Spatial considerations of stream hydraulics in reach scale temperature modeling

Abstract

While a myriad of processes control water temperature, the most significant in streams without notable shading or groundwater inputs are surface heat fluxes at the air-water interface. These fluxes are particularly sensitive to parameters representing the water surface area to volume ratio. Channel geometry dictates this ratio; however, it is currently unclear how spatial variability in stream hydraulics influences temperature predictions or how the contribution of the boundary condition influences interpretation of processes most sensitive to this variability. To investigate these influences over long reach scales, we used high-resolution spatial observations collected over a 25 km reach within a Laplace-domain solution to a two-zone temperature transient storage model. We found that for the study reach and flow condition, changes in the surface area to volume ratio did not generally coincide with changes in stream temperature. Though, notable changes in cumulative mean residence time corresponded with changes in the temperature extremes throughout the study reach. The surface heat fluxes were clearly the most sensitive to spatially variable hydraulics that translated into high residence times once the contribution of the boundary condition decayed. Consistent with solute transport, reach segment lengths that reflect the spatial correlation in observations were necessary to capture the spatial influences of hydraulics on temperature predictions. This approach provides a fundamental step for determining whether spatial detail related to stream hydraulics is important to support accurate temperature predictions and how best to represent that detail. Key Points: The contribution of the boundary condition persisted over several kilometers Channel surface heat fluxes were most sensitive to spatially variable hydraulics Temperature predictions converged at scales of spatial correlation