Preprints
https://doi.org/10.5194/hess-2021-459
https://doi.org/10.5194/hess-2021-459

  17 Sep 2021

17 Sep 2021

Review status: this preprint is currently under review for the journal HESS.

Reconstructing Continuous Vegetation Water Content To Understand Sub-daily Backscatter Variations

Paul C. Vermunt1, Susan C. Steele-Dunne1, Saeed Khabbazan1, Jasmeet Judge2, and Nick C. van de Giesen1 Paul C. Vermunt et al.
  • 1Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2628 CN Delft, The Netherlands
  • 2Center for Remote Sensing, Agricultural and Biological Engineering Department, University of Florida, Gainesville, FL 32611 USA

Abstract. Microwave observations are sensitive to vegetation water content (VWC). Consequently, the increasing temporal and spatial resolution of spaceborne microwave observations creates a unique opportunity to study vegetation water dynamics and its role in the diurnal water cycle. However, we currently have a limited understanding of sub-daily variations in VWC and how they affect passive and active microwave observations. This is partly due to the challenges associated with measuring internal VWC for validation, particularly non-destructively and at timescales of less than a day. In this study, we aimed to (1) use field sensors to reconstruct diurnal and continuous records of internal VWC of corn, and (2) use these records to interpret the sub-daily behaviour of a 10-day time series of polarimetric L-band backscatter with high temporal resolution. Sub-daily variations of internal VWC were calculated based on the cumulative difference between estimated transpiration and sap flow rates at the base of the stems. Destructive samples were used to constrain the estimates and for validation. The inclusion of continuous surface canopy water estimates (dew or interception) and surface soil moisture allowed us to attribute hour-to-hour backscatter dynamics to either internal VWC, surface canopy water or soil moisture variations. Our results showed that internal VWC varied with 10–20 % during the day in non-stressed conditions, and the effect on backscatter was significant. Diurnal variations of internal VWC and nocturnal dew formation affected vertically polarized backscatter most. Moreover, on a typical dry day, backscatter variations were 1.5 (HH-pol) to 3 (VV-pol) times more sensitive to VWC than to soil moisture. These results demonstrate that radar observations have the potential to provide unprecedented insight into the role of vegetation water dynamics in land-atmosphere interactions at sub-daily timescales.

Paul C. Vermunt et al.

Status: open (until 12 Nov 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-459', Andrew Feldman, 08 Oct 2021 reply

Paul C. Vermunt et al.

Paul C. Vermunt et al.

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Short summary
This study investigates the use of hydrometeorological sensors to reconstruct variations in internal vegetation water content of corn, and relates these variations to the sub-daily behaviour of polarimetric L-band backscatter. The results show significant sensitivity of backscatter to the daily cycles of vegetation water content and dew, particularly on dry days and for vertical and cross-polarizations, which demonstrates the potential for using radar for studies on vegetation water dynamics.