Relationship Between Water Temperatures and Air Temperatures for Central U.S. Streams

An analysis of the relationship between air and stream water temperature records for 11 rivers located in the central United States was conducted. The reliability of commonly available water temperature records was shown to be of unequal quality. Simple linear relationships between air (Ta) and water (Tw) temperatures were developed for daily and weekly average temperatures and showed some level of accuracy, especially for weekly average temperatures and for small streams. The general equations Tw:= 5.0 + 0.75 Ta and Tw = 2.9 + 0.86 Ta simulated the daily and weekly temperatures of the 11 streams studied with a standard deviation between measurements and prediction of 2.7· C and 2.1° C, respectively. Predictions were better for shallow streams than for deep streams. Better predictions were obtained with equations derived for each specific stream individually. Time lags between water and air temperatures ranged from hours to days. In first approximation, the lag time between water temperature response to air temperature changes is found to be proportional to average water depth. For a 2 ft deep stream, it is on the order of hours, for a 15 ftdeep river it is on the order of days. Taking into account these time lags slightly improved daily water temperature. predictions. Periods of ice cover were excluded from the analysis.Environmental Research Laboratory, US Environmental Protection Agenc