DEVELOPING A STATISTICAL MODEL FOR PREDICTING
RATIOS OF RIVERS IN THE SOUTH ISLAND HEADWATERS
18O
AND
2H
Sarah M MAGER,1 Emily E. DIACK,1 Robert VAN HALE,2
1
Department of Geography, University of Otago, New Zealand
2
Department of Chemistry, University of Otago, New Zealand
Aims
The aim of this paper is to characterise the spatial and temporal range in δ18O and δ2H isotopes
of rivers draining the Southern Alps of the South Island of New Zealand.
Method
Discrete water samples have been collected every 3 to 6 months for 55 to 71 rivers draining the
Southern Alps of New Zealand from January 2012 to August 2015. Grab samples were collected
directly from the river and stored in 2 mL glass vials with polypropylene septa caps and measured
for δ18O and δ2H using a Picarro CRDS analyser in the Department of Chemistry at the University
of Otago. The precision for these analyses is δ18O 0.1 ‰ and δ2H 0.9 ‰.
Results
These water samples have been analysed for their stable isotope ratios of δ 18O and δ2H and
show marked variability across the range front that is strongly associated with variations in
catchment mean elevation, distance from the coast, and latitude. Lower-elevation catchments
typically have the most enriched δ18O and δ2H ratios, whereas the greatest depletion occurs on
the east of the main divide, particularly in the interior of central Otago. West Coast catchments
are enriched in δ18O and δ2H compared to the East Coast catchments likely due to the combined
effects of close proximity to the humid air masses that form over the Tasman Sea, and orographic
uplift driving isotopic fractionation over the Southern Alps. The ratios of δ18O and δ2H measured
in these alpine rivers are consistent with mean rainfall ratios, suggesting little attenuation of the
isotopic ratios as water is routed through the surface and shallow-subsurface pathways under
base flow. In the headwater catchments it appears as the routing of water through the catchment
has the effect of homogenising short-scale variations in isotopic ratios that occurs between
discrete rain events, or during a single storm event. For the majority of catchments there is no
significant difference in the isotopic ratios of δ18O and δ2H between seasons even in catchments
that have significant snow and/or ice storage (e.g. Franz Josef, Fox, Tasman, Hooker).
A variety of morphological catchment characteristics were extracted using ArcGIS 10.1 and input
into a dual direction step-wise regression to determine the key predictors of the isotopic ratios of
the river water. The resultant multivariate regression (r-squared of 91% at the 99% confidence
interval) determined that the mean δ18O and δ 2H ratios are predictable at the catchment scale by
a) the central point of each catchments’ distance from the coast b) mean catchment elevation and
c) latitude or northing. The statistical output model was applied to the River Environment
Classification stream orders of 5, 6, and 7 across the South Island by generating metrics of
distance from coast, mean elevation and northing for catchments using the regression
relationship determined from this study as the basis of a predictive model using semivariogram
krigging in ArcGIS. The modelled δ18O and δ2H are compared to known values for rivers across
the Southern Alps, and show some deviations occur (as in parts of Southland and Canterbury),
and may reflect additional sources of water into these systems, such as groundwater interaction.
Conclusions
The stable isotopes from rivers draining the Southern Alps vary across the main divide; with the
most enriched values occuring in low elevation catchments on the West Coast. The most
depleted isotopes occur in the headwaters of Otago catchments. Isotopic ratios of 18O and 2H
can be predicted using three catchment variables: distance from coast, mean elevation and
latitude.