In this paper we reviewed the use of microwave remote sensing methods for characterizing crop canopies and vegetation water stress related phenomena. Our analysis includes both active and passive systems that are ground-based, airborne, or spaceborne. Most of the published results that have examined crop canopy characterization and water stress have used active microwave systems. In general, quantifying the effect of dynamic vegetation properties, and water stress related processes in particular, on the measured microwave signals can still benefit from improved models and more observational data. Integrated data sets providing information on both soil status and plant status are lacking, which has hampered the development and validation of mathematical models. There is a need to link three-dimensional functional, structural crop models with radiative transfer models to better understand the effect of environmental and related physiological processes on microwave signals and to better quantify the impact of water stress on microwave signals. Such modeling approaches should incorporate both passive and active microwave methods. Studies that combine different sensor technologies that cover the full spectral range from optical to microwave have the potential to move forward our knowledge of the status of crop canopies and particularly water related stress phenomena. Assimilation of remotely sensed properties, such as backscattering coefficient or brightness temperature, in terms of estimating biophysical crop properties using mathematical models is also an unexplored avenue.

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