Baltimore Ecosystem Study Institute of Ecosystem Studies

2013 BES Annual Meeting Presentation and Poster Abstracts

Analysis of Water Isotope Data to Determine Mean Transit Times in Dead Run
Welty, Claire
Co-Authors: Claire Welty and John T. Kemper

As a demonstration project for a Picarro L2120-i water isotope analyzer recently purchased with a NSF MRI grant, we have conducted a time series analysis of the water isotope content of precipitation and Dead Run streamflow. Composite samples of precipitation events were collected in Oella, MD beginning January 1, 2013; instantaneous stream samples for Dead Run at Franklintown and its DR1-DR5 subwatersheds were collected weekly beginning March 5, 2013. 2013 precipitation was characterized by highly variable isotopic content, as a number of winter and late-spring frontal storms were overall much more depleted in d18O and d2H compared to summer convective storms. The isotopic contents of stream time series from six nested watersheds were dampened in amplitude and shifted in phase relative to the precipitation time series; this behavior is commonly observed for pristine forested watersheds where precipitation is known to be the sole source of streamflow. A simple first cut analysis fitting sine wave functions to the two data sets yielded mean watershed transit times of about 15 days; the results were insensitive to subwatershed scale. The impact of a broken water supply pipe contributing approximately one third of streamflow at DR5 is reflected in the DR5 isotopic time series, which exhibits less variability compared to the other (DR1 and DR2) headwater streams. Evidence of this anthropogenic perturbation diminishes as the DR5 flow becomes a smaller percentage of total streamflow downstream. The mean transit time calculated for DR Franklintown agrees with previous calculations using spectral analysis of precipitation and stream chloride over a multiple-year period.