Niko Wanders et al. published a paper in HESS investigating the improvement in flood forecasting when assimilating simultaneously different soil moisture products. See his paper at:

http://www.hydrol-earth-syst-sci.net/18/2343/2014/hess-18-2343-2014.pdf

Wanders, N., Karssenberg, D., de Roo, A., de Jong, S. M., and Bierkens, M. F. P., 2014. The suitability of remotely sensed soil moisture for improving operational flood forecasting, Hydrol. Earth Syst. Sci., 18, 2343-2357.