U.S. climate reference network soil moisture observations with triple redundancy: Measurement variability

NOAA U.S. Climate Reference Network Program has deployed triple redundant soil moisture and soil temperature observation instruments at 114 climate stations nationwide at 5 standard depths, providing real time insights into measurement variability. Some initial findings gathered from this unique configuration are discussed in this paper. Between 2009 and 2011, the U.S. Climate Reference Network (USCRN) was augmented with soil moisture/soil temperature probes and atmospheric relative humidity instruments as part of a programmatic expansion in support of the National Integrated Drought Information System (NIDIS). The 114 sites in this sparse network are well distributed across the conterminous United States in open, rural locations expected to remain unchanged in land use for many decades into the future. Soil probes are installed in triplicate redundancy, similar to the air temperature and precipitation measurements, at either five standard World Meteorological Organization (WMO) depths (5, 10, 20, 50, and 100 cm) or only two depths (5 and 10 cm) depending on the nature of the underlying materials. Stations also measure air temperature, surface skin temperature, precipitation, solar radiation, and 1.5-m wind speed. In addition to sensor failure, the triplicate design of USCRN soil probes have allowed for an initial characterization of variability of soil moisture measurements. Nationwide analysis of soil moisture during early-to-mid growing season in 2011 and 2012 was performed to examine the differences in response to the widespread drought of 2012. The redundancy of the network helps retain the continuity of the record over time, and also provides key insights into the variations of measurements at a single location that are related to a combination of installation effects and the impacts of soil differences at the local level. This article highlights the usefulness of deploying triplicate configurations of soil probes for detecting faulty sensors and for better understanding the nature of soil moisture measurement variability.