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| Titel |
An assessment of the isotopic (2H/18O) integrity of water samples collected and stored by unattended precipitation totalizers |
| VerfasserIn |
Stefan Terzer, Leonard I. Wassenaar, Cedric Douence, Luis Araguas-Araguas |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135159
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| Publikation (Nr.) |
 EGU/EGU2016-15992.pdf |
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| Zusammenfassung |
| The IAEA-WMO Global Network of Isotopes in Precipitation (GNIP) provides worldwide
δ18O and δ2H data for numerous hydrological and climatological studies. The traditional
GNIP sample collection method relies on weather station operators to accumulate
precipitation obtained from manual rain gauges. Over the past decades, widespread weather
station automatization resulted in the increased use of unattended precipitation totalizers that
accumulate and store the rainwater in the field for up to one month. Several low-tech
measures were adopted to prevent in situ secondary evaporative isotopic enrichment (SEE) of
totalized water samples (i.e. disequilibrium isotopic fractionation after precipitation is stored
in the collection device). These include: (a) adding a 0.5-1 cm floating layer of paraffin oil to
the totalizer bottle, (b) using an intake tube leading from the collection funnel and submerged
to the bottom of the totalizer bottle, or (c) placing a table tennis ball in the funnel
aiming to reduce evaporation of the collected water from the receiving bottle to the
atmosphere.
We assessed the isotopic integrity of stored rainwater samples for three totalizers
under controlled settings: each aforementioned totalizer was filled with a 100 or 500
mL of isotopically known water and installed in the field with the intake funnels
sheltered to prevent rainwater collection. Potential evapotranspiration (PET) was
obtained from on-site meteorological recordings. Stored evaporative loss from each
totalizer was evaluated on a monthly basis; gravimetrically and by analysing δ18O and
δ2H of the stored water, for a period of 6 months and a cumulative PET of ∼500
mm.
The gravimetric and isotope results revealed that for smaller water volumes (100 ml,
corresponding to ca. 5 mm of monthly precipitation), negligible isotope enrichment (δ18O)
was observed in the paraffin-oil based totalizer, whereas unacceptable evaporative isotope
effects were observed for the ball-in-funnel collector. For the submerged-tube sampler,
the evaporative effect depended on the amount of stored water: 100 ml showed
unacceptable isotopic enrichment, whereas the SEE of 500 ml stored water was
acceptable.
These data allowed us to estimate the impact of secondary evaporative enrichment on a
device-specific basis as a function of PET. Based on global PET grids (e.g. CGIAR
data), and benchmarking the expected SEE against the reasonable uncertainty of
isotope spectrometry (< ±0.1‰ for δ18O), these findings reveal the most suitable
totalizer device for any given climatic condition. Under extreme conditions (e.g.
high aridity, little precipitation vs. high PET), a paraffin-oil based rain totalizer
is most appropriate for monthly collections. Submerged-tube samplers may be
considered if either a higher frequency of collection were possible, or monthly
under pluvial/temperate climate conditions. The use of ball-in-funnel type totalizers
are not recommended at all, unless samples could be collected on a daily basis. |
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