A new precipitation weighted method for determining the meteoric water line for hydrological applications demonstrated using Australian and global GNIP data

SummaryThe relationship between δ2H and δ18O in precipitation at a site, known as the local meteoric water line (LMWL), is normally defined using an ordinary least squares regression (OLSR) which gives equal weighting to all data points regardless of the precipitation amount they represent. However, smaller precipitation amounts are more likely to have a lower D-excess due to re-evaporation of raindrops below the cloud-base or biases in the sampling method. In this paper we present an equation for a precipitation amount weighted least squares regression (PWLSR) that will correct these biases for use in groundwater and surface hydrology applications.New LMWL equations are presented for Australian sites in the Global Network of Isotopes in Precipitation (GNIP), where the PWLSR consistently produces a LMWL with a larger gradient than the OLSR. Perth and Alice Springs exhibit the largest change in slope. This is consistent with the higher frequency of small monthly precipitation amounts with low D-excess values occurring at these sites in summer for Perth and throughout the year for Alice Springs.The PWLSR method was also applied to 288 stations in the GNIP data base (N > 36) and the difference between the slopes of the LMWLs (Δa = slopePWLSR-slopeOLSR) calculated for these stations. The mean change in slope, Δa was 0.12 with 56% of sites showing an increase in slope or positive Δa value and 44% having a decrease in slope or negative Δa. Sites with Mediterranean climates showed the greatest increase in slope. The magnitude of the change in slope followed some general trends showing a positive correlation with average δ2H and δ18O composition and rainfall variability, and negative correlation with period of record (N).

Highlights► A precipitation weighted method to define a local meteoric water line (LMWL) is proposed. ► This new method minimises the impact of small rainfall events on the LMLW. ► In Australia this increases the slope of the LMWL at all GNIP sites. ► New local meteoric water line equations are presented for Australian GNIP sites. ► Global distribution is shown and Mediterranean climates found to have most impact.