The Global Meteoric Water Line is an equation defined by the geochemist Harmon Craig that states the average relationship between hydrogen and oxygen isotope ratios in natural terrestrial waters, expressed as a worldwide average.
A meteoric water line can also be calculated for a given area, and used as a baseline within that area. Kinetic fractionation will cause the isotope ratios to vary between localities within that area. This relationship is used within the field of isotope hydrology. Craig's original assertion is that the isotopic enrichments, relative to ocean water, display a linear correlation over the entire range for waters which have not undergone excessive evaporation'.