Technical note: Lessons from and best practices for the deployment of the SoilWater Isotope Storage System

Soil water isotope datasets are useful for understanding connections between the hydrosphere, atmosphere, biosphere, and geosphere. However, they have been under-produced because of the technical challenges associated with collecting those datasets. Here, we present the results of testing and automation of the SoilWater Isotope Storage System (SWISS). The unique innovation of the SWISS is that we are able to automatically collect water vapor from the critical zone at a regular time interval and then store that water vapor until it can be measured back in a laboratory setting. Through a series of quality assurance and quality control tests, we tested whether the SWISS is resistant to both atmospheric intrusion and leaking in both laboratory and field settings. We assessed the accuracy and precision of the SWISS through a series of experiments in which water vapor of known composition was introduced into the flasks, stored for 14 d, and then measured. From these experiments, after applying an offset correction to report our values relative to Vienna Standard Mean Ocean Water (VSMOW), we assess the precision of the SWISS to be +/- 0.9 parts per thousand and +/- 3.7 parts per thousand for delta O-18 and delta H-2, respectively. We deployed three SWISS units at three different field sites to demonstrate that the SWISS stores water vapor reliably enough that we are able to differentiate dynamics both between the sites as well within a single soil column. Overall, we demonstrate that the SWISS retains the stable isotope composition of soil water vapor for long enough to allow researchers to address a wide range of ecohydrologic questions.