Using the spatial analyst hydrology tools it is quite simple to create streams, basins, watersheds, sinks etc. What I'm missing is a way to fill the different basins: if I've got my flow direction raster I can create a flow accumulation raster and a basin raster. The assumption is that the cell with the highest flow accumulation value within a basin is the lowest point within that basin. What I would like to do is, based on a certain amount of 'rainfall' (value in millimeters x cellsize x highest flow accumulation value) determine which part of the basin is filled with that volume. Since the resulting level in the basin is unknown (that should be the output) it is not a simple case of extracting two grids from each other. Does anyone know a method to achieve this?