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:
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.