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

GPS Soil Moisture Network - NWOT-Niwot Ridge P.S.
Start: 02-Sep-2009
End: 29-May-2020
Larson, Kristine, 2009, GPS Soil Moisture Network - NWOT-Niwot Ridge P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5VQ30RT