code: def _l_skew_weight(self, donor_catchment):
          
          try:
              dist = donor_catchment.similarity_dist
          except AttributeError:
              dist = self._similarity_distance(self.catchment, donor_catchment)
          b = 0.0219 * (1 - exp(-dist / 0.2360))
          c = 0.2743 / (donor_catchment.record_length - 2)
          return 1 / (b + c)
docs: Return L-SKEW weighting for donor catchment.
  
          Methodology source: Science Report SC050050, eqn. 6.19 and 6.22b