The effect of projected global climate change due to a doubling of atmoapheric CO2 on water temperatures in five streams in Minnesota is estimated using a deterministic heat transport model. The model calculates heat exchange between the atmosphere and the water and is driven by climate parameters and stream hydrologic parametera. The model is based on a finite difference solution of the unsteady heat advection-dispersion equation. An energy balance at the water surface 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. The energy balance at the bottom of the stream requires modeling of diurnal heat exchange between water and streambed. The model was calibrated against detailed measurements to account for seasonally variable shading and wind sheltering shown to be dependent on leaf cover of trees on stream banks. Measurements were made in 5 streams at 2 minute intervals over periods of up to 4 weeks. After calibration, accuracies of hourly and daily water temperature predictions over periods of several weeks are on the order of 0.2 to 1Â°C. The model is sensitive to each of the aforementioned weather parameters to different degrees. Sensitivity coefficients are calculated by two different methods which gave same order of magnitude results. The mean and/or the standard deviation of each of the weather parameters are combined with the sensitivity coefficients to establish the sensitivity of the model to each parameter. The sensitivity analysis showed that stream water temperature is most sensitive to air temperature and solar radiation. Using climate projections from the GISS (Goddard Institute for Space Studies), GFDL (Geophysical Fluid Dynamics Laboratory) and OSU (Oregon State University) Global Circulation Models (GCM's) as input; stream temperature simulations predict a warming of freely flowing river reaches by 2.4Â°C to 4.7Â°C when atmospheric CO2 doubles. In small shaded streams water temperatures are predicted to rise by an additional 6Â° C in summer if trees along stream banks should be lost due to climate change. These projected water temperature changes have significant consequences for survival and growth of fishes in different temperature guilds (cold-, cool- and warmwater fishes). A model developed by the USEPA, relating fish survival and fish habitat to water temperatures, was applied to make the first assessment. The simulation results obtained with the complete heat budget equations are also used to examine simplified water temperature/air temperature correlations.
|Original language||English (US)|
|State||Published - Dec 1992|