The NOAA-CREST L-band microwave observation unit will be used to establish three temporal SM observational sites in Western Puerto Rico. The selected sites (Isabela, Lajas, and Mayagüez) are part of the Agricultural Experiment Station program and are under the footprint of the Puerto Rico Weather Radar Network. The main objective of this work is to generate a data set for calibration and validation activities of the recently launched NASA Soil Moisture Active Passive (SMAP) mission. As well, this research investigate the spatiotemporal variability of SM in a field-scale area, with particular focus on the effect of land surface heterogeneity (important over tropical domains) on the retrieval of SM from L-band passive microwave observations. Important focus is given to the characterization of crop canopies and water stress related phenomena using MRS methods. Lastly, a framework will be developed to integrate satellite based SM data with the NWS's current and future operational forecast system for Puerto Rico as it will accounts for surface layer SM. The satellite based SM data will replace the model calculated SM state to the HL-RDHM by simultaneous horizontal, vertical, and temporal data assimilations. The coarse spatial resolution (30 km x 9 km) of current data sets will be downscaled to 4 km x 4 km Hydrologic Rainfall Analysis Project (HRAP) grid cells using physical parameters (Seo, Lakhankar, et al 2014). To our knowledge, this study constitutes the first attempt to study, in the Caribbean, the performance of SM retrievals using passive microwave observations form a ground-based L-band radiometer that mimics the SMAP mission.