Interferometric Synthetic Aperture Radar (InSAR) involves the study of a rapidly growing archive of data from satellite, airborne and ground platforms, at a range of wavelengths and resolutions.  InSAR is often used to estimate ground displacements over time, due to earthquakes, volcanic activity, glacier motion, or anthropogenic activity such as the withdrawal or injection of subsurface fluids.  In some areas, such as central California, groundwater depletion has resulted in many meters of subsidence with current rates of up to 30 cm/yr.  In places where there are no requirements on the reporting of groundwater withdrawal (such as California) monitoring ground subsidence from space is one of the few tools we have at our disposal as we try to characterize the water budget.  However, subsidence is not the only factor that contributes to the InSAR observations.  Rapid groundwater withdrawal is often associated with agriculture, and the cycles of tilling, irrigation and harvest change the ground surface in sometimes unexpected ways.  Our research shows that biases can be introduced that are related to nonlinearities introduced in standard InSAR processing approaches and characteristics of the crops themselves. These biases can reach 1 cm/yr in some crop types, depending on the time period of observation.  This is small relative to the 30 cm/yr rates observed in central California, but may be significant in smaller hydrologic basins, or for studies that focus on the temporal variability of such signals.
 

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