Return L-CV weighting for a donor catchment.

        Methodology source: Science Report SC050050, eqn. 6.18 and 6.22a

def _l_cv_weight(self, donor_catchment):
        """
        Return L-CV weighting for a donor catchment.

        Methodology source: Science Report SC050050, eqn. 6.18 and 6.22a
        """
        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)