You have to prepare a 2D unstructured data file (dfsu) or a 2D grid data file (dfs2) that contains the flow properties. The area in the data file must cover the model area. If a dfsu-file is used, a piecewice constant interpolation is used to map the data. If a dfs2-file is used, a bilinear interpolation is used to map the data. The program will interpolate if the time step is not equal to that of the simulation.