Thermal pollution from urban runoff is considered to be a significant contributor to the
degradation of coldwater ecosystems. Impervious surfaces (streets, parking lots and buildings)
are characteristic of urban watersheds. A model for predicting temperature time series for dry
and wet ground surfaces is described in this report. The model has been developed from basic
principles. It is a portion of a larger project to develop a modeling tool to assess the impact of
urban development on the temperature of coldwater streams. Heat transfer processes on
impervious and pervious ground surfaces were investigated for both dry and wet weather
periods. The principal goal of the effort was to formulate and test equations that quantify the heat
fluxes across a ground surface before, during and after a rainfall event. These equations were
combined with a numerical approximation of the 1-D unsteady heat diffusion equation to
calculate temperature distributions in the ground beginning at the ground surface. Equations to
predict the magnitude of the radiative, convective, conductive and evaporative heat fluxes at a
dry or wet surface, using standard climate data as input, were developed. Plant canopies were
included for surfaces covered by vegetation. The model can simulate the ground surface and subsurface
temperatures continuously throughout a specified time period (e.g. a summer season) or
for a single rainfall event.
Ground temperatures have been successfully simulated for pavements, bare soil, short and tall
grass, trees and two agricultural crops (corn and soybeans). The simulations were first run for
different locations and different years as imposed by the availability of measured soil
temperature and climate data. Data came from sites in Minnesota, Illinois and Vermont. To
clarify the effect of different land uses on ground temperatures, the calibrated coefficients for
each land use and the same soil coefficients were used to simulate surface temperatures for a
single climate data set from St. Paul, MN (2004). Asphalt and concrete give the highest surface
temperatures, as expected, while vegetated surfaces gave the lowest. Bare soil gives surface
temperatures that lie between those for pavements and plant-covered surfaces. The soil
temperature and moisture model appears to model surface temperatures of bare soil and
pavement with RMSEs of 1 to 2°C, and surface temperatures of vegetation-covered surfaces with
RMSEs of 1 to 3oC.
The plant canopy model used in this study, based on the work of Best and Deardorff, provides an
adequate approximation for the effect of vegetation on surface heat transfer, using only a few
additional parameters compared to bare surfaces. While further simplifications of the model are
possible, such simplifications do not reduce the number of required input parameters, and do not
eliminate the need for estimating the seasonal variation of the vegetation density.
A model for roof temperatures was also developed, based on the surface heat transfer
formulations used for pavement. The model has been calibrated for both a commercial tar/gravel
roof and a residential roof. Compared to pavement, the roof surface reach similarly high
maximum temperatures, but reach lower minimum temperature at night cool due to their lower
Herb, William R.; Janke, Ben; Mohseni, Omid; Stefan, Heinz G..
All-Weather Ground Surface Temperature Simulation.
St. Anthony Falls Laboratory.
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