This study presents a novel performance evaluation algorithm for horizontal GCHPs (ground coupled heat pump systems) that considers rainfall infiltration. The influence of rainfall infiltration on the thermal characteristics of shallow trenches is examined using infiltration analyses, and then a numerical analysis study is conducted in order to investigate how rainfall infiltration affects the performance of HGHEs (horizontal ground heat exchangers). According to the thermal performance test results in unsaturated ground with a varying thermal conductivity profile, the rainfall infiltration results in a widening fluid temperature gap between the inlet and outlet, and it increases the thermal efficiency compared with that without rainfall. Furthermore, in fully saturated ground, groundwater advection has a positive influence on the performance of the heat exchanger, and the advection effect varies with the local ground conditions such as hydraulic conductivity and void ratios. In the cooling mode, the free convection phenomenon occurs in shallow trenches, and this fluid circulation attenuates the ground temperature increases due to the heat source, which leads to a significantly faster heat steady state. However, noticeable free convection only occurs if the ground has a high permeability coefficient. (C) 2015 Elsevier Ltd. All rights reserved.