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Many hydrologic studies require data on the oxygen and hydrogen stable isotope composition ( delta O-18,d(2)Pi) of precipitation and various collector designs have been suggested for gathering corresponding samples. Yet, it is crucial that these collectors, also known as totalizers, prevent evaporation and associated isotope fractionation. Surprisingly, we were unable to find a comprehensive collector intercomparison in the literature, and much less one that addressed hot and arid conditions.

In this study, we tested six different collector designs over a period of 32 days. They were filled to 20% of their total volume with water of known isotope composition and placed in a modified laboratory drying oven with a low relative humidity (5%) and a diurnal temperature change of 26 to 45 degrees C. Evaporative mass losses were determined gravimetrically daily while samples for isotope analyses were collected every four days.

The classic Oil collector, featuring a layer of paraffin oil to prevent evaporation, showed the smallest mass losses and no detectable isotope shift. The Tube-dip-in-water collector with pressure equilibration tube also performed well, although we noted somewhat larger mass losses and isotope shifts. The latter accounted for 0.28 parts per thousand (Delta delta O-18) and 0.9 parts per thousand (Delta delta H-2) after 32 days, which is significant, but in view of the extreme conditions, these changes should be still acceptable for most studies. The remaining collectors (Ball-in-funnel, Floating balls, and Float based) all failed.

Under the prevailing conditions, the Tube-dip-in-water collector with pressure equilibration tube seems to represent a good, oil-and thus contamination-free compromise. Nonetheless, also in this system small precipitation amounts are problematic. This may necessitate modifications of the standard design in terms of funnel diameter and bottle geometry. We strongly advise researchers to conduct own tests with their intended collector before field deployment.