Constraining Plant Hydraulics With Microwave Radiometry in a Land Surface Model: Impacts of Temporal Resolution

Abstract

Vegetation water content (VWC) plays a key role in transpiration, plant mortality, and wildfire risk. Although land surface models now often contain plant hydraulics schemes, there are few direct VWC measurements to constrain these models at global scale. One proposed solution to this data gap is passive microwave remote sensing, which is sensitive to temporal changes in VWC. Here, we test that approach by using synthetic microwave observations to constrain VWC and surface soil moisture within the Climate Modeling Alliance Land model. We further investigate the possible utility of sub‐daily observations of VWC, which could be obtained through a satellite in geostationary orbit or combinations of multiple satellites. These high‐temporal‐resolution observations could allow for improved determination of ecosystem parameters, carbon and water fluxes, and subsurface hydraulics, relative to the currently available twice‐daily sun‐synchronous observational patterns. We find that incorporating observations at four different times in the diurnal cycle (such as could be available from two sun‐synchronous satellites) provides a significantly better constraint on water and carbon fluxes than twice‐daily observations do. For example, the root mean square error of projected evapotranspiration and gross primary productivity during drought periods was reduced by approximately 40%, when using four‐times‐daily relative to twice‐daily observations. Adding hourly observations of the entire diurnal cycle did not further improve the inferred parameters and fluxes. Our comparison of observational strategies may be informative in the design of future satellite missions to study plant hydraulics, as well as when using existing remotely sensed data to study vegetation water stress response.

Investigating Diurnal and Seasonal Cycles of Vegetation Optical Depth Retrieved From GNSS Signals in a Broadleaf Forest

“…We also used simulations from the Climate Model Alliance Land model (CliMA‐Land) (Holtzman et al., 2023; Wang et al., 2021). CliMA‐Land is a highly scalable model that incorporates key processes within the Soil‐Plant‐Atmosphere‐Continuum (Wang et al., 2021).…”

“…Vertically resolved plant hydraulics are incorporated into the model, with each organ characterized by its water potential and hydraulic conductivity. To capture the dynamic water storage within vegetation, CliMA‐Land introduces a parameter linked to capacitance, representing a range of hydraulic strategies (Holtzman et al., 2023). In the context of simulating vegetation water content, the model enables a linear pressure‐volume curve on leaf water potential to derive VOD in forms of VOD = (1 + a * ψ leaf )*( b + c *LAI), where a , b , c are unknown parameters, which were retrieved using microwave brightness temperature from the AMSR‐E and AMSR2 satellites as constraints.…”

“…For more details, we refer the reader to Holtzman et al. (2023). The model simulation is used to investigate the mechanistic relationship between leaf water potential, plant transpiration and VOD.…”

Emerging Methods to Validate Remotely Sensed Vegetation Water Content

“…For more rigorous evaluations between data sets, triple collocation analyses can be used to characterize VOD errors from several data streams simultaneously (McColl et al., 2016). Finally, with further development of plant hydraulic schemes and plant water status as an explicit state variable in models, satellite VOD should be more widely assimilated into model frameworks (Holtzman et al., 2023; Konings et al., 2019; Kumar et al., 2020).…”