A coupled equilibrium boundary layer model with stable water isotopes and its application to local water recycling

Authors: XIAO, KE - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; LEE, XUHUI - Yale University; XIAO, WEI - Nanjing University; Baker, John

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2023
Publication Date: 8/15/2023
Citation: Xiao, K., Griffis, T.J., Lee, X., Xiao, W., Baker, J.M. 2023. A coupled equilibrium boundary layer model with stable water isotopes and its application to local water recycling. Agricultural and Forest Meteorology. 339. Article 109572. https://doi.org/10.1016/j.agrformet.2023.109572.
DOI: https://doi.org/10.1016/j.agrformet.2023.109572

Interpretive Summary: The contribution of regional-scale evapotranspiration to regional precipitation, known as local recycling ratio (LRR) is an important variable in climate modeling, but is difficult to measure. We made water vapor isotope measurements in air sampled in a corn field near Rosemount, MN, along with similar measurements from the top of a nearby 285 m radio tower and used those in conjunction with an equilibrium atmospheric boundary layer model to estimate water vapor transport in the planetary boundary layer (PBL). We found that the summer values of LRR for this region (Upper Midwestern US) varied between 0.17 and 0.36, with the lowest value corresponding to the drought year of 2008. Thus, we conclude that in most years, regional evapotranspiration provides approximately 30% of the summer precipitation, while the remainder has a continental-scale source. These data will be useful in improving atmospheric transport models that can be used to assess the impact of expected changes in regional climate on regional evapotranspiration and ultimately crop production.

Technical Abstract: The contribution of evapotranspiration (ET) to regional precipitation, known as the “local water recycling”, is a key process that affects the water cycle and water management. However, how much planetary boundary layer (PBL) moisture arises from ET is highly uncertain due to complex atmosphere and land surface conditions. In this study, an idealized two-layer equilibrium boundary layer model was coupled with a stable water isotope module including HDO and H218O to constrain PBL growing season water transport processes. The model was validated using turbulent heat fluxes and isotope measurements of water vapor (delta m) and precipitation (delta P) measured at a cropland site and a nearby tall tower in the Upper Midwest, United States. The results show that the PBL equilibrium features of delta m and delta P are well-constrained by thermal and moisture equilibrium in the PBL. For this study region, the summer values of f and LRR are estimated to be 0.09 and 0.29 ± 0.12, respectively. The summer LRR values for the years 2006–2010 were 0.35, 0.36, 0.17, 0.29, and 0.29, respectively. The small recycling ratio in 2008 corresponded to a drought condition with the lowest precipitation and second lowest ET among the five years. The summer magnitude of the amount effect is -2.8% (mm day-1)-1 and -0.8% (mm day-1)-1 for delta DP and delta 18OP, respectively. The local water recycling is identified as a significant factor influencing the continental effect. The feedback processes revealed here indicate that the local water recycling is expected to be weakened under drought conditions, but it will be enhanced if irrigation is applied more intensely with more frequent drought events as the climate continues to warm.