Methodology source: FEH, Vol. 3, p. 14

def _residual_soil(self):
        """
        Methodology source: FEH, Vol. 3, p. 14
        """
        return self.catchment.descriptors.bfihost \
               + 1.3 * (0.01 * self.catchment.descriptors.sprhost) \
               - 0.987