Evaluating a coupled phenology-surface energy balance model to understand stream-subsurface temperature dynamics in a mixed-use farmland catchment

Stream temperature is a key variable that controls both physical and biogeochemical processes in aquatic ecosystems. Complex physical interactions between land surface and subsurface processes make accurate simulations of stream temperature dynamics at catchment scales a challenging task. In this study we propose an integrated, catchment-scale framework to model stream, soil, streambed, and groundwater temperatures under the influence of hydrologic and vegetation dynamics in a mixed land use catchment in central England. The phenology and surface energy modules in the coupled model were used to quantify the impacts of vegetation processes on radiation fluxes (e.g., canopy shading and the effect of vegetation growth on optical parameters). The model enabled accurate simulations of the movement and partitioning of water and thermal fluxes in different hydrologic domains with R ² values of observed and simulated temperatures in the range 0.60–0.87. Simulated groundwater heads and stream stages allowed the identification of gaining and losing portions of stream reaches and the estimation of Darcy fluxes. Simulation results show significantly dampened diel streambed temperature fluctuations below 0.3 m in gaining reaches, while in losing reaches the diel fluctuations showed relatively strong fluctuations below 0.3 m. The model enabled evaluation of the relative contributions of different processes to the stream thermal budget. Results indicate that net radiation was the dominant heat source, while latent heat flux was the primary heat sink. The model provides a useful tool to explicitly simulate water and heat fluxes as well as temperature-dependent reaction rates in biogeochemical analyses.