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<p>The Keeling plot approach, a general method to identify the isotopic composition of source atmospheric <span class="inline-formula">CO₂</span> and water vapor (i.e., evapotranspiration), has been widely used in terrestrial ecosystems. The isotopic composition of ambient water vapor (<span class="inline-formula"><i>δ</i>_a</span>), an important source of atmospheric water vapor, is not able to be estimated to date using the Keeling plot approach. Here we proposed two new methods to estimate <span class="inline-formula"><i>δ</i>_a</span> using the Keeling plots: one using an intersection point method and another relying on the intermediate value theorem. As the actual <span class="inline-formula"><i>δ</i>_a</span> value was difficult to measure directly, we used two indirect approaches to validate our new methods. First, we performed external vapor tracking using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to facilitate explaining the variations of <span class="inline-formula"><i>δ</i>_a</span>. The trajectory vapor origin results were consistent with the expectations of the <span class="inline-formula"><i>δ</i>_a</span> values estimated by these two methods. Second, regression analysis was used to evaluate the relationship between <span class="inline-formula"><i>δ</i>_a</span> values estimated from these two independent methods, and they are in strong agreement. This study provides an analytical framework to estimate <span class="inline-formula"><i>δ</i>_a</span> using existing facilities and provides important insights into the traditional Keeling plot approach by showing (a) a possibility to calculate the proportion of evapotranspiration fluxes to total atmospheric vapor using the same instrumental setup for the traditional Keeling plot investigations and (b) perspectives on the estimation of isotope composition of ambient <span class="inline-formula">CO₂</span> (<span class="inline-formula"><i>δ</i>_a¹³C</span>).</p>