ON THE USE OF STREAMBED TEMPERATURES FOR ESTIMATING BASEFLOW VELOCITY

Hydrogeologists use several methods to estimate baseflow velocity, including the variation of temperatures with depth into the streambed. Temperature is particularly useful in this setting because it is a natural tracer and in most instances the streambed experiences diurnal temperature variations. We use these diurnal temperature variations and the attenuation of the signal with depth to calculate baseflow velocity from streambed temperature profiles. We programmed the amplitude ratio method of Hatch et al. (2006, Water Resour. Res., 42, W10410) into a Matlab script, which we use to estimate daily-average baseflow velocities of both synthetic and field data. Our primary goal is to test the consistency of the baseflow velocity calculations with a variety of sampling intervals. The synthetic data consist of idealized sinusoids of thermal fluctuations at a shallow and deep piezometer location. The field data derive from piezometer nests installed at 18 locations in the streambed of Boone Creek in northwestern North Carolina. In both cases we employ a piezometer nest spacing of 25 cm. Synthetic and field sampling occurred at one-minute intervals; we then resampled these data at a variety of intervals, from five minutes up to 2 hours. The results of our calculations with the synthetic data indicate that velocities derived from data sampled at intervals of one minute to 15 minutes yield similar velocities; however, the results of our calculations with the noisier field data are less definitive. Our calculations with the field data suggest that one-minute intervals yield less consistent velocities than our synthetic data set; the most consistent velocity calculations occur with five to 15-minute time intervals. Large storm events, which may cause reversed gradients in the streambed, render some portions of the thermal time series unusable, resulting in an inability to estimate baseflow velocity on the day of the storm.