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Diurnal variations in water temperature in a section of the West Branch Susquehanna River near Lewisburg, PADiurnal variations in water temperature in a section of the West Branch Susquehanna River near Lewisburg, PA

    Foley, Tessa, Department of Environmental Studies, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, tbf005@bucknell.edu; Hayes, Benjamin R., Watershed Sciences and Engineering Program, Bucknell Center for Sustainability and the Environment, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837, brh010@bucknell.edu.

    To assess the spatial and temporal variability in water temperatures in the West Branch of the Susquehanna River, a network of nine buoys were deployed at three cross-section locations near Milton, Lewisburg, and Chillisquaque. The buoys were equipped with data loggers and sensors to record relative humidity, air temperature (℃), solar radiation (W m-2), and water temperature (℃) at 0.15-meter depth as well as 0.61-meter depth. Readings were collected every 15 minutes from May to August. The buoys were anchored to the bed of the river with cinder blocks with sensors to record water temperature (℃) at bed of the river and total water depth.

    Longitudinally, water temperatures consistently increased by 0.2 to 0.5 ℃ km-1 as a result of shortwave radiation from the sun and latent heat transfer from warmer air immediately above the water. Lateral variations in temperatures were considerably more complex due to changes in water depth, riparian shading, and mixing of water currents. Vertically, temperatures in the deepest section of the channel typically increase 1-2 °C from the water surface to the riverbed, reflecting long-wave radiation conduction that penetrates the water column and warms the low albedo and dark, iron and magnesium-stained cobbles that cover the bed of the river. Generally, water temperatures are a subdued reflection of air temperatures, with atmospheric conditions and solar radiation exerting significant influence over the diurnal fluctuations in river temperatures. Typically, peak diurnal water temperatures occur several hours later than peak solar radiation each day. Data from buoys positioned approximately fifteen meters from the channel banks indicate that shading from the riparian corridor is the primary factor affecting temperature variability along the river’s edges. For example, for the network of three buoys deployed at the Milton cross-section, solar radiation reached a maximum of 913 W/m² at the right bank, compared to a minimum of 856 W/m² at the left bank on June 13, 2024. Water temperatures decreased by as much as 5.4 ℃ after the sun descended below the shaded areas of the channel.

    river  temperature, solar radiation, heat transfer