IDENTIFYING SEASONAL VARIATIONS IN HYPORHEIC TEMPERATURE PROFILES IN A LOW-GRADIENT THIRD ORDER AGRICULTURAL STREAM USING 2-D THERMAL MODELING

The thermal profile of a streambed is affected by a number of factors including temperature of both stream water and ground water, hydraulic conductivity, thermal conductivity and heat capacity of the streambed, and the nature of hyporheic flowpaths. Seasonal changes in thermal profiles could therefore be documented in terms of seasonal changes in these parameters. In the study, temperature data were collected at depths of 30, 60, 90 and 150 cm at six hyporheic wells 5 meters apart along the thalweg of a third-order, low gradient stream. Thermal models of the streambed were built using VS2DHI to simulate the thermal profiles observed in the field. Deviations from observed temperature were calculated using mean absolute error (mae). Sensitivities of hydraulic and thermal parameters used in the model were identified and the more sensitive parameters were first corrected before adjusting for the less sensitive parameters. Models for a week long time period in April suggested a higher sensitivity of the model to saturated thermal conductivity, vertical flux and low sensitivity for porosity, dry heat capacity of the streambed, and vertical to horizontal hydraulic conductivity ratio. After adjusting parameter values the model improved to a mae of 0.248 °C. During the latter part of the study, this model will be used to simulate streambed temperatures for summer and winter time periods. It is anticipated that the model from April will not be a best fit model for either summer or winter time periods. Seasonal changes in parameters like hydraulic conductivity are expected due to changes in water viscosity or due to changes in the streambed properties as a result of scour and fill events. Sensitivity analysis for hydraulic and thermal parameters will be done to identify how these parameters behave differently from the original model and from each other. Lastly, adjustments to the parameters will be made to improve the model for summer and winter time periods by selecting parameter values associated with the least mae and the differences in these values will be documented to underline the seasonal variations in thermal profiles of the streambed.