Return vector alpha which is the weights for donor model errors

        Methodology source: Kjeldsen, Jones & Morris 2014, eq 10

        :param donor_catchments: Catchments to use as donors
        :type donor_catchments: list of :class:`Catchment`
        :return: Vector of donor weights
        :rtype: :class:`numpy.ndarray`

def _vec_alpha(self, donor_catchments):
        """
        Return vector alpha which is the weights for donor model errors

        Methodology source: Kjeldsen, Jones & Morris 2014, eq 10

        :param donor_catchments: Catchments to use as donors
        :type donor_catchments: list of :class:`Catchment`
        :return: Vector of donor weights
        :rtype: :class:`numpy.ndarray`
        """
        return np.dot(linalg.inv(self._matrix_omega(donor_catchments)), self._vec_b(donor_catchments))