Currently it is not possible to read one input series for one element from several input files.
Reason is that in the method adjust_short_series all values which are within the general timegrid but not within the timegrid of the input file are set to nan or the default value respectively. A new option could allow that the values not within the timegrid are not set to the default, but to the old series. This could look like:

        if hydpy.pub.options.resetseries == False:
            values = self.series
        else:
            values = numpy.full(self.seriesshape, float(self.initinfo[0]))