Utilizing a Campbell Scientific 229 Matric Potential Sensor as an Operational Soil Temperature Sensor

The Oklahoma Mesonet is an automated network of 120 remote, meteorological stations across Oklahoma. Each station measures core parameters that include: air temperature and relative humidity at 1.5 m, wind speed and direction at 10 m, atmospheric pressure, downwelling solar radiation, rainfall, and bare and vegetated soil temperatures at 10 cm below ground level. In addition, over 100 sites measure air temperature at 9 m. In an effort to avoid anthropogenic influences, most Oklahoma Mesonet sites are located in rural areas. Mesonet data are collected and transmitted to a central point every 5 minutes where they are quality controlled, distributed and archived. To provide a more robust network design of soil temperature measurements, the Oklahoma Mesonet analyzed the potential of using the Campbell Scientific 229 heat dissipation water matric potential sensor as dual purpose sensor to also measure soil temperature. First, an improved wiring technique for the 229 sensor greatly reduced the mean squared error of temperature measurements from 0.20°C to 0.01°C. To put the sensor accuracy into perspective, a field experiment was conducted to quantify the expected variability of soil temperature across a 10 m strip of land similar to a Mesonet site. The experiment identified that soil temperatures at the 5 cm depth frequently vary by more than 0.75°C, while less variability is observed at the 10 and 30 cm depths. Additionally, a comparative analysis of past Mesonet soil temperature values via BetaTHERM soil temperature sensors and the 229 measurements yielded strong correlations and linearities between the daily average values and errors of less than 1°C. However, laboratory test of both sensors in calibrated temperature chambers demonstrated near exact agreement. The overall results of the experiments demonstrated that the 229 is a viable sensor for use as a soil temperature sensor when properly calibrated and installed.