def _vec_b(self, donor_catchments):
        """
        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`
        """
        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