The Importance of Maintaining a Total System Energy Balance for Predicting Stream Temperatures Using a Fully-Integrated Surface/subsurface Modeling Framework

Climate change is forecast to have a significant impact on surface and groundwater water availability in certain regions, but it may also have a detrimental effect on the thermal exchange fluxes between surface and subsurface flow regimes. While there has been an increase in field-based research directed towards characterizing thermal energy exchange processes that occur at the surface water/groundwater interface within streams, relatively little work has been performed to simulate these exchanges in a fully-integrated surface/subsurface computational framework. A fully-integrated, physically-based modeling framework is needed to elucidate the roles of the various meteorological, surface/subsurface hydrological and thermal energy exchange processes in mediating the temperature of streams and surface water bodies. To address this issue, HydroGeoSphere, a physically-based fully-integrated surface/subsurface flow and transport model, was enhanced in order to simulate water flow, evapotranspiration and advective-dispersive heat transport over the 2D land surface and in the 3D subsurface under variably-saturated conditions. Results from high-resolution 3D numerical simulations are presented to illustrate the importance of explicitly accounting for temporal variations in incoming longwave and shortwave solar radiation and latent and sensible heat fluxes over the entire land surface when simulating the thermal evolution of stream waters. The need to account for these surface heat fluxes in a manner that preserves energy balances within both the surface and subsurface flow regimes highlights the importance of taking a holistic fully-integrated modeling approach when predicting the impacts of, for example, climate change on stream water temperatures.