The correlation uses three parameters for ground temperature at the surface to define a correlation for undisturbed ground temperatures as a function of depth and time. The Kusuda-Achebach method provides an undisturbed ground temperature based on the correlation developed by Kusuda T. The model does not consider the effects of vegetative canopy layers, snow cover, ground water flow, ground moisture transport, or surface runoff. Soil freezing given the assumed stagnant soil moisture content is also considered. Evapotranspiration is considered as described by Allen et al., 1998. Surface heat balance boundary conditions are similar what is described in Herb et al., 2008. The basis for the model was taken from Xing, 2014, however, the numerical methods were adapted from those described in Lee, 2013 the latter uses an implicit numerically stable finite difference method, whereas the former utilized an explicit, conditionally stable method. Once the steady-periodic ground temperature has been determined, the temperatures are cached for use later in the simulation. The model, which uses a daily timestep, is run through an annual simulation using the user provided weather file to determine daily averages for global horizontal radiation, air temperature, relative humidity, and wind speed. This model uses a 1-D implicit finite difference heat transfer model to determine the steady periodic annual ground temperature. No additional data needs to be entered for this option. Once steady periodic behaviour is reached, the ground temperatures are cached for retrieval during the rest of the simulation. An annual simulation is run on the ground model during it’s initialization until the annual ground temperature profile has reached steady periodic behaviour. The Finite Difference method uses a 1-D finite difference heat transfer model which uses the weather file to obtain surface boundary conditions. Note that all of the calculation methods also require the ground thermal properties as defined by the soil material on the Soil tab. More information on these options is provided below.
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