Return percentage runoff urban adjustment factor.

        Methodology source: eqn. 6, Kjeldsen 2010

def _pruaf(self):
        """
        Return percentage runoff urban adjustment factor.

        Methodology source: eqn. 6, Kjeldsen 2010
        """
        return 1 + 0.47 * self.catchment.descriptors.urbext(self.year) \
                   * self.catchment.descriptors.bfihost / (1 - self.catchment.descriptors.bfihost)