code: def _vec_b(self, donor_catchments):
          
          p = len(donor_catchments)
          b = 0.1175 * np.ones(p)
          for i in range(p):
              b[i] *= self._model_error_corr(self.catchment, donor_catchments[i])
          return b
docs: Return vector ``b`` of model error covariances to estimate weights
  
          Methodology source: Kjeldsen, Jones and Morris, 2009, eqs 3 and 10
  
          :param donor_catchments: Catchments to use as donors
          :type donor_catchments: list of :class:`Catchment`
          :return: Model error covariance vector
          :rtype: :class:`numpy.ndarray`