Development of an experimental approach to study coupled soil‐plant‐atmosphere processes using plant analogs

Abstract

The atmosphere, soils, and vegetation near the land‐atmosphere interface are in a state of continuous dynamic interaction via a myriad of complex interrelated feedback processes which collectively, remain poorly understood. Studying the fundamental nature and dynamics of such processes in atmospheric, ecological, and/or hydrological contexts in the field setting presents many challenges; current experimental approaches are an important factor given a general lack of control and high measurement uncertainty. In an effort to address these issues and reduce overall complexity, new experimental design considerations (two‐dimensional intermediate‐scale coupled wind tunnel‐synthetic aquifer testing using synthetic plants) for studying soil‐plant‐atmosphere continuum soil moisture dynamics are introduced and tested in this study. Validation of these experimental considerations, particularly the adoption of synthetic plants, is required prior to their application in future research. A comparison of three experiments with bare soil surfaces or transplanted with a Stargazer lily/limestone block was used to evaluate the feasibility of the proposed approaches. Results demonstrate that coupled wind tunnel‐porous media experimentation, used to simulate field conditions, reduces complexity, and enhances control while allowing fine spatial‐temporal resolution measurements to be made using state‐of‐the‐art technologies. Synthetic plants further help reduce system complexity (e.g., airflow) while preserving the basic hydrodynamic functions of plants (e.g., water uptake and transpiration). The trends and distributions of key measured atmospheric and subsurface spatial and temporal variables (e.g., soil moisture, relative humidity, temperature, air velocity) were comparable, showing that synthetic plants can be used as simple, idealized, nonbiological analogs for living vegetation in fundamental hydrodynamic studies.

Experimental Testing Scale Considerations for the Investigation of Bare‐Soil Evaporation Dynamics in the Presence of Sustained Above‐Ground Airflow

“…This would create conditions favorable for diffusion dominated water vapor transport and accumulation (Haghighi & Or, ; McInnes et al, ; Wagner et al, ) as well as local soil water preservation (Veihmeyer, ). This is a localized small‐scale phenomenon referred to as microclimatic amelioration in the micrometeorological and ecohydrological communities (e.g., Allegrini et al, ; Cleugh & Hughes, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ). As in the case of air velocity, a thickening of the concentration boundary layer could be observed above the top of the surface undulations.…”

“…The fundamental study of interrelated heat and mass transfer processes that occur within Earth's near‐surface environment is complex, requiring highly controlled test conditions and fine spatiotemporal resolution data that are often not feasible or impossible to collect in field settings where natural variations in subsurface conditions (e.g., heterogeneity) and atmospheric dynamics (e.g., weather and solar radiation) can lead to considerable data set noise and uncertainty and the extent and the resolution of sensor/monitoring networks are constrained by labor and cost (Betts et al, ; Famigletti et al, ; Western et al, , ). Many of these issues can be circumvented or mediated to some degree with the CESEP wind tunnel‐porous media test facility which was designed to reproduce scale‐dependent field phenomena associated with soil‐plant‐atmosphere continuum dynamics in a laboratory setting (e.g., Trautz, Illangasekare, & Rodriguez‐Iturbe, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ). Research of this nature can be conducted in this facility because it was designed to adhere to the principles of intermediate‐scale experimentation in the subsurface (Lenhard et al, ) and similarity theory in the atmosphere (Monin & Obukhov, ).…”

“…This test scale was first introduced in the late 1980s to study a class of problems related to flow, fate, and transport in the subsurface; see Oostrom et al () for review. This approach has more recently been adopted in the study of evaporation from bare soils (e.g., Haghighi & Or, , ; Trautz, ) and evapotranspiration (e.g., Trautz, Illangasekare, & Rodriguez‐Iturbe, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ). Intermediate‐scale experimental test systems are designed so that individual processes can be isolated and their overall contributions to flow and transport quantified (Trautz, Illangasekare, & Rodriguez‐Iturbe, ).…”

Sensitivity of a Continuum‐Scale Porous Media Heat and Mass Transfer Model to the Spatial‐Discretization Length‐Scale of Applied Atmospheric Forcing Data

“…A significant portion of research investigating impacts of atmospheric and strength of soil‐atmosphere coupling on the subsurface or atmospheric state is conducted numerically (e.g., Davarzani et al, ; Entekhabi et al, ; Fetzer et al, ; Knist et al, ; Mosthaf et al, ; Santanello et al, ). This is due in part to the fact that the generation of high‐resolution data sets needed to explore such coupling experimentally or validate model theory is difficult, if not at times impossible, to obtain in small laboratory column apparatuses or in the field setting which are constrained by issues related to scale, system control (e.g., soil property and climate variability), and cost (i.e., monetary, time) (Betts et al, ; Ferguson et al, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ; Western et al, ). Many of these issues are now being overcome with the development of specialized laboratory systems such as the Center for Experimental Study of Subsurface Processes wind tunnel‐porous media test facility (e.g., Trautz, Illangasekare, & Rodriguez‐Iturbe, ; Trautz et al, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ) and refinement of remote sensing (e.g., satellite, eddy‐covariance) techniques (e.g., Ferguson et al, ; Hirschi et al, ; Hohenegger et al, ; Trigo et al, ).…”

“…This is due in part to the fact that the generation of high‐resolution data sets needed to explore such coupling experimentally or validate model theory is difficult, if not at times impossible, to obtain in small laboratory column apparatuses or in the field setting which are constrained by issues related to scale, system control (e.g., soil property and climate variability), and cost (i.e., monetary, time) (Betts et al, ; Ferguson et al, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ; Western et al, ). Many of these issues are now being overcome with the development of specialized laboratory systems such as the Center for Experimental Study of Subsurface Processes wind tunnel‐porous media test facility (e.g., Trautz, Illangasekare, & Rodriguez‐Iturbe, ; Trautz et al, ; Trautz, Illangasekare, Rodriguez‐Iturbe, Heck, & Helmig, ) and refinement of remote sensing (e.g., satellite, eddy‐covariance) techniques (e.g., Ferguson et al, ; Hirschi et al, ; Hohenegger et al, ; Trigo et al, ). As more high‐resolution data sets become readily available, new questions regarding model data assimilation will continue to emerge—for example, how much data are needed to test, refine, and drive numerical heat and mass transfer models across a wide range of spatiotemporal scales.…”