Heat transfer takes place simultaneously with moisture transfer in unsaturated soil and the transport processes interact with a ground heat exchanger (GHX) installed for a ground source heat pump. This paper presents a numerical methodology to produce spatiotemporally varying soil properties and assess their interactions with a horizontal GHX and the impacts on its dynamic performance. First, moisture and temperature profiles are generated from the numerical solution of coupled one-dimensional heat and moisture transfer equations and using soil properties that vary with time and depth. Next, governing equations for a three-dimensional mathematical model are solved numerically for a system of atmosphere, soil and horizontally coupled GHX. The model is then used to assess the thermal performance of the GHX under different atmospheric conditions at five installation depths in five soil types. Results demonstrate the importance of coupling moisture transfer with heat transfer in a mathematical model for dynamic thermal simulation of heat exchangers in shallow ground. The atmospheric conditions have been shown to have significant impacts on the dynamic performance which cannot be predicted using a model for heat transfer only. The heat transfer rate of a horizontal GHX varies with time and the depth of installation and the total amount of heat transfer for an operating season increases with installation depth. Heat transfer in moist sandy soil is found to be higher than that in loamy sand soil and much higher than that in loam, clay loam or clay soil.