anomalous Stream Temperature Response to Storms in a Forested Headwater Stream in Central Pennsylvania

Open Access
Author:
Gerecht, Katelyn Elizabeth
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 29, 2012
Committee Members:
  • Michael Gooseff, Thesis Advisor
Keywords:
  • stream temperature
  • storm response
  • headwater
  • runoff
Abstract:
Contrary to previous investigations of stream temperature response to storms in forested headwater streams, we observed sharp increases of up to 3.8 °C in response to storm events in a small 1.5 km2 watershed that is more than 90% forested. Stream temperature data collected from April through September of 2011 show that during storm events this headwater stream can exhibit a more urban-like stream temperature response. Sudden temperature increases of this magnitude may have negative impacts on the stream’s ecological and biogeochemical services. There is a statistically significant difference in the stream’s mean temperature response to storms during spring months versus during fall months, suggesting a seasonal relationship. The largest and most intense storms between April and June cause the largest increases in stream temperature, while the largest and most intense storms between July and September cause the largest decreases in stream temperature. We investigate the physical mechanisms of the stream’s response to a large storm in May to better understand the cause of the rapid temperature increases. We conclude that climatic variables in this humid climate zone during the spring months cause the precipitation temperature to be much warmer than the pre-storm stream temperature. Low permeability soils and high intensity precipitation cause fast overland runoff to carry this warm “new” water to the stream causing the unexpected temperature anomaly. We propose that a new conceptual model for explaining storm response in small forested watersheds may be applicable here. Previous research has shown that the bulk of storm response in headwater streams is “older” groundwater being displaced into the stream by the “new” precipitation infiltrating into the subsurface. Instead, here we show that both stream temperature and fluid electrical conductivity observations suggest that the water input to this stream is primarily “new” water directly from the storm. We see both a freshening and a warming of the stream, whereas if the predicted conceptual model applied to this system we would expect to see the stream cool with an increase in fluid electrical conductivity.