STREAM TEMPERATURE DYNAMICS - MEASUREMENTS AND MODELING

A numerical model based on a finite difference solution of the unsteady heat advection-dispersion equation is formulated to predict water temperatures in streams at time increments of 1 hour. An energy balance accounts for the effects of air temperature, solar radiation, relative humidity, cloud cover, and wind speed on the net rate of heat exchange through the water surface, and heat conduction between water and streambed. Continuous stream temperature recordings in shallow streams show strong dynamic behavior including diurnal variations of several degrees Celsius which are lost in the standard daily records. These measured water temperatures are used to calibrate the model for the optimum percentages of Sun shading and wind sheltering. Stream exposure to solar radiation is shown to vary from 30 to 100% and wind exposure from 10 to 30% depending on the character of the stream. Values are related to stream width and season because of variable leaf cover of trees on stream banks. After calibration, accuracies of hourly and daily water temperature predictions over periods of several weeks are of the order of 0.2-degrees to 1-degrees-C. Solar (shortwave) radiation is shown to be the most important component of the heat flux across the stream water surface, but none of the other components, i.e., long-wave radiation, evaporation, and convection to the atmosphere, are negligible. Conductive heat exchange between the streambed and the water is a significant heat balance component in shallow streams.