Development of an autonomous, monthly and daily, rainfall sampler for isotope research

Abstract

An autonomous, low cost (<US$750), and open source rainfall sampler has been developed for hydrogen and oxygen isotope research, able to sample daily and monthly for up to 60 days of rainfall, over a three month period. The sampler is designed to use modern fabrication methods such as 3D printing and laser cutting to minimise the need for machined and injection molded components. The sampler can use either paraffin oil or a submerged inlet tube (also known as tube-dip-in samplers) to prevent evaporation, with the use of the inlet tube method facilitated by 3D printed bottle caps. An experiment was performed to identify the most suitable plastic for these caps, with acetone treated ABS (Acrylonitrile Butadiene Styrene) being most suitable, followed by PETG (Polyethylene Terephthalate Glycol), untreated ABS, and PLA (Polylactic acid). In addition, the effectiveness of both paraffin oil and the inlet tube method for preventing evaporation was quantified, with paraffin identified as being the most effective at present. During a 90 day outdoor experiment, the ¹⁸O/¹⁶O vs. ²H/¹H ratios of some water samples evolved along a local evaporation line, with increased isotopic enrichment of samples correlating to water loss. A coupled hydrologic-isotopic model was applied to these data, and successfully predicted the change in isotope ratios based on the amount of water lost from each sample. This modelling approach, combined with daily and monthly sample collection and quantification of evaporation rates within the sheltered environment of the sampler allows for back calculation of the original volume and isotopic composition of daily and monthly rainfall samples. The rainfall sampler thus facilitates cost -and time- effective remote monitoring of the isotopic composition of precipitation to support an array of Earth system research.