Model Overview
Microclimate model
The Niche Mapper microclimate model was first described in detail by Porter et al. (1973) and Beckman
et al. (1973), with further descriptions in Porter and Mitchell (2006) and Fuentes and Porter (2013). It
has been show to compute soil surface conditions with 2-3° C accuracy (Porter et al. 1973; Porter &
James 1979). The model includes routines for hourly calculations of solar and infra-red radiation
intensities, above-ground profiles of air temperature, wind velocity, and relative humidity, and soil
temperature profiles at 10 nodes from the surface down to a user-specified maximum depth. The
present study used soil nodes of 0, 2.5, 5, 10, 15, 20, 30, 50, 100 and 200cm, with wind speed and air
temperature adjusted from the reference height of 1.2m to the local height of 1cm. These calculations
are made for two user-provided extremes of shading by vegetation for each run of the model.
The model requires as input the maximum and minimum daily values of air temperature, wind speed,
relative humidity and cloud cover, the timing of the maxima and minima relative to dawn or solar noon,
soil properties (conductivity, specific heat, density, solar reflectivity, emissivity) as well as the roughness
height, slope and aspect. Clear sky solar radiation is computed based on latitude and longitude using the
algorithm described in McCullough and Porter (1971). In the present version, aerosol attenuation is
computed based on the Aerosol Data Set (GADS) (Koepke et al. 1997), instead of the original profile
from Elterman (1968, 1970). Climatic data were drawn from a global data set of monthly mean daily
maximum and minimum air temperature and monthly mean daily relative humidity, wind speed and
cloud cover (1961–1990, 10’ resolution, New et al. 2002).
Sky temperature was related to air temperature, cloud cover and humidity according to Campbell and
Norman (Campbell & Norman 1998 eqn. 10.10). Air and wind velocity profiles are assumed to follow
logarithmic profiles (Porter et al. 1973 eqns. 2-3). Soil temperatures are computed based on a onedimensional partial differential equation that uses two boundary conditions – the soil surface
temperature and the deep soil temperature (Porter et al. 1973 eqn. 4). The soil surface temperature is
computed via a heat balance equation, accounting for heat exchange via radiation, convection,
conduction and evaporation (Porter et al. 1973 eqns. 5-11). The present version of the model allows soil
properties to vary with depth, as described in Fuentes and Porter (2013). In the calculations for this
study, no evaporative heat exchange was assumed and the deep soil temperature was assumed to be
the mean annual air temperature.
Ectotherm model
The ectotherm model of Niche Mapper solves a steady-state heat budget on the hour as a function of
available microclimatic variation in solar and infra-red radiation, air temperature, wind speed, relative
humidity and soil temperature. The following equations for the heat budget terms are found in Porter et
al. (1973): solar radiation (eqn. 13), infra-red radiation (eqn. 14), convection (eqn. 15). For convection,
the Nusselt-Reynolds number relationship derived for Dipsosaurus dorsalis developed by Porter et al.
(1973) was applied. Conduction was assumed to be minimal (10% of body surface area in contact with
ground, Av) and was modelled as Qcond = AV(ksub/D2)(Tb-Tsub) where ksub is the substrate thermal
conductivity, Tb is body temperature and Tsub is substrate temperature. No evaporative heat loss from
the skin was assumed. The model has been demonstrated to predict operative temperatures for small (5
g) lizard-shaped objects within 1 °C on average under a wide range of weather conditions (Kearney et al.
2009).
For each hour of the simulation, the model checks the daylight conditions (day, night, dusk/dawn) as
reported by the microclimate model (solar zenith angle). If light conditions are unsuitable for activity,
the animal is assumed to remain in its retreat site which, in the present study, was underground. During
‘inactivity’, if conditions are within voluntary limits at the shallowest depth, then the animal is assumed
to remain there, but will otherwise move to deeper nodes in the soil. If light conditions are suitable for
activity, a heat budget is solved for surface conditions starting at the minimum shade level. If conditions
are within the voluntary range, the animal is considered active and is simulated to be on the surface
with the resultant body temperature. If conditions are too hot, shade is incrementally increased by 3%
intervals until the maximum shade is reached. If shade selection is sufficient for body temperature to
remain within voluntary limits on the surface, the animal is simulated to remain on the surface.
Otherwise, the animal is simulated to retreat and is recorded as inactive.
References
Beckman W.A., Mitchell J.W. & Porter W.P. (1973). Thermal Model for Prediction of a Desert Iguana's
Daily and Seasonal Behavior. Journal of Heat Transfer, 95, 257-262.
Campbell G.S. & Norman J.M. (1998). Environmental Biophysics. Springer, New York.
Elterman L. (1968). UV, visible and IR attenuation for altitudes to 59 km. In. U. S. Airforce Cambridge
Research Laboratory Bedford, Mass.
Elterman L. (1970). Vertical-attenuation model with eight surface meteorological ranges 2 to 13
kilometers. In. U. S. Airforce Cambridge Research Laboratory Bedford, Mass.
Fuentes M.M.P.B. & Porter W.P. (2013). Using a microclimate model to evaluate impacts of climate
change on sea turtles. Ecological Modelling, 251, 150-157.
Kearney M., Shine R. & Porter W.P. (2009). The potential for behavioral thermoregulation to buffer
‘cold-blooded’ animals against climate warming. Proceedings of the National Academy of
Science, 106, 3835-3840.
Koepke P., Hess M., Schult I. & Shettle E.P. (1997). Global Aerosol Data Set. In. Max-Planck-Institut für
Meteorologie Hamburg.
McCullough E.C. & Porter W.P. (1971). Computing clear day solar radiation spectra for the terrestrial
ecological environment. Ecology, 52, 1008-1015.
New M., Lister D., Hulme M. & Makin I. (2002). A high-resolution data set of surface climate over global
land areas. Climate Research, 21, 1-25.
Porter W.P. & James F.C. (1979). Behavioral implications of mechanistic ecology II: the African Rainbow
Lizard, Agama agama. Copeia, 1979, 594-619.
Porter W.P. & Mitchell J.W. (2006). Method and system for calculating the spatial-temporal effects of
climate and other environmental conditions on animals. In:
http://www.patentstorm.us/patents/7155377-fulltext.html (ed. Office UP). Wisconsin Alumni
Research Foundation USA.
Porter W.P., Mitchell J.W., Beckman W.A. & DeWitt C.B. (1973). Behavioral implications of mechanistic
ecology - Thermal and behavioral modeling of desert ectotherms and their microenvironment.
Oecologia, 13, 1-54.