Measuring and Modeling Stream Temperature in a Forested Ozark Border Stream: An Energy Balance Approach

Forested riparian buffers play an important role in modulating stream water quality, including temperature. Studies are needed to characterize canopy energy attenuation and thus buffering of stream temperature. This is particularly the case in the central hardwood forest regions of the United States where relationships between canopy density and stream temperature in karst terrain is not well understood. Data were collected from two intensively instrumented study sites along two stream reaches of opposing orientation in a semi-karst watershed on the border of southern Missouri’s Ozark region, USA, during the 2010 water year. Data supplied the necessary information to calculate an energy budget for each stream reach. From October 2009- July 2010, total precipitation was 110 cm as measured from a nearby reference flux tower (US DOE). Average estimated stream discharge was 0.20 m3/s in the E-W oriented reach and 0.27 m3/s in the N-S oriented reach. Average air temperature was 9.3 °C along the E-W reach and 10.0 °C along the N-S reach, while measured average incident shortwave radiation was 45.2 W/m2 along the E-W reach and 48.5 W/m2 along the N-S reach. Temporal and spatial variation in the site’s multilayered forest canopy was quantified via ceptometer and hemispherical photography. Average measured leaf area index (LAI) in April-May 2010 (ceptometer) was 1.89 within the riparian zone (n=77) and 1.48 within the canopy gap created by the stream (n=14), while average LAI from June-July was 4.04 within the riparian zone and 2.80 within the gap. Stream water latent and sensible heat exchange with the overlying air was modeled based on hourly averages of microclimate measurements taken along each reach. Average latent heat flux from the stream was -16.7 W/m2 within the E-W reach and -17.6 W/m2 within the N-S reach (negative values indicate energy loss). Average sensible heat flux from the stream was -1.34 W/m2 within the E-W reach and -2.27 W/m2 within the N-S reach. Average streambed heat flux was 41.1 W/m2 along the E-W reach, and -15.5 W/m2 along the N-S reach. Incident net radiation on the stream was modeled based on measured values from each climate station and measured values of leaf area index. Average net radiation within the E-W reach was 31.8 W/m2 and 72.0 W/m2 within the N-S reach, with differences occurring largely due to topographic shading and canopy density. On average, stream temperature decreased by 0.12 °C within the E-W reach and increased by 0.14 °C within the N-S reach. This finding holds important implications for forest stand management and stream orientation. Differences between modeled available energy for heating and temperature behavior may be partially attributed to hyporheic and groundwater exchange with the stream, not quantified in this work. On-going analysis will better elucidate the relationships between canopy density, energy attenuation, and groundwater influences on stream water temperature. Study results will be immediately used to improve riparian forest management in this distinct hydrogeological region.