Multiple SMAP Validation data sets are now available at NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC). These data sets contain soil moisture, soil temperature and precipitation measurements recorded in 2016 in Iowa, USA.
SMAPVEX16-IA Permanent Soil Moisture Network, Version 1 Data set DOI: https://doi.org/10.5067/P3ZUS77FW7JL
SMAPVEX16-IA Manual ThetaProbe Soil Moisture, Version 1 Data set DOI: https://doi.org/10.5067/QUZD7OEEVRFZ
SMAPVEX16-IA Temporary Network Soil Moisture and Temperature, Version 1 Data set DOI: https://doi.org/10.5067/CP6MNQ7I7ZRC