code: def update(self):
          
          con = self.subpars.pars.control
          temp = con.zonearea.values.copy()
          temp[con.zonetype.values == GLACIER] = 0.
          temp[con.zonetype.values == ILAKE] = 0.
          self(numpy.sum(temp)/con.area)
docs: Update |RelSoilArea| based on |Area|, |ZoneArea|, and |ZoneType|.
  
          >>> from hydpy.models.hland import *
          >>> parameterstep('1d')
          >>> nmbzones(4)
          >>> zonetype(FIELD, FOREST, GLACIER, ILAKE)
          >>> area(100.0)
          >>> zonearea(10.0, 20.0, 30.0, 40.0)
          >>> derived.relsoilarea.update()
          >>> derived.relsoilarea
          relsoilarea(0.3)