Modeling the spatial distribution of the meteoric water line of modern precipitation across the broader Mediterranean region

Abstract

The covariance of stable hydrogen (δ2H) and oxygen (δ18O) isotopes in local precipitation (the local meteoric water line – LMWL) serves as a benchmark in isotope hydrological or paleoclimatological applications. However, the isotope hydrometeorological monitoring network is still sparse in many parts of the Mediterranean, and the degree of spatial data coverage is insufficient to address current needs. To remedy this weakness a spatially continuous interpolated geostatistical product of the LMWLs of modern precipitation across the Mediterranean has been developed. The LMWLs of the stations with data for more than four years between 2000 and 2015 were calculated using reduced major axis regression, and then interpolated across the region using statistical and machine learning methods. Random forest interpolation with buffer distance and elevation gave the best performance in the out-of-sample verification, and was thus used to derive the final interpolated product. The slope and intercept of the LMWLs ranged between ∼5.9 to 8.2 and −3.9 to 16.1‰, respectively. A detailed comparison with previous local/regional estimations showed that the model presented here concurs with those, albeit with minor deviations in certain regions. With the present results, it then becomes possible to assess how well grounded the ‘Eastern- and Western Mediterranean Meteoric Water Lines’ (EMMWL and WMMWL) for the 21st century are. In the eastern Mediterranean, the current model shows a slope (∼6.9) and the intercept (∼15‰) concurring with local studies, but does not reproduce the frequently cited benchmark values of the EMMWL; slope: 8, intercept ≥20‰. The EMMWL may be considered an idealized isotope-hydrological benchmark useful in regional studies; nonetheless, it cannot be taken as a valid representation of the actual empirical description of the δ2H – δ18O covariance of local precipitation in the eastern Mediterranean. Defining a uniform regional MWL in the western Mediterranean is not supported by the spatial heterogeneity of LMWL parameters based on the present estimations, calling into question the utility of the ‘WMMWL’ as an isotope hydrological benchmark.