Toward estimation of seasonal water dynamics of winter wheat from ground-based L-band radiometry: a concept study
Authors: Jagdhuber Thomas , Jonard François , Fluhrer Anke , Chaparro David , Baur Martin J. , Meyer Thomas , Piles María 
Published: April 2022

Abstract. The vegetation optical depth (VOD) variable
contains information on plant water content and biomass. It 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
water potential gradients and flow resistances in the soil, the vegetation
and the atmosphere. Yet water flux estimation remains an elusive challenge
especially on a global scale. In this concept study, we conduct a
field-scale experiment to test mechanistic models for the estimation of
seasonal water fluxes (PWU and TR) of a winter wheat stand using
measurements of soil moisture, VOD, and relative air humidity (RH) in a
controlled environment. We utilize microwave L-band observations from a
tower-based radiometer to estimate VOD of a wheat stand during the 2017
growing season at the Selhausen test site in Germany. From VOD,
we first extract the gravimetric moisture of vegetation and then determine
the relative water content (RWC) and vegetation water potential (VWP) of
the wheat field. Although the relative water content could be directly
estimated from VOD, our results indicate this may be challenging for the
phenological phases, when rapid biomass and plant structure development take
place within the wheat canopy. We estimate water uptake from the soil to the
wheat plants from the difference between the soil and vegetation potentials
divided by the flow resistance from soil into wheat plants. The
TR from the wheat plants into the atmosphere was obtained
from the difference between the vegetation and atmosphere water potentials
divided by the flow resistances from plants to the atmosphere. For this, the
required soil matric potential (SMP), the vapor pressure deficit (VPD),
and the flow resistances were obtained from on-site observations of soil,
plant, and atmosphere together with 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 have to be made when estimating the vegetation
water potential from relative water content as well as the water flow
resistances between soil, wheat plants, and atmosphere. Moreover, direct
validation of water flux estimates for the assessment of 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, known from the literature, and
intercompare reasonably well in absolute terms with independent TR
estimates of the NASA ECOSTRESS mission, which relies on a Priestly–Taylor
type of retrieval model. Our findings support that passive microwave remote-sensing techniques qualify for the estimation of vegetation water dynamics
next to traditionally measured stand-scale or plot-scale techniques. They
might shed light on future capabilities of monitoring water dynamics in the
soil–plant–atmosphere system including wide-area, remote-sensing-based earth
observation data.