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.
Janke, Ben; Mohseni, Omid; Herb, William R.; Stefan, Heinz G..
Heating of Rainfall Runoff on Residential and Commercial Roofs.
St. Anthony Falls Laboratory.
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