Stream-air temperature relations to classify stream-ground water interactions in a karst setting, central Pennsylvania, USA

Summary Stream–ground water interactions in karst vary from complete losses through swallow holes, to reemergences from springs. Our study objective was to compare stream–air temperature and energy exchange relationships across various stream–ground water relationships in a carbonate watershed. It was hypothesized that ground water-fed stream segments could be distinguished from perched/losing segments using stream–air temperature relationships. Two types of computations were conducted: (1) comparisons of stream–air temperature relationships for the period of October 1999–September 2002 at 12 sites in the Spring Creek drainage and (2) detailed energy budget computations for the same period for ground water-dominated Thompson Run and Lower Buffalo Run, a stream with negligible ground water inputs. Weekly average air temperatures and stream temperatures were highly correlated, but slopes and intercepts of the relationship varied for the 12 sites. Slopes ranged from 0.19 to 0.67 and intercepts ranged from 3.23 to 9.07 °C. A two-component mixing model with end members of ground water and actual stream temperatures indicated that the slope and intercept of the stream–air temperature relationship was controlled by ground water inputs. Streams with large ground water inputs had greater intercepts and lesser slopes than streams that were seasonally losing, perched, and/or distant from ground water inputs. Energy fluxes across the air–water interface were greatest for the ground water-fed stream due to increased longwave, latent, and sensible heat losses from the stream in winter when large temperature and vapor pressure differences existed between the stream and air. Advection of ground water was an important source and sink for heat in the ground water-fed stream, depending on season. In contrast, along the seasonally losing stream reach, advection was of minimal importance and stream temperatures were dominated by energy exchange across the air- water interface. Overall, results showed that stream temperatures and energy exchange in this carbonate watershed were strongly related to stream–ground water interactions.