Toward estimation of seasonal water dynamics of winter wheat from ground-based L-band radiometry: a concept study

[摘要] The vegetation optical depth (VOD) variablecontains information on plant water content and biomass. It can be estimatedalongside soil moisture from currently operating satellite radiometermissions, such as SMOS (ESA) and SMAP (NASA). The estimation of waterfluxes, such as plant water uptake (PWU) and transpiration rate (TR),from these earth system parameters (VOD, soil moisture) requires assessingwater potential gradients and flow resistances in the soil, the vegetationand the atmosphere. Yet water flux estimation remains an elusive challengeespecially on a global scale. In this concept study, we conduct afield-scale experiment to test mechanistic models for the estimation ofseasonal water fluxes (PWU and TR) of a winter wheat stand usingmeasurements of soil moisture, VOD, and relative air humidity (RH) in acontrolled environment. We utilize microwave L-band observations from atower-based radiometer to estimate VOD of a wheat stand during the 2017growing season at the Selhausen test site in Germany. From VOD,we first extract the gravimetric moisture of vegetation and then determinethe relative water content (RWC) and vegetation water potential (VWP) ofthe wheat field. Although the relative water content could be directlyestimated from VOD, our results indicate this may be challenging for thephenological phases, when rapid biomass and plant structure development takeplace within the wheat canopy. We estimate water uptake from the soil to thewheat plants from the difference between the soil and vegetation potentialsdivided by the flow resistance from soil into wheat plants. TheTR from the wheat plants into the atmosphere was obtainedfrom the difference between the vegetation and atmosphere water potentialsdivided by the flow resistances from plants to the atmosphere. For this, therequired 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. Thispathfinder study shows that the L-band microwave radiation contains valuableinformation on vegetation water status that enables the estimation of waterdynamics (up to fluxes) from the soil via wheat plants into the atmosphere,when combined with additional information of soil and atmosphere watercontent. Still, assumptions have to be made when estimating the vegetationwater potential from relative water content as well as the water flowresistances between soil, wheat plants, and atmosphere. Moreover, directvalidation of water flux estimates for the assessment of their absoluteaccuracy could not be performed due to a lack of in situ PWU and TRmeasurements. Nonetheless, our estimates of water status, potentials, andfluxes show the expected temporal dynamics, known from the literature, andintercompare reasonably well in absolute terms with independent TRestimates of the NASA ECOSTRESS mission, which relies on a Priestly–Taylortype of retrieval model. Our findings support that passive microwave remote-sensing techniques qualify for the estimation of vegetation water dynamicsnext to traditionally measured stand-scale or plot-scale techniques. Theymight shed light on future capabilities of monitoring water dynamics in thesoil–plant–atmosphere system including wide-area, remote-sensing-based earthobservation data.

[学科分类] 大气科学