The isotopic composition of hydrogen and oxygen in water varies in waters of different sources, and isotopic composition changes through mixing of water masses and through processes such as evaporation. For instance, the isotope composition in surface water is often different from the isotope composition in precipitation and groundwater. This isotopic variation can be useful in a variety of hydrological, ecological, and climate science studies. Bowen et al. developed a model based on geographic information system data to predict long-term annual average surface water isotope ratios across the United States. They validated their model with data from a water monitoring program and showed that their map reproduced the observed pattern of isotopic variation across the United States.