Estimation of spatial covariance structures with application to hydrological, hydrochemical, and isotopic data from aquifers: state-of-the-art and adjoint state maximum likelihood cross-validation methods : Samper, F J; Neuman, S P Proc NATO Advanced Research Workshop on Advances in Analytical and Numerical Groundwater Flow and Quality Modelling, Lisbon, 2–6 June 1987P585–620. Publ Dordrecht: Reidel, 1987