LONG-TERM TEMPERATURE TRENDS IN INTERMITTENT HEADWATER STREAMS IN THE FERNOW EXPERIMENTAL FOREST, WEST VIRGINIA, USA

Increasing air temperatures accompanying climate disruption are projected to increase perennial surface water temperature resulting in decreased water quality and increased risk to aquatic species and habitat. However, the effect of increasing air temperatures on intermittent headwater streams has received less attention. In the Fernow Experimental Forest (“the Fernow”), a 4,600-acre research forest in the Monongahela National Forest, at least 10 experimental research watersheds have been established in the last ~60 years to evaluate the effects of forestry management practices on water quantity and quality. As a result, these watersheds have nearly continuous records of flow and temperature, with measurements on at least weekly frequencies. The streams in these watersheds have increased flow during the growing season due to more intense rain events and decreased flow during the dormant season. Average yearly air temperature in the Fernow increased by 1.4 C from 9.9 C in 1959 to 11.3 C in 2017 over which time surface water temperatures decreased by 1.6 C from 14.1 C in 1959 to 12.5 C in 2017. A cosine model was applied to seven of the research watersheds to evaluate seasonal trends, amplitude dampening, and phase lag of stream temperature relative to air temperature. A seasonal sinusoidal temperature pattern was present in all watersheds (p < 0.01). Average water temperature damping was 0.52 and average phase-lag (number of days between peak air temperature and peak water temperature) was 14.8 days. These analyses indicate that while air temperature is increasing in the Fernow, water temperatures are decreasing and converging with average air temperature. The stream temperature patterns observed in this study show how complex interactions between timing and temperature of groundwater recharge and discharge affect stream temperatures. Thus, understanding how the combination of forestry management practices, underlying geology, and increasing air temperature affect is crucial for better understanding watershed temperatures.