Refining Isotopic Parameterization Choices Using Stable Water Isotope Profiles In Surface Layer And Soil To Improve Modeling Of Mid-Latitude Continental Moisture Cycling

Abstract:

The moisture balance of the continental boundary layer plays an important role in regulating the exchange of water and energy between the land surface and atmosphere. The surface layer moisture balance is controlled by a number of factors including precipitation, infiltration, evaporation and transpiration. Measurements of stable isotope ratios in water can be exploited to better understand the mechanisms controlling atmosphere-land surface water fluxes. We present three years of in situ tower-based measurements of stable isotope ratios of water (δD and δ¹⁸O) in vapor, precipitation, vegetation and soil from the Boulder Atmospheric Observatory, a semi-arid 300 meter tall-tower site in Erie, Colorado. Co-located meteorological and disdrometer measurements at the surface and 300m allow us to explore key aspects of continental moisture cycling in a semi-arid environment such as the important contribution of sub-surface vapor diffusion to the surface water vapor budget and its implications for partitioning in dry ecosystems, and the role of rain evaporation during precipitation events on inter-event and seasonal time scales. We use our observations to constrain a Craig-Gordon evaporation model at the land surface to weight the contributions of rainfall, surface water vapor exchange and sub-surface vapor diffusion to soil water isotope values. A multi-season in situ time series of surface vapor isotope profiles in conjunction with soil and precipitation allows us to field-validate choices for parameters such as the kinetic fractionation factor for each process. This has implications both for modeling short-term gas exchange at the land surface in modern-day climate models as well as for refining paleoclimate interpretations of stable oxygen and hydrogen isotope-based proxies. Tall-tower precipitation and vapor isotope profile data can also be analyzed in conjunction with disdrometer data on inter-event and seasonal time scales to quantify the role of rain evaporation in the surface layer water balance. These studies can help refine fractionation factor choices in land surface and boundary layer parameterizations in isotope-enable climate models.