TRENDS IN VERTICAL PORE WATER FLUX IN THE HYPORHEIC ZONE OF A LOW GRADIENT THIRD ORDER STREAM

The relationship between fluid flow and heat transfer is a powerful tool used to delineate and to quantify water movement in the near-surface streambed or hyporheic zone. This study tests the viability of a heat tracing method through the characterization of one-dimensional, vertical, flux rates in the top 150cm of the hyporheic zone of a low-gradient, third-order perennial stream. Six wells installed along the thalweg of a 25-meter stretch of the stream recorded temperature. From February 2009 to March 2010, temperatures were logged at 15-minute intervals in the stream and at depths of 30, 60, 90, 150 cm. Stream stage was also recorded every 15 minutes. To determine if stage is a control on the direction and magnitude of vertical flux and to visualize how flux changes temporally and spatially, vertical flux rates were calculated using temperature sensor pairs at depth with the one-dimensional conduction-advection-dispersion equation utilized in VFLUX. Flux is calculated at the midpoint between a sensor pair, e.g., a flux is calculated at a depth of 45cm the midpoint between the 30cm and 60cm sensors. Model results indicate that the flux direction varies with depth at all six of the streambed wells. The dominant flux direction at a depth of 15cm is upward (negative) while the dominant flux direction at a depth of 45 cm, 75 cm, and 120 cm is downward (positive). At a 15 cm depth, the average flux at each well ranges from -2*10^-6 to 5*10^-7 m/s. The average flux at 45 cm ranges from -1*10^-7 to 7*10^-6 m/s. The average flux at 75 cm ranges from 5*10^-7 to 8*10^-6 m/s and at 120cm depth the average flux rate ranges from 3*10^-6 to 1*10^-6 m/s. We posited that there is a positive linear relationship between stage and flux; i.e. with a rise (fall) in stage, the stream loses (gains) water to (from) the streambed, a positive or downward flux (negative, upward flux). The strength of the bivariate relationship between stage and flux was determined by the Pearson coefficient of correlation at each midpoint between sensor pairs. The p-values for 21 of the 24 well midpoints are less than a a of 0.05, indicating a statistically significant correlation between stage and flux at these depths. Of the 21 statistically significant correlations, 15 wells have a weak positive correlation between stage and flux with correlation coefficients ranging from 0.050 to 0.377 with an average of 0.159.