Fig. 1: The diagram of water vapor and δD (δD_004) dynamics.

a The processes influencing δD_004 variability, shown on a plot of water vapor volume mixing ratio (y-axis) versus δD (x-axis). For a constant water vapor volume mixing ratio (4 mmol mol⁻¹, flat gray line labeled “Reference VMR”), variations in the hydrogen isotope ratio (δD_004, shown in the Figure as δD₀₀₄) represent the shifting importance of precipitation vs. evapotranspiration (ET-P). The precise scaling of δD_004 to ET-P will be modified by the efficiency of rainout (i.e., the efficiency with which cloud condensate is converted to rain) and by the source of moisture to the atmosphere. For example, the two-sided black arrow shows the expected range of δD_004 if oceanic evaporation is the sole source of moisture to the atmosphere (blue line) and condensate formed during convection is immediately removed from the atmosphere by precipitation (red line). The intersection of the blue and red lines with the “Reference VMR” line explicitly shows the expected δD_004 values if P = 0 or ET = 0, respectively. As the contribution of transpiration to atmospheric moistening increases (green line), the δD_004 range will extend to the right, causing the expected δD_004 value to be higher when P = 0. Contrastingly, as either rain evaporation (purple dashed line) or remote moisture convergence (orange line) becomes important, the δD_004 range will extend to lower isotope ratios, causing the expected δD_004 value to be lower when ET = 0. Decreasing the efficiency with which condensate forms precipitation (pink dotted line) will, in comparison, increase the expected δD_004 value when ET = 0, limiting the expected δD_004 range for a given set of ET-P states. b The schematic illustrates the key processes in (a).