code: def update_watervolume_v1(self):
      
      der = self.parameters.derived.fastaccess
      flu = self.sequences.fluxes.fastaccess
      old = self.sequences.states.fastaccess_old
      new = self.sequences.states.fastaccess_new
      new.watervolume = (old.watervolume +
                         der.seconds*(flu.inflow-flu.outflow)/1e6)
docs: Update the actual water volume.
  
      Required derived parameter:
        |Seconds|
  
      Required flux sequences:
        |Inflow|
        |Outflow|
  
      Updated state sequence:
        |WaterVolume|
  
      Basic equation:
        :math:`\\frac{d}{dt}WaterVolume = 1e-6 \\cdot (Inflow-Outflow)`
  
      Example:
  
          >>> from hydpy.models.dam import *
          >>> parameterstep()
          >>> derived.seconds = 2e6
          >>> states.watervolume.old = 5.0
          >>> fluxes.inflow = 2.0
          >>> fluxes.outflow = 3.0
          >>> model.update_watervolume_v1()
          >>> states.watervolume
          watervolume(3.0)