Preprints
https://doi.org/10.5194/bg-2021-71
https://doi.org/10.5194/bg-2021-71

  24 Mar 2021

24 Mar 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Towards Estimation of Seasonal Water Dynamics of Winter Wheat from Ground-Based L-Band Radiometry 

Thomas Jagdhuber1,2, François Jonard3,4, Anke Fluhrer1,2, David Chaparro5, Martin J. Baur6, Thomas Meyer4, and María Piles7 Thomas Jagdhuber et al.
  • 1German Aerospace Center, Microwaves and Radar Institute, Münchener Strasse 20, 82234 Wessling, Germany
  • 2Institute of Geography, University of Augsburg, Alter Postweg 118, 86159 Augsburg, Germany
  • 3Agrosphere (IBG-3), Institute of Bio- and Geosciences, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
  • 4Earth and Life Institute, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium
  • 5Universitat Politècnica de Catalunya, CommSensLab & IEEC/UPC, Jordi Girona 1-3, 08034 Barcelona, Spain
  • 6University of Cambridge, Department of Geography, Philippa Fawcett Dr, CB3 0AS, Cambridge, U.K.
  • 7Image Processing Lab, Universitat de València, 46980 Valencia, Spain

Abstract. The vegetation optical depth (VOD) parameter contains information on plant water content and biomass, and can be estimated alongside soil moisture from currently operating satellite radiometer missions, such as SMOS (ESA) and SMAP (NASA). The estimation of water fluxes, such as plant water uptake (PWU) and transpiration rate (TR), from these Earth system parameters (VOD, soil moisture) requires assessing potential (suction tension) gradients of water and flow resistances in the soil, the vegetation and the atmosphere, yet it remains an elusive challenge especially on global scale. Here, we used a field-scale experiment to test mechanistic models for the estimation of seasonal water fluxes (PWU and TR) of a winter wheat stand including measurements of soil moisture, VOD, and relative air humidity (RH) under a controlled environment. We utilized microwave L-band observations from a tower-based radiometer to estimate VOD of a wheat stand during the 2017 growing season at the Selhausen laboratory in Germany. From VOD, we first extracted the gravimetric moisture of vegetation and then determined subsequently the relative water content (RWC) and the vegetation water potential (VWP) of the wheat field. Although the relative water content could directly be estimated from VOD, our results indicate this may be problematic for the phenological phases, when rapid biomass and plant structure development take place in the wheat canopy. The water uptake from the soil to the wheat plants was estimated from the difference between the soil and vegetation potentials divided by flow resistance from soil into wheat plants. The transpiration rate from the wheat plants into the atmosphere was obtained from the difference between the vegetation and atmosphere potentials divided by flow resistances from plants to the atmosphere. For this, the required soil matric potential (SMP), the vapor pressure deficit and the flow resistances were obtained from on-site observations of soil, plant and atmosphere and simple mechanistic models. This pathfinder study shows that the L-band microwave radiation contains valuable information on vegetation water status that enables the estimation of water dynamics (up to fluxes) from the soil via wheat plants into the atmosphere, when combined with additional information of soil and atmosphere water content. Still, assumptions when estimating the vegetation water potential from relative water content as well as when estimating the water flow resistances between soil, wheat plants and atmosphere had to be made. Moreover, validation of water flux estimates for assessing their absolute accuracy could not be performed due to a lack of in situ PWU and TR measurements. Nonetheless, our estimates of water status, potentials and fluxes show the expected temporal dynamics and intercompare reasonably well in absolute terms, providing confidence in further developing the proposed approach. Our findings support that passive microwave remote sensing techniques allow for the estimation of vegetation water dynamics next to traditionally measured stand-scale or plot-scale techniques. This might shed light on the potential capabilities of monitoring water dynamics in the soil-plant-atmosphere system using wide-area, remote sensing-based Earth observation data.

Thomas Jagdhuber et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-71', Anonymous Referee #1, 27 Apr 2021
    • AC1: 'Reply on RC1', Thomas Jagdhuber, 29 Jun 2021
  • RC2: 'Comment on bg-2021-71', Mostafa Momen, 10 May 2021
    • AC2: 'Reply on RC2', Thomas Jagdhuber, 29 Jun 2021

Thomas Jagdhuber et al.

Thomas Jagdhuber et al.

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Short summary
This is a path finder study to water dynamics across winter wheat starting from ground-based L-band radiometry in combination with on-site measurements of soil and atmosphere. We research the feasibility to estimate potentials and seasonal flux rates of water (water uptake from soil and transpiration rates into the atmosphere) within the soil-plant-atmosphere system (SPAS) of a winter wheat field. The main finding is that L-band radiometry can be integrated into a field-based SPAS assessment.
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