You should if possible avoid placing sources and sinks in points that are subjected to flooding and drying. If you have a source or sink in such a point, it will be inactivated when the point is dry during a sweep. But a separate mass budget is performed at all dried source/sink points such that the mass outlet/intake at a source/sink is correct. That is, for sources at dried points the mass outlet is accounted for until the particular point eventually floods (i.e. water depth increases the flooding depth) and thus enters the computations in the usual way. For sinks at dried points the sink intake is subtracted from the particular point until the water depths becomes very small (MIKE 21 cannot handle artificially generated non-positive water depths).