AN30 - Measuring water vapor with Picarro’s water isotope analyzer: hardware configuration and data analysis recommendations

Stable isotopes (δ18O and δD) of water vapor provide insight into the hydrologic cycle. In particular, geographic origin of water, energy balance, evapotranspiration (ET) of crops, and climate can be investigated with these measurements. While stable isotope techniques have been previously used in these studies, it has only recently become possible to take in situ, long-term, continuous (every 10 seconds) measurements of stable water vapor isotopologues. A Picarro water vapor isotope analyzer (L1115-i) was recently deployed in China with the goal of measuring ET from a corn crop. Measurements were conducted continuously over several days, sampling from five different heights within and above the canopy of a corn ecosystem. In this example, the isotope analyses can allow the partitioning of ET into its components: soil evaporation and leaf transpiration, though there are many other similar motivations for measuring water vapor isotopes for which the setup described here would be ideal. Once daily, during the vapor measurements, liquid water isotope standards were measured by the Picarro analyzer using its included autosampler and subsequently used to calibrate the vapor-phase data. Described here are the analyzer and sampling system and relevant aspects of the data analysis.