Scientific References

Poster: Real-Time Field-Based Water Vapor Isotope Measurements with a CRDS Analyzer: Probing Cropland Evapotranspiration

Authors Aaron Van Pelt, David G. Williams, Wei Sun, Leo Mayr


While stable isotope techniques have been previously applied to partition evapotranspiration (ET) fluxes in crops, 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) based on cavity ringdown spectroscopy (CRDS) was recently deployed at China’s National Experimental Station for Precision Agriculture during the FAO/IAEA 2nd Research Coordination Meeting on “Managing Irrigation Water to Enhance Crop Productivity under Water-Limiting Conditions using Nuclear Techniques.” Measurements were conducted continuously over several days, sampling from five different heights within and above the canopy of a corn (Zeamays) ecosystem. The continuous measurements by the Picarro analyzer were complimented by additional measurements from the same sampling points, wherein the vapor was cryogenically trapped for later laboratory quantification of the water isotopologues. Stable isotope measurements were taken concurrently with conventional ET flux measurements. The isotope analyses can allow the partitioning of ET into its components: soil evaporation and leaf transpiration. 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 as well field data and comparison of the vapor-phase results with the off-line liquid analysis of the cryogenically-trapped vapor.