Simulation of heat export by rainfall-runoff from a paved surface

The overall goal of our research has been to create a hydro-thermal model to quantify the impact on stream temperature of urban development in watersheds of cold-water streams. In this paper we describe a significant component of that modeling effort, namely a sub-model for predicting storm water runoff rates and temperatures from paved surfaces during a rainfall event. An unsteady 1-D model has been formulated from basic principles of heat transfer and runoff processes on planar impervious surfaces for both dry- and wet-weather periods. The model predicts runoff flow rates and temperatures as a function of both distance and time on a paved surface, taking into account the magnitude of the radiative, convective, evaporative and conductive heat fluxes at the surface. It also predicts the 'total heat export' for an event, which is defined as the heat contained in the runoff above a reference temperature. A case study is presented in which the model is applied to a parking lot, and simulated runoff flow, temperature, and heat export closely match observations of these quantities for an early-evening rainfall event in August. A sensitivity study was performed to investigate to which extent heat export is affected by antecedent pavement temperature, characteristics of the rainfall event, and physical parameters of the paved surface. It was found that heat export is more sensitive to rainfall intensity, rainfall duration, and antecedent pavement temperature than the physical properties (slope, roughness, and length) of the paved surface. An increase in rainfall duration increases the total event heat export, especially for high-intensity events. Heat exchange with the atmosphere typically reduces runoff temperature and heat export versus a case in which atmospheric heat fluxes are neglected, with greater effect occurring as rainfall intensity decreases.