Precipitation isoscapes for New Zealand: enhanced temporal detail using precipitation-weighted daily climatology^†

† Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

ABSTRACT

Predictive understanding of precipitation δ²H and δ¹⁸O in New Zealand faces unique challenges, including high spatial variability in precipitation amounts, alternation between subtropical and sub-Antarctic precipitation sources, and a compressed latitudinal range of 34 to 47 °S. To map the precipitation isotope ratios across New Zealand, three years of integrated monthly precipitation samples were acquired from >50 stations. Conventional mean-annual precipitation δ²H and δ¹⁸O maps were produced by regressions using geographic and annual climate variables. Incomplete data and short-term variation in climate and precipitation sources limited the utility of this approach. We overcome these difficulties by calculating precipitation-weighted monthly climate parameters using national 5-km-gridded daily climate data. This data plus geographic variables were regressed to predict δ²H, δ¹⁸O, and d-excess at all sites. The procedure yields statistically-valid predictions of the isotope composition of precipitation (long-term average root mean square error (RMSE) for δ¹⁸O = 0.6 ‰; δ²H = 5.5 ‰); and monthly RMSE δ¹⁸O = 1.9 ‰, δ²H = 16 ‰. This approach has substantial benefits for studies that require the isotope composition of precipitation during specific time intervals, and may be further improved by comparison to daily and event-based precipitation samples as well as the use of back-trajectory calculations.

KEYWORDS:

• Hydrogen-2
• isoscapes
• isotope hydrology
• New Zealand
• oxygen-18
• precipitation
• spatial depiction

Acknowledgements

The authors gratefully thank anonymous reviewers, and many volunteers for operating the precipitation collection stations, as well as laboratory staff for their assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

W. Troy Baisden http://orcid.org/0000-0003-1814-1306

Elizabeth D. Keller http://orcid.org/0000-0002-9408-9215

Russell D. Frew http://orcid.org/0000-0002-3492-9454

Leonard I. Wassenaar http://orcid.org/0000-0001-5532-0771

Notes

† Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

Additional information

Funding

Precipitation collection was funded by a New Zealand Cross Departmental Research Pool grant to R.D.F., and GNS Science Core Funding in the Global Change Through Time Programme (GCT84) to W.T.B.