STREAM TEMPERATURE ESTIMATION FROM AIR-TEMPERATURE

Air temperatures are sometimes used as substitutes for stream temperatures. To examine the errors associated with this procedure, linear relationships between stream temperatures, T(w), and air temperatures, T(a), recorded for 11 streams in the central U.S. (Mississippi River basin) were analyzed. Weather stations were an average 42 miles (range 0 to 144 miles) from the rivers. The general equations, T(w) = 5.0 + 0.75 T(a) and T(w) = 2.9 + 0.86 T(a) with temperatures in degrees-C, were derived for daily and weekly water temperatures, respectively, for the 11 streams studied. The simulations had a standard deviation between measurements and predictions of 2.7-degrees-C (daily) and 2.1-degrees-C (weekly). Equations derived for each specific stream individually gave lower standard deviations, i.e., 2.1-degrees-C and 1.4-degrees-C, respectively. Small, shallow streams had smaller deviations than large, deep rivers. The measured water temperatures follow the air temperatures closely with some time lag. time lags ranged from hours to days, increasing with stream depth. Taking into account these time lags improved the daily temperature predictions slightly. Periods of ice cover were excluded from the analysis,