Comparison of heat tracer models in the estimation of upward flux through streambed sediments

Abstract

Although many studies examine the use of heat as a tracer to determine vertical infiltrationof water into the streambed, few consider the case where both water and heatflow (advection and conduction) are in the upwards direction. In this study, we comparethe usefulness of both a one-dimensional numerical model and two analytical solutionsfor the case where water movement is upwards and the (flat) groundwater temperatureis greater than the diel temperatures at the surface. We first create a theoretical testcase to compare expected temperature traces at various depths within the sedimentcolumn for each model both in the presence and absence of a vertical temperaturegradient. These theoretical results are discussed in light of the assumptions inherentin the models. Then the models are applied to a study area located along a reach ofthe Truckee River in Nevada, USA, during the winter season and flux estimates bothbetween models and between sensor depths are compared. Our results show that despiteviolation of some assumptions inherent in the analytical models, flux estimatesover the entire vertical streambed column can be within one order of magnitude ofthe numerical model under some conditions. Further, predictions of downwards fluxobtained using only the shallow sensors highlight the need to consider the physicalprocesses to be measured when choosing sensor depth, especially when advectionand conduction are upwards.