The following information obtained from UNAVCO's online DOI system:
(https://www.unavco.org/data/doi/search/search.html)

GPS Soil Moisture Network - AMES-ames P.S.
Start: 04-Apr-2012
End: 05-Apr-2016
Braun, John, 2012, GPS Soil Moisture Network - AMES-ames P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5MK6B14