Concurrent On‐Site Measurement of Local Meteoric Water Line Crucial for Short‐Term Isotope Ecohydrological Studies

Abstract Accurate determination of the contemporaneous local meteoric water line (LMWL) is critical for isotope ecohydrological studies, as it forms the baseline for identifying water sources and assessing the degree of evaporation in water bodies. Some studies, despite requiring contemporaneous LMWLs, have used approximations from past local or nearby records, or from isotopic landscapes (isoscapes); however, the uncertainties of such approximations remain poorly understood. Using monthly Global Network of Isotopes in Precipitation data, we quantify the likelihood that LMWLs are misrepresented by past records from the same site, past or concurrent records from nearby sites, and concurrent outputs from isoscapes and isotope‐enabled atmospheric general circulation models (iGCMs). Overall, past records underestimate both slopes and intercepts of LMWLs for one to 8 years. The likelihood of obtaining significantly different LMWLs at the same site is 16.1 ± 3.1% for slopes and 30.4 ± 6.0% for intercepts. For nearby sites, these likelihoods rise to 21.6 ± 5.6% (slopes) and 52.9 ± 6.1% (intercepts) for different periods, and 20.9 ± 8.1% (slopes) and 49.3 ± 7.0% (intercepts) for the same period, both increasing with the distance between sites. Longer records increase the likelihood of obtaining significantly different LMWLs, as they may smooth out interannual variations. Under similar precipitation, temperature and vapor pressure deficit, the likelihood of obtaining significantly different LMWLs decreases marginally. Additionally, over 60% of LMWLs from isoscapes and iGCMs differ significantly from observations. Using biased LMWLs can lead to misinterpretations of, for example, ecohydrological separation between plant water and groundwater. These results underscore the importance of developing concurrent on‐site LMWLs in isotope‐based ecohydrological studies whenever feasible.