Heat Export and Runoff Temperature Analysis for Rainfall Event Selection

Thermal pollution by surface runoff from urban areas can contribute to the degradation of coldwater ecosystems. The hydrothermal characteristics of surface runoff from rainfall are therefore of interest. Three hydrothermal parameters of surface runoff have been studied: runoff temperature (oC), heat flux (W/m2) and total heat export (J/m2). Heat fluxes were defined above a reference temperature of 20oC. The results can be used to identify storm events that have the potential for the largest heat export from a watershed and consequently the strongest thermal pollution of a receiving coldwater stream. In this study, records of rainfall events and weather data are used to estimate the three hydrothermal parameters by model simulation. The model for predicting rainfall runoff temperatures and rates from an impervious surface (parking lot) has been described in Project Report No. 484 from the St. Anthony Falls Laboratory, University of Minnesota (Herb et al 2006). The weather data came from the MnROAD test site in Albertville, MN, and from the SAMSON data set. Runoff temperatures and heat export were calculated for a 100x100m paved surface using 6 years of 15 minute weather data or 30 years of 1-hour weather data. The 6-year data set contained 280 rainfall events from April through October. The 280 values of the three hydrothermal parameters were related to basic rainfall event parameters such as total rainfall, duration, and rainfall temperature (dew point). Average runoff temperature was found to be well correlated to dew point temperature during the storm, and air temperature and solar radiation prior to the storm. 20 extreme values of the hydrothermal parameters were ranked and also related to basic rainfall parameters. Partial duration series of hydrothermal parameters were analyzed separately for frequency of occurrence (return periods).