precipitation at 15-minute intervals. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Spring and Fall leaf phenology observations have been made at 9 locations at the Hubbard Brook Experimental Forest since 1989. Timing and progression of spring leaf out and fall senescence are recorded for 3 dominant tree species, sugar maple, yellow birch, and beech, in treated and untreated watersheds and high and low elevations. Weekly measurements are taken during the active period of the fall or spring season. These data were gathered at the Hubbard Brook Experimental Forest in North Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Wind data have been measured by an anemometer mounted 3 m above the ground at Hubbard Brook Experimental Forest Headquarters since 1965. Prior to 1981, every mile of wind movement caused a tick mark on a strip-chart recorder, and wind direction as N, S, E, W, or a combination, was recorded continuously. From 1981-2003, wind speed and direction were measured with a MetOne wind speed sensor. In June 2003 the MetOne was replaced with a R.M. Young company wind speed direction sensor (model 05103). Since that time, wind direction (azimuth) is based on a 0 to 360 degree scale. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Relative humidity has been measured within the Hubbard Brook Experimental Forest since 1956. Prior to 1981, relative humidity was measured at seven hygrothermographs within the forest, and minimum relative humidity at the time of maximum daily temperature was converted to vapor pressure. Digital data from the hygrothermograph charts are only available for three short periods prior to 1981: January 1966 to April 1966; June 1967 to Sept. 1967 and; May 1971 to April 1973. Beginning in 1981, a humidity sensor has operated as part of the automatic weather station at Headquarters. After installation of the digital sensor in 1981, mean daily air temperature and mean daily relative humidity were converted to vapor pressure. These data were gathered at the Hubbard Brook Experimental Forest in Woodstock, NH, which is operated and maintained by the USDA Forest Service, Northern Research Station.