Influence of topography on stream insolation and thermal regimes in a mid-latitude, montane, forested headwater

Forested mountain headwaters are important as ecosystems and to downstream water quality, but for many headwater streams limited temperature data is available, representing few locations over short periods. We combined high-resolution geospatial modeling of solar radiation, limited field data, and deterministic stream temperature modeling to gain insights into the influence of topography and tree cover on stream insolation and thermal regimes in the forested headwaters of the southern Appalachian Mountains, USA. In summer, daily maximum temperatures were consistently higher in streams draining north-facing slopes than those draining south-facing slopes, however evaluation of the daily stream temperature gain (afternoon maximum - morning minimum) and the duration of the warming period revealed north-facing streams experienced lower temperature gains over a longer warming period, consistent with the effect of aspect on slopes at mid-latitudes < 40° N in summer. The net influence of solar incidence angles (which vary with slope, aspect, latitude, date, and time of day) on the solar radiation flux of the forested streams were further evaluated with a north and south-facing stream pair using the deterministic stream temperature model (HFLUX) parameterized with measured solar radiation from a local climate station, located in the valley bottom, and with spatially and temporally explicit estimates of stream insolation computed with an open source geospatial model (r.sun). The results demonstrate influence of topography on stream insolation and temperatures, even in densely forested landscapes, and provide a generalizable process-based method for estimating insolation along headwater streams.