Stream temperature correlations with air temperatures in Minnesota: Implications for climate warming

Air temperatures are sometimes used as easy substitutes for stream temperatures. To examine the errors associated with this substitution, linear relationships between 39 Minnesota stream water temperature records and associated air temperature records were analyzed. From the lumped data set (38,082 daily data pairs), equations were derived for daily, weekly, monthly, and annual mean temperatures. Standard deviations between all measured and predicted water temperatures were 3.5 degrees C (daily), 2.6 degrees C (weekly), 1.9 degrees C (monthly), and 1.3 degrees C (annual). Separate analyses for each stream gaging station gave substantially lower standard deviations. Weather monitoring stations were, on average, 37.5 km from the stream. The measured water temperatures follow the annual air temperature cycle closely. No time lags were taken into account, and periods of ice cover were excluded from the analysis. If atmospheric CO2 doubles in the future, air temperatures in Minnesota are projected (CCC GCM) to rise by 4.3 degrees C in the warm season (April-October). This would translate into an average 4.1 degrees C stream temperature rise, provided that stream shading would remain unaltered.