The Oklahoma Mesonet’s Soil Temperature and Soil Moisture Networks Chris Fiebrich Manager, OK Mesonet

Soil Temperature Observations  Mesonet Sites:  120 sites with 10-cm soil temperatures under bare and sod  116 sites with 5-cm soil temperatures under bare soil  113 sites with 5-cm soil temperatures under sod  112 sites with 30-cm soil temperatures under sod  Micronet Sites:  35 sites with 5, 10, 15, and 30-cm soil temperatures under sod (pre-2009); at 5, 25, and 45 cm ( since 2009).

Soil Temperature Sensors  Mesonet Sites  BetaTHERM sensor that uses a chip thermistor, housed in a sealed stainless steel tube filled with electronics potting material.  Calibrated using a bath of 50% water and 50% antifreeze, compared with NIST- certified Hart Scientific reference thermometer between -25 and 60°C. Every sensor must meet ±0.5°C accuracy over entire range.  Micronet Sites  Pre 2009, used same BetaTHERM sensor as Mesonet, now uses Stevens Hydra Probe.  Calibration process is evolving. For now, only verify that sensor in deionized water produces real dielectric constant around 80 and imaginary dielectric constant less than 5.

Soil Temperature Calibrations

Soil Temperature Data  Sampled every 30 seconds, averaged every 15 minutes

Soil Temperature Data  Good agreement between the Hydra Probe soil temperatures and the BetaTHERM thermistor measurements

Soil Temperature Considerations  Soil Temperature gradients can be substantial in the top ~10 cm of soil. Field technicians monitor depth using PVC guide. I personally have lower confidence in 5-cm temperatures.  Heaving occasionally occurs in winter.

Soil Temperature Considerations  On rare occasions, solar panel casts afternoon shadow on soil temperature plot for up to 30 minutes. In extreme instances, this shading results in an artificial, 1-2°C decrease in 5-cm soil temperature.

Soil Temperature QA  Soil Temperature Data  Automated tests include range, step, persistence, spatial, like-instrument, heat transfer, climate, and various adjustment tests  Manual analysis includes looking at plots of average values and cumulative differences between sensors at different depths.

Soil Temperatures Can Be Quite Heterogeneous  Monthly Average Soil Temperature at 5 cm, September 2008 (~5 km spacing)  Monthly Average Soil Temperature at 5 cm, September 2008 (~30 km spacing)

Moving on to Soil Moisture…

Soil Moisture Observations  Mesonet Sites:  108 sites with 5-cm soil moisture  106 sites with 25-cm soil moisture  80 sites with 60-cm soil moisture  33 sites with 75-cm soil moisture (this depth is being decommissioned as sensors fail)  Micronet Sites:  35 sites with 5, 25, 45-cm soil moisture

Soil Moisture Sensors  Mesonet Sites  Campbell Scientific 229-L Water Matric Potential Sensor: heat dissipation sensor that utilizes a thermocouple as a temperature sensor and a resistor as a heating element, both housed within a hypodermic needle embedded within a porous ceramic matrix.  Calibrated by immersing in distilled water for 5 days to achieve wet point and then sealing in desiccant for 5 days to achieve dry point. After the sensors are deployed in the field, individual coefficients are updated if the soil creates drier or wetter situations than observed in the lab.  Micronet Sites  Stevens Hydra Probe  Tested by placing in deionized water and ensuring that real dielectric constant is ~80 and imaginary dielectric constant is less than 5.

229-L Mesonet Calibration Certificate

Soil Moisture Data: August 2008  Top plot: Fractional Water Index as measured by 229-L at Mesonet Site Apache, OK. “Saturates”  Bottom plot: Volumetric Water as measured by Hydra Probe at ARS Site A152 (approximately 7 km away). “Pulsates” Red: 5 cm depth Green: 25 cm depth Black: 45/60 cm depth Blue: Rainfall (in)

Soil Moisture Quality Assurance  Soil Moisture Data  After installation, all data is manually flagged for 21 days to allow the soil to heal (sometimes longer during extended drought periods)  Range tests, calibration tests, step tests, frozen soil tests, preferential flow tests  Manual inspection and monthly analysis (max and min for each depth), time series analysis

Oklahoma Mesonet Soil Moisture Data, Monthly Analysis  Monthly Total Rainfall (mm), December 2008  Maximum Fractional Water Index at 5 cm, December 2008

ARS Soil Moisture Data, Monthly Analysis  Monthly Total Rainfall (mm), September 2008  Maximum Volumetric Water at 5 cm, September 2008

Hydra Probe Sensitivity to Soil Classification

Maintenance Metrics 229-LHydra Probe # of Sensors Used # of Failures301 (13 years of use)38 (3 years of use) Common ProblemsMice, gophers, deer, and other cable damage; lightning, ceramic hardening Cable damage, lightning

Common Symptoms of Failed Hydra Probes  Sensor voltages report out of range: either zero or -9.23*10 18  Derived volumetric water values have large oscillations within range, but are not associated with rain or other phenomena

Hydra Probe Wiring Problems/Errors  Mistakes in wiring can cause depths to be “crossed”  Wiring errors can cause soil moisture to decrease during rain events and increase during dry-down periods  A good verification of the wiring is to plot the derived soil temperatures from the sensor

229-L Considerations  Need valid thermocouple reference junction temperature (noise in reference temperature can cause noise in the 229-L measurement) because we use separate multiplexer  Resistive heater can fail or the device supplying power to the resistive heater can fail

Measurement Representativeness  Soil Temperatures:  Vegetation differences can cause soil temperatures to vary > 15°C between neighboring stations  Some soil types are more likely to have erosion problems, cracking, and heaving  Soil Moisture  Vegetation differences, slope, and soil type can make the difference between a dry and a moist soil  If you thought rainfall was heterogeneous between neighboring stations, wait until you look at soil moisture data!

 Samples collected from 12 locations within 20 m of the site

Journal of Oceanic and Atmospheric Technology 2008