Heating of Rainfall Runoff on Residential and Commercial Roofs

A common assumption in stream water temperature modeling is that rooftops of all types contribute very little heat to runoff from rainfall. In this report we examine the accuracy of this assumption (a) by analyzing temperature data which we recorded on a residential rooftop, a commercial rooftop, and a concrete driveway, and (b) by simulating temperature profiles within rooftops and pavements, and estimating heat transfer from these surfaces to rainfall runoff. Analysis of both wet- and dry-weather temperature data which we recorded over periods of several months allowed us to conclude that a driveway has a far greater capacity for heat storage and release than a rooftop, although the commercial rooftop was able to store and release more heat than the residential rooftop. On sunny days and prior to rainfall, rooftops can reach higher temperatures than paved surfaces, but not much heat is stored, and roof temperatures drop rapidly as cloud cover increases with an approaching storm. Interestingly, weather events leading to the highest dew point (rainfall) and surface temperatures often occurred during late night or early morning hours, contrary to the expectation that the worst-case runoff heating events would occur during daylight hours. The analysis conducted for three rainfall events showed that the heat export from the commercial rooftop was roughly three times that of the residential rooftop, but only 30%-90% of the heat export from the concrete driveway. Potential heat export was significantly higher for the driveway than for either rooftop. In conclusion, the results of the data analysis and heat export simulations support the assumption that residential rooftops contribute very little heating to runoff from rainfall. Commercial rooftops may have a thermal impact on rainfall runoff because of their greater thermal storage capacity. An asphalt pavement, (road or driveway) is expected to have a greater thermal impact than a concrete pavement. Commercial rooftops in addition to asphalt and concrete pavements should be considered when the water temperatures of rainfall runoff from highly urbanized areas are estimated.