The starting head (L) consists of one IDF (or a constant value) for each modellayer specifying for each cell the initial head to start the model simulation. The starting heads correspond with the initial heads specified in the MODFLOW BAS package.
For different grid resolutions the starting head is scaled internally via iMOD or iMODFLOW. For downscaling (e.g. from an input of 100 x 100 meter to a finer resolution of 10 x 10 meter) all finer grid cells obtain an interpolated value of the original cell. A four-point polynomial interpolation is used for this, which gives a smooth interpolation based upon the direct neighbouring grid cell centres that are not equal to the NodataValue. If none is found, the up-scaled value remains equal to the NodataValue. It might be advisable to store starting head conditions on a coarse scale, as they will be smoothed anyhow, prior to the simulation. For up-scaling, a simple approach is applied by computing the average value of all finer grid cells inside the coarse cell. Values that are equal to the NodataValue are excluded and if they all appear to be equal to the NodataValue, the up-scaled value becomes NodataValue as well. Bear in mind that iMOD does not take into account whether grid cells are partially overlapping cells. It simply uses grid cells that are inside a coarse grid cell based upon their grid centres.