Heat tracer test in a riparian zone: Laboratory experiments and numerical modelling

Abstract

The use of water temperature as a tracer has been widely utilized for characterizing the dynamics of water flow and heat transport in riparian zones. By using experimental approaches, we investigated the effects of water temperature, hydraulic head, and heat radiation on water flow and thermal dynamics in riparian zones through sand tank experiments, which simulate the dynamics of water flow and heat transport in riparian zones. Water of low temperature was pumped into the sand tank, and changes of temperature at different locations of the sand tank were measured. Temperature data was examined for three different water temperatures (4.0 °C, 6.0 °C, and 9.5 °C), two different hydraulic heads (25 cm and 45 cm), and two different radiation temperatures (no radiation and 22 °C). The thermal dynamic variation pattern in different types of water temperatures, hydraulic heads, and radiation combinations, was also analyzed using a HYDRUS-2D model. The temperature sensors, located near the inlet infiltration boundary, required a shorter time to reach the steady state, because the temperature declined more rapidly near the inlet. The effect of lateral inflow on the temperature gradient was obvious. The temperature gradient in the horizontal direction gradually decreased, and the vertical temperature gradient gradually increased. In the initial stage of infiltration, the temperature gradient in the horizontal direction was larger than the temperature gradient in the vertical direction, however, as time goes on, the temperature gradient in the vertical direction was larger than the temperature gradient in the horizontal direction. In addition, the horizontal temperature gradient of the top sand layer was less affected by the water temperature. The closer the temperature observation point of the same horizontal section was to the infiltration boundary, the higher the rate of temperature difference changes. Comparison of the predicted and observed thermal dynamics variation of the 2-D sand tank shows good agreement, indicating that the major mechanism for water flow and thermal dynamics variation was hydraulic head. The sensitivity analysis results illustrate that the model was most sensitive to hydraulic head (H), followed by Van Genuchten parameter (α), permeability coefficient (Ks), water temperature (T), Van Genuchten parameter (n), residual moisture content (θr), and saturated moisture content (θs). The variation of each parameter was linear with the change of temperature field. Parameters which were positively related to the temperature field were T, α, θr and θs, which means the parameter value becomes larger as the temperature becomes larger, and vice versa. The parameters which were negatively related to the temperature field were H, Ks and n.