Simulation models can be efficient tools, provided they are sufficiently validated. In the United States, the HELP model was developed to simulate the water balance of landfills, especially of liner systems. HELP version 3 was examined under the humid climate of Germany in an extensive validation study. The aim of this paper is to put the validation results into practice by showing the potential and limitations of applying the HELP model for surface cover systems using examples of practical planning. The examples cover the components of surface covers: topsoil and vegetation, a compacted layer at the bottom of the topsoil, drainage layers and drainage nets, geomembranes, and several liners. Potential errors are shown and classified, and recommendations for critical application cases are given. In summary, the HELP model is a suitable tool for instructed users, but good knowledge of the model and its behavior and critical review of the simulation results are essential. A German enhancement of the HELP model, HELP 3.50 D, was developed. It fixes errors shown in this article and enhances some processes.