Global prediction of δA and δ²H‐δ¹⁸O evaporation slopes for lakes and soil water accounting for seasonality

Global trends in the δ 2 H‐ δ 18 O enrichment slope of continental lakes and shallow soil water undergoing natural evaporation are predicted on the basis of a steady state isotope balance model using basic monthly climate data (i.e., temperature and humidity), isotopes in precipitation data, and a simple equilibrium liquid‐vapor model to estimate isotopes in atmospheric moisture. The approach, which demonstrates the extension of well‐known conceptual models in stable isotope hydrology to the global scale, is intended to serve as a baseline reference for evaluating field‐based isotope measurements of vapor, surface water, and soil water and as a diagnostic tool for more complex ecosystem models, including isotope‐equipped climate models. Our simulations reproduce the observed local evaporation line slopes (4–5 range for lakes and 2–3 range for soil water) for South America, Africa, Australia, and Europe. A systematic increase in slopes (5–8 range for lakes) toward the high latitudes is also predicted for lakes and soil water in northern North America, Asia, and Antarctica illustrating a latitudinal (mainly seasonality‐related) control on the evaporation signals that has not been widely reported. The over‐riding control on the poleward steepening of the local evaporation lines is found to be the isotopic separation between evaporation‐flux‐weighted atmospheric moisture and annual precipitation, and to lesser extents temperature and humidity, all of which are influenced by enhanced seasonality in cold regions.