Two Simple Models for Estimating Daily Mean Water Temperatures and Diel Variations in a Danish Low Gradient Stream

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Abstract
The Danish River Suså is at the studied reaches a third-fourth order low gradient stream with a large summer biomass of macrophytes. In a simple model daily mean water temperature was described as a function of daily mean air temperature at the two localities studied. A linear regression analysis showed that the slopes of the two regression lines were significantly different, which could be explained by the relative proportion of incoming groundwater. A simple model for estimating the diel variations in stream water temperature was established from submodels of incoming and outgoing radiation and of the exchange of heat, mainly with the atmosphere. The only input to the model is air temperature and photosynthetic active radiation, PAR. Two parameters, A sol and A con were calibrated. The parameter A sol expresses the proportion of PAR used to heat the water. At the upstream locality A sol varied with season due to shading from the herbaceous bank vegetation, whereas A sol at the downstream locality was constant. A con expresses the rate of heat exchange between air and water, and A con was found to be linearly related to the reaeration coefficient, which was estimated independently. The model simulated the diel variations in water temperatures quite accurately. Even on days when air temperature and PAR showed large fluctuations did simulated water temperatures deviate less than 1°C from actual measurements during 90% of the time. Using the model at the downstream reach, with an average A sol and A con as estimated from the reaeration coefficient simulated temperatures did never deviate more than 2°C from actual measurements and less than 1°C in 84% of the time. It is concluded that the establishment of such simple temperature models would be very useful in a wide range of ecological studies in streams.