Investigation of the Relative Effects of Insolation, Groundwater, and Spatial Variability in Temperature Dynamics of Two Headwater Streams

Forested headwater streams account for much of the aquatic species diversity and contribute valuable recreational fisheries in the mountains of the Southeastern United States. Stream temperature is key regulator of headwater ecosystems and thermal regime is such a critical factor that it limits where many species can survive, grow, and successfully reproduce. Forested headwater streams are dynamic systems located in complex terrain. The variability of heat exchange between a stream and its surroundings results in thermal variations along its course. Understanding the spatial variability of heat fluxes along headwater streams is important to understanding the thermal dynamics and their effects on the biota. Solar radiation and groundwater inflow are two primary components of the heat budget of headwater streams and are spatially variable over short distances. A comparative analysis of north-facing and south-facing watersheds the Coweeta Basin of the Southern Appalachian Mountains found that temperatures of north-facing streams were cooler than south-facing streams for most of the year, but were warmer in summer. A north-south watershed pair with similar discharge, drainage areas, elevation, slope, and landcover characteristics was selected for further study. Water temperature was monitored longitudinally from the stream origins to the outlets beginning in late 2014. Preliminary data suggested variation in solar radiation resulting from the spatial heterogeneity of evergreen and deciduous trees and seasonal changes in leaf density could explain temperature patterns. We used the Subcanopy Solar Radiation Model, which accounts for topographic and vegetative shading by using a light penetration index derived from LIDAR data, to produce spatially explicit estimates of solar radiation and elucidate spatial and temporal variations in solar radiation along the two streams. Groundwater influence on stream temperature dynamics was investigated using salt-dilution gaging methods. Combined with the longitudinal water temperature study the results were used to examine the relative influences of solar radiation and groundwater inflow, and better understand the role of spatial variation of the two heat fluxes, on the temperature dynamics of the headwater streams.