code: def _pruaf(self):
          
          return 1 + 0.47 * self.catchment.descriptors.urbext(self.year) \
                     * self.catchment.descriptors.bfihost / (1 - self.catchment.descriptors.bfihost)
docs: Return percentage runoff urban adjustment factor.
  
          Methodology source: eqn. 6, Kjeldsen 2010