Although horizontal flow constructed wetlands (HF CWs) are usually subjected to unsteady loads in real application, the modelling of HF CW response to time-variable loads has been scarcely studied in literature yet. The aim of this study is to test the capability of HYDRUS-CWM1 to simulate the behavior of HF CWs subjected to unsteady loads. Hence, we applied HYDRUS-CWM1 to simulate laboratory results of HF CWs subjected to variable COD inflows. The modelling results adequately fit the experimental data, with an almost perfect agreement in global COD removal efficiencies (67 and 68% from laboratory experiments and simulations, respectively), and mean percent error equal to 20 and 31% for effluent COD and NHView the MathML source4+ concentrations, respectively. The obtained results suggest that HYDRUS-CWM1 can be a powerful tool to simulate the response of HF CWs under time-variable loads. Additionally, more detailed data are shown to be crucial in order to better exploit process-based model tools