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The following is an excerpt from the introduction to Essig (1998): The Dilemma of Applying Uniform Temperature Criteria in a Diverse Environment: An Issue Analysis 

"Bartholow (1989) describes the physical factors affecting stream temperatures as used in the Instream Water Temperature Model (SNTEMP) (Theurer and others 1984), and reports the results of a sensitivity analysis of the SNTEMP. When predicting mean daily water temperature, air temperature is the most sensitive input variable. Relative humidity is the next most sensitive input variable, accounting for less than half as much change in stream temperature. Percent shade follows a close third to relative humidity. When predicting maximum daily water temperature, air temperature is just as important, but percent shade, which affects diurnal range, overtakes relative humidity as the second most sensitive variable. For both measures, stream flow is the fourth most sensitive variable and 'water temperature is very sensitive to changes in air temperature when stream flow is low'."