Network Design and Implementation
Texas Soil Moisture Observation Network (TxSON) is an intensively-monitored 36 km grid cell (1300 km2) located near Fredericksburg, Texas, along the Pedernales River and within the middle reaches of the lower Colorado River. The network consists of over 40 monitoring stations that measure in situ soil moisture and precipitation, mission and its Calibration and Validation Program. Using a nested design (>7 stations per cell), TxSON will replicate soil moisture at 3, 9 and 36 km satellite pixels in support of NASA’s Soil Moisture Active/Passive (SMAP) Cal/Val Program.
The grid location was determined through temporal stability analysis of soil moisture from the North American Land Data Assimilation System (NLDAS-2) across each Hydrologic Unit Code (HUC8) within the Middle Colorado basin. Subgrid locations at 3- and 9-km cells were based primarily on land accessibility and secondarily on soil type and geomorphic settings including soil thickness, bedrock geology, and terrain. The end goal is to establish areally-averaged soil moisture estimates at each spatial scale through additional field measurement and airborne campaigns.
TxSON uses three different system designs. A micro-soil moisture station consists of a micrologger (Campbell Scientific CR200), soil moisture sensors (CS-655) installed at 5, 10, 20, and 50-cm depths, an 8” raingage (TE525), a 10W solar panel, and cellular data modem. In addition, 6 TxSON Meteorological Stations and soil moisture sensors were distributed across the entire 36km cell. These use a data logger (CR1000), air temperature and humidity (Rotonic HC2S3), wind speed and direction (RM Young Wind Sentry), and solar radiation (Hukesflux LP02) along with soil moisture sensors (CS-655). Additionally, 7 exiting LCRA Hydromet stations were augmented with soil moisture sensors (CS-655).
Each water content sensor is installed horizontally into the undisturbed face of a trench excavated by a portable soil auger. The complete excavation is to 36 inches or to the limestone bedrock; whichever comes first. The hole is carefully backfilled and the sensor wires or leads (~20 feet) are routed through PVC conduit into a data logger housed above ground on a secured tripod. Power is maintained using a 10W solar panel and a 12V, 7AH sealed battery. This area is protected from livestock using 8’ cattle panels.
Once installed, stations require little physical upkeep but periodic access is required. Site access will be requested at least 24 hour in advance and the landowners’ convenience. All sites transmit data hourly to BEG and will be made available for UT researchers, as well as interested stakeholders. We do not plan to remove any stations over the short-term (3 years). We also plan to keep the system in place for the long-term (10+ years) to build an adequate data set.