code: def _l_cv_weight(self, donor_catchment):
          
          try:
              dist = donor_catchment.similarity_dist
          except AttributeError:
              dist = self._similarity_distance(self.catchment, donor_catchment)
          b = 0.0047 * sqrt(dist) + 0.0023 / 2
          c = 0.02609 / (donor_catchment.record_length - 1)
          return 1 / (b + c)
docs: Return L-CV weighting for a donor catchment.
  
          Methodology source: Science Report SC050050, eqn. 6.18 and 6.22a