Abstract
Sediment size governs advection, controlling the hydraulic conductivity of the stratum, and conduction, influencing the amount of surface area in contact between the sediment particles. To understand the role of sediment particle size on thermal profiles within the hyporheic zone, a statistical approach, involving general summary statistics and time series cross-correlation, was employed. Data were collected along two riffles: Site 1: gravel (d50 = 3.9 mm) and Site 2: sand (d50 = 0.94 mm).Temperature probe grids collected 15-minute temperature data at 30, 60, 90, and 140 cm below the streambed surface over a 6 month period. Surface water and air temperature was recorded. Diel temperature signal penetration depth was limited to the upper 30 cm of the streambed and was driven by advection. Surface seasonal trends were detected at greater depths, indicating that thermal pulses are transmitted initially by advection and by conduction to areas deeper in the hyporheic zone. Site 1 showed a high degree of thermal heterogeneity via a localized downwelling zone within a gaining stream environment. Site 2 exhibited a vertically and horizontally homogenized thermal environment attributed to an increased amount of sand sediments that limited advection and significant groundwater discharge that mediated the effects of downwelling surface water.
Highlights
Temperature is a basic parameter that controls physical, ecological, and biogeochemical activities in aquatic systems [1,2,3]
This study focuses on variations in temperature profiles of the hyporheic zone (HZ) at two sites: a gravel dominated HZ and a sand dominated HZ, with the hope of furthering existing knowledge of water temperature in the environment and providing another tool for characterizing HZs
Time series cross-correlation, as described by Mangin [20], was used to understand the relationships between streambed temperatures within each site, as well as between sites in more detail.Time series cross-correlation measures the relationship between two quantitative time series, i.e. surface water temperature compared to hyporheic water temperature.The observations of two series are correlated as various lags and leads, where the relationship is expressed by a cross-correlation coefficient (r) equal to a value between −1 and 1, with values closest to 1 indicating the strongest relationship between the time series
Summary
Temperature is a basic parameter that controls physical, ecological, and biogeochemical activities in aquatic systems [1,2,3]. HZ temperatures are controlled by the mixing of groundwater and surface water, reflecting the rates of infiltrating surface water and upwelling groundwater [10], disregarding geothermal influences, and surface water temperatures show both diel and seasonal fluctuations [11,12,13]. Dogwiler and Wicks [15] show that with increasing depth and/or distance from infiltration sites, the diel and seasonal fluctuations of surface water become attenuated and lagged. These patterns can be a valuable tool in defining HZ depth and extent [13,16,17,18]. Delineations of HZ extent are not constant through time, as shown by Fraser and Williams [19], whose results suggest that the extent of the HZ varies seasonally as well as with eventbased fluctuations [20]
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