Estimation of Volume Fraction and Gravimetric Moisture of Winter Wheat Based on Microwave Attenuation: A Field Scale Study

A considerable amount of water can be stored in vegetation, especially in regions experiencing large quantities of precipitation (mid-latitudes). In this context, an accurate estimate of the actual water status of the vegetation could lead to an improved understanding of the effect of plant water on the water budget. In this study, we developed and validated a novel approach to retrieve the vegetation volume fraction (δ) (i.e., volume percentage of solid plant material of a canopy in air) and the gravimetric vegetation water content (mg) (i.e., amount of water per wet biomass) for winter wheat. The estimation was based on the attenuation of L-band microwave measurements through vegetation (vegetation optical depth, (τ)-parameter). Ground-based L-band microwave measurements over an entire growing cycle together with in situ measured vegetation characteristics have been used for this purpose. Retrieved δ- and mg-values revealed to be comparable to literature and in situ measurements (i.e., δ was within the range between 0 and 0.01 and the retrieved mg had a mean value of 0.58 (0.55 (in situ) and 0.54 (literature))). Finally, we also tested the sensitivity of the δ- and mg-retrievals to their input-values to investigate their possible mutual dependencies. The analysis showed that already small changes in the input-mg or -δ result in relatively large changes in the retrieved δ or mg. © 2019 IEEE.