You can check these highly-cited papers to get information on the basics on the problem.
Moulin, S., A. Bondeau & R. Delecolle, 1998: Combining agricultural crop models and satellite observations: From field to regional scales. Int. J. Remote Sensing, Volume 19, Issue 6.
de Wit, A. J. W. & C.A. van Diepen, 2007: Crop model data assimilation with the Ensemble Kalman filter for improving regional crop yield forecasts. Agricultural and Forest Meteorology, Volume 146, Issues 1–2, Pages 38–56
Bolten, J.D., W.T. Crow, T.J. Jackson, X. Zhan and C.A. Reynolds,"Evaluating the utility of remotely-sensed soil moisture retrievals for operational agricultural drought monitoring," IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 3, 57-66, 10.1109/JSTARS.2009.2037163, 2010.
Bolten, J.D. and W.T. Crow, "Improved prediction of quasi-global vegetation conditions using remotely-sensed surface soil moisture," Geophysical Research Letters, 39, L19406, doi:10.1029/2012GL053470, 2012.